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Title: Meteorology - The Science of the Atmosphere
Author: Talman, Charles Fitzhugh
Language: English
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[Illustration: _A tornado near Elmwood, Nebraska. A painting made from
what is probably one of the most remarkable photographs ever taken
of a tornado. The original photograph was made in two parts, as the
photographer was too close to get the whole funnel cloud into the field
of his camera._

(Photograph by G. B. Pickwell)]



                        POPULAR SCIENCE LIBRARY

                            EDITOR-IN-CHIEF
                           GARRETT P. SERVISS


                                AUTHORS

  WILLIAM J. MILLER          HIPPOLYTE GRUENER         A. RUSSELL BOND
    D. W. HERING             LOOMIS HAVEMEYER       ERNEST G. MARTIN
      ARTHUR SELWYN-BROWN                   ROBERT CHENAULT GIVLER
        ERNEST INGERSOLL                    WILFRED MASON BARTON
          WILLIAM B. SCOTT                  ERNEST J. STREUBEL
            NORMAN TAYLOR                         DAVID TODD
                        CHARLES FITZHUGH TALMAN
                              ROBIN BEACH

                      ARRANGED IN SIXTEEN VOLUMES
                 WITH A HISTORY OF SCIENCE, GLOSSARIES
                          AND A GENERAL INDEX

                             _ILLUSTRATED_

                             [Illustration]

                               VOLUME ONE

                      P. F. COLLIER & SON COMPANY
                                NEW YORK



                             Copyright 1922
                     BY P. F. COLLIER & SON COMPANY

                        MANUFACTURED IN U. S. A.



                              METEOROLOGY
                     THE SCIENCE OF THE ATMOSPHERE

                                   BY
                        CHARLES FITZHUGH TALMAN

               CHAIRMAN, COMMITTEE OF PUBLIC INFORMATION,
                    AMERICAN METEOROLOGICAL SOCIETY

                             [Illustration]

                      P. F. COLLIER & SON COMPANY
                                NEW YORK



PREFACE


Meteorology is the science of the atmosphere and its phenomena,
including weather.

Nowadays, when we speak of a “meteor,” we generally mean a shooting
star; but formerly this term was applied (and it still often is in
technical literature) to a great variety of phenomena and appearances
in the atmosphere, including clouds, rain, snow, rainbows, and so
forth. That is how the science of the atmosphere came to have its
present name.

Meteorology is not a branch of astronomy. These two sciences are as
different from each other as zoölogy is from botany. They are both
founded on physics, and they “overlap” each other to some extent, just
as every science does certain others; but if you want information about
the atmosphere, weather and climate, an astronomical observatory is not
the place to seek it; while if you wish to make inquiries about comets,
sun spots, eclipses, standard time, or the date on which Easter fell in
the year 1666, do not apply to the Weather Bureau.

In the city of Washington the Government maintains an astronomical
and timekeeping institution known as the Naval Observatory, and it
maintains in the same city the central office of the United States
Weather Bureau. The two establishments are a mile apart in space and
nearly a whole library apart in the subjects with which they are
concerned. The fact that their functions are persistently confounded by
the public indicates the necessity of writing this preface to a popular
book on meteorology.



CONTENTS


  CHAPTER                                           PAGE
      I.  THE ANATOMY OF THE ATMOSPHERE                9
     II.  THE RESOURCES OF THE ATMOSPHERE             24
    III.  THE ATMOSPHERE AS A HIGHWAY                 39
     IV.  DUST AND SMOKE IN THE ATMOSPHERE            52
      V.  WEATHER AND WEATHER INSTRUMENTS             66
     VI.  CLOUDLAND                                   90
    VII.  PRECIPITATION                              106
   VIII.  WINDS AND STORMS                           123
     IX.  ATMOSPHERIC ELECTRICITY                    141
      X.  ATMOSPHERIC OPTICS                         164
     XI.  ATMOSPHERIC ACOUSTICS                      186
    XII.  CLIMATE AND CLIMATES                       197
   XIII.  ORGANIZED METEOROLOGY                      212
    XIV.  WEATHER MAPS AND FORECASTS                 224
     XV.  AGRICULTURAL METEOROLOGY                   245
    XVI.  COMMERCIAL METEOROLOGY                     261
   XVII.  MARINE METEOROLOGY                         271
  XVIII.  AERONAUTICAL METEOROLOGY                   284
    XIX.  MILITARY METEOROLOGY                       306
     XX.  MEDICAL AND PHYSIOLOGICAL METEOROLOGY      316
    XXI.  WEATHER-MAKING                             332
   XXII.  ATMOSPHERIC BYWAYS                         346
          GLOSSARY                                   365



LIST OF ILLUSTRATIONS


  FUNNEL-SHAPED CLOUD OF A TORNADO                     _Frontispiece_
          _Painted from an unusual Photograph_

                                                          FACING PAGE

  EXPLORING THE UPPER AIR--BEGINNING OF A PILOT BALLOON FLIGHT     32

  EXPLORING THE UPPER AIR--SENDING UP A SOUNDING BALLOON           32

  METEOROGRAPH FOR USE WITH A SOUNDING BALLOON                     33

  KITE METEOROGRAPH TO BE SENT ALOFT ATTACHED TO A KITE            33

  ALTO-CUMULUS--FLEECY MASSES SHOWING BLUE SKY BETWEEN             96

  CUMULUS, OR WOOL-PACK CLOUD                                      96

  MAMMATO-CUMULUS, OR “RAIN BALLS”                                 97

  CUMULO-NIMBUS--THE THUNDERCLOUD                                  97

  CLOUD BANNER OVER MOUNT ASSINIBOINE, CANADIAN ROCKIES           100

  CIRRUS, WITH LOWER CLOUDS IN THE FOREGROUND                     100

  LENTICULAR CLOUD OVER MOUNT RAINIER                             101

  ICE STORM AT PHILADELPHIA--TREES COVERED WITH GLAZE             116

  NIEVE PENITENTE IN THE ARGENTINE ANDES                          117

  CLOUDS OR FOG CASCADING THROUGH LAST FORK CAÑON INTO SANTA
    ANITA CAÑON                                                   136

  CLOUDBURST IN SOUTHERN UTAH                                     137

  PHOTOGRAPHIC ANALYSIS OF LIGHTNING--SINGLE AND DOUBLE
    DISCHARGES                                                    160

  PHOTOGRAPH OF LIGHTNING, SHOWING “BLACK” FLASHES                161

  ATMOSPHERIC ELECTRICITY INSTRUMENTS                             161

  GLASS WEATHER MAP IN THE UNITED STATES WEATHER BUREAU           224

  THE SUN DRAWING WATER                                           225

  ORCHARD HEATERS IN OPERATION AS PROTECTION AGAINST EXPECTED
    FROST                                                         256

  SNOW SURVEYOR AT WORK, USING A CYLINDRICAL SNOW SAMPLER         257

  SNOW ROLLERS, OR WIND-BLOWN SNOWBALLS                           257

  BED OF THE POTOMAC RIVER PHOTOGRAPHED FROM THE AIR              288

  DRILLING WITH COMPRESSED AIR IN A COPPER MINE                   289

  LAUNCHING A WEATHER BUREAU KITE TO EXPLORE THE AIR OVER THE
    OCEAN                                                         289

  WEATHER BUREAU KIOSK, UNION SQUARE, SAN FRANCISCO               320

  UNITED STATES WEATHER BUREAU STATION AT PEORIA, ILLINOIS        320

  CENTRAL OFFICE OF THE UNITED STATES WEATHER BUREAU AT
    WASHINGTON                                                    321



CHAPTER I

THE ANATOMY OF THE ATMOSPHERE


Two quite different conceptions of the substance called “air” are
current in the world. One has prevailed from time immemorial. The other
is wholly modern. One is the popular view, the other the scientific.

Ancient philosophers regarded air as one of the four “elements” of
which all things were supposed to be made. Average humanity, though it
did not concern itself with philosophy, must have begun, almost as soon
as it realized the existence of air at all, to think of it as something
that, however it changed its state from hot to cold, dry to moist, pure
to impure, was fundamentally uniform--a single entity. Certainly this
idea is in full vigor today. The air that we breathe, supply to our
fires, stir with fans, pump into bicycle tires, fly in--the air that
asserts its independence of our will in the wind and the weather--gives
us the impression of individuality. We instinctively rank it with water
among the simple, definite things in the repertory of nature.

Even the man of science often finds it convenient to discuss and deal
with air as if it were a single substance, but he is well aware that
it is nothing of the kind. He knows that it is, in fact, a jumble of
gases having very different properties. Some are heavy, others light.
Some are chemically very active, others extremely inactive. Some
are abundant, others very rare. These gases constitute the earth’s
atmosphere. Other planets have atmospheres that are quite different in
composition from ours. The sun itself has a very complex atmosphere.

The earth’s atmosphere is, then, a collection of gases, which are mixed
but not chemically combined. Some of them are themselves chemical
compounds. Each of these gases behaves very much the same as if the
others were not present, and each of them has its separate business to
perform in the economy of nature. For example, a tree draws upon the
store of carbon dioxide gas in the atmosphere to build up its tissues.
Presently the tree is cut down and its wood is burned for fuel. In this
process a different atmospheric gas is brought into play. We often say
that the “air” supports combustion--that we supply “air” with a bellows
to make a fire burn more brightly--but it is not the air as a whole
that enables things to burn. Four-fifths of the atmospheric substance
takes no part in the process. We burn with oxygen alone. So it is with
breathing. Oxygen and not air constitutes the breath of life.

Near the surface of the earth the proportions of the more abundant
gases mixed together in the air are remarkably constant. Ignoring a
variable admixture of water vapor, oxygen is always about 21 per cent,
by volume, and nitrogen about 78 per cent. The remaining 1 per cent
is mainly argon. At great altitudes, however, these percentages no
longer obtain. The atmospheric gases differ greatly among themselves
in weight, and in the high atmosphere, where they are not mixed by
the winds, as they are below, the heavier tend to settle to the
bottom and the lighter to float on top, as oil floats on water. It
is calculated that at a height of thirty miles above sea level the
percentage of nitrogen is about 86½ and of oxygen only 10, while at
the same altitude the gas hydrogen, which at low levels constitutes
less than one-hundredth of 1 per cent of the atmosphere amounts to more
than 2½ per cent. Going higher, the percentage of hydrogen is supposed
to increase rapidly, until, at an altitude of forty-eight miles, the
atmosphere is more than half hydrogen, and at eighty miles above the
earth this gas forms 99 per cent of the whole. These figures are not
necessarily final; for some authorities believe that the atmosphere
contains an unknown gas lighter than hydrogen, while others think that
the hydrogen found in the lower air enters into chemical combinations
before it can reach the higher levels; but it is beyond doubt that the
composition of the upper atmosphere is quite different from that of the
lower.

Of course almost any gas may be found locally and occasionally in the
atmosphere, but there are several that are always found wherever a
refined analysis of the air is made, and others that are generally
present. The following is a fairly complete list: Nitrogen, oxygen,
water vapor, argon, carbon dioxide, hydrogen, helium, neon, krypton,
xenon, niton (radium emanation), ozone, hydrogen dioxide, ammonia and
other compounds of nitrogen.

A number of these substances have only become known to science within
the last quarter of a century. Argon, though it constitutes nearly 1
per cent of the atmosphere, escaped detection until the year 1894. The
investigation of argon led to the discovery of some of the others. In
1895 it was found that the air, as well as certain minerals, contains
helium. This substance was not new to science, but it had never before
been found on earth. It was discovered in the atmosphere of the sun, by
means of the spectroscope, as early as 1868. Terrestrial helium, neon,
krypton, and xenon were all discovered by Sir William Ramsay, who also
shared with Lord Rayleigh the distinction of discovering argon.

Ramsay has published the following figures for the proportions in which
some of the rare gases exist in the atmosphere:

  Helium     1 part in     245,320 by volume
  Neon       1   “          80,800  “    “
  Krypton    1   “       2,000,000  “    “
  Xenon      1   “      17,000,000  “    “

Niton, or radium emanation, is one of the products of the
disintegration of radium. Niton itself disintegrates very rapidly,
one-half of any given quantity disappearing in about four days, and one
of its products is helium. The amount of niton in the atmosphere is
never more than an infinitesimal trace. Thus we are told that the total
quantity of this substance present in the atmosphere of the whole earth
up to an altitude of one kilometer (0.6 mile) weighs less than nine
ounces, and that each cubic centimeter of air contains among its thirty
million million million molecules only between one and two molecules of
niton, on an average.

Turning, now, to the more abundant constituents of the atmosphere, we
find that oxygen and nitrogen differ strikingly from each other in
the fact that, while the former has a strong chemical affinity for
nearly all other elements, the latter is chemically inert, having
little tendency to unite directly with other elements, though by
indirect processes, and chiefly through the agency of plants and
animals, a large number of nitrogen compounds are produced. Oxides of
nitrogen are formed directly from the atmospheric gases by lightning
discharges, and these unite with the moisture of the air to form
nitric and nitrous acids. A certain amount of ammonia (a compound of
nitrogen and hydrogen) may also be formed by lightning from nitrogen
and atmospheric water, but most of the ammonia in the air is derived
from the decomposition of plant and animal matters. The compounds of
nitrogen that occur in the air are washed down by rain in considerable
quantities. Analyses of rain water made in different parts of the world
show from one to nine pounds of such substances per acre per annum.

Carbon dioxide (more familiarly known as carbonic acid gas) occurs in
the atmosphere in the almost constant proportion of three parts in
10,000 by volume. It is a little more abundant in the air of towns
than in the open country or over the ocean, and it undergoes slight
periodic variations, but the fact that it is not much more variable
is rather surprising, considering that it is continually being added
to and abstracted from the air by numerous agencies that have no
dependence upon one another. It is supplied to the air by volcanoes,
mineral springs, the combustion of fuel, the respiration of animals and
plants, and the decay of organic matter. The amount supplied annually
by the burning of coal alone is estimated to be equivalent to more
than one-thousandth of the total volume of the gas present in the
atmosphere at any one time. On the other hand, all green plants, in the
presence of sunlight, withdraw carbon dioxide from the air, abstract
the carbon from it for the use of the plant, and return the oxygen to
the atmosphere. Thus it is estimated that an acre of beech forest takes
a ton of carbon out of the air annually. A vast amount of atmospheric
carbon dioxide enters into chemical combination with certain rocks
at the earth’s surface. Lastly, a large quota of this atmospheric
gas is absorbed by sea water, and certain authorities have seen in
this process a regulator of the total amount in the atmosphere, the
hypothesis being that the ocean gives back some of the carbon dioxide
whenever this substance becomes deficient in the air.

Water vapor--i. e., water in an invisible gaseous form--is always
present in the atmosphere, but its amount is subject to wide
fluctuations. An important fact in this connection is that, at any
given temperature, the air can hold only a definite amount of this
vapor. This maximum amount increases rapidly with temperature. When the
air is fully charged with water vapor it is said to be “saturated.”
Properly speaking, the temperature limits the amount of the vapor that
can occur in a given space, regardless of the presence of the other
constituents of air, and in scientific language it is the vapor itself
that is said to be saturated, and not the air; but in a popular book
about the atmosphere, where much has to be said about atmospheric water
vapor, adherence to scientific usage in this matter invariably leads
to awkward complications. Speaking, then, in familiar terms--when the
air is saturated with water vapor, a fall in temperature causes some
of the vapor to condense in visible form, as cloud, fog, rain, dew,
snow, hail, etc. As the sole source of these various forms of moisture,
and on account of the important part it plays in many atmospheric
processes, water vapor is, from a meteorological point of view, the
most interesting constituent of the atmosphere.

One more atmospheric gas requires notice here, both on account of
the great popular interest attaching to it, and because of recent
scientific discoveries concerning it--viz., ozone. This substance may
be described, in nontechnical language, as a concentrated form of
oxygen. It is one of the most powerful oxidizing agencies known, and
has found useful applications in medicine and various industries. Its
popular renown, however, is due to the fact that for many years it was
regarded as a great natural purifier of the atmosphere. “Life-giving
ozone” was reputed to be abundant in the air of forests, mountains, and
the seashore. Systematic observations were made of the prevalence of
ozone at different places throughout the world, generally by noting the
change of color of test-papers exposed to the air. These “ozonometric”
observations are now a closed chapter in the history of meteorology,
for it has been found that the reactions of so-called ozone papers
are due chiefly or entirely to atmospheric substances other than
ozone. Moreover, direct examination of the air by more accurate
methods--including samples collected with the aid of kites and balloons
up to a height of several thousand feet above the earth--shows that the
amount of ozone in the whole of the lower atmosphere is exceedingly
small--much too small to be of hygienic significance. Whatever ozone is
produced from oxygen at such levels by lightning discharges or other
possible agencies probably enters promptly into chemical union with
oxidizable substances and therefore has only a brief existence.

On the other hand, the spectroscope has brought us evidence that far
aloft in the atmosphere, many miles above the earth, ozone is quite
abundant. Here it is supposed to be generated by two agencies--the
electrical discharges of the aurora and ultra-violet radiations from
the sun. The ultra-violet rays that help to produce it are prevented
from reaching the earth, and astronomers are thus deprived of much
interesting information they might otherwise obtain concerning the
spectra of the sun and stars. However, as the present Lord Rayleigh
has pointed out, we can console ourselves for this fact by reflecting
that if the ozone did not shut off much of the ultra-violet light from
the sun, this light would probably ruin our eyesight; or, rather, we
should be put to the inconvenience of constantly wearing some sort of
protective spectacles in the daytime.

The high-level ozone is further interesting because of exercising a
certain control over the temperature of the lower air. It is more
transparent for incoming solar radiation than for outgoing earth
radiation. Hence, when it is unusually abundant, it should raise the
general temperature of the earth. This presumably happens when the
condition of the sun is such that an unusual amount of ultra-violet
radiation reaches the upper atmosphere, a fact that must be taken into
consideration in any attempt to establish a relation between climatic
fluctuations and the sun-spot period.

The lowest part of our atmosphere is the densest because it is
compressed by the weight of the air above it. Thus it happens that,
although the atmosphere is at least several hundred miles in height,
one-half of its mass--i. e., one-half of the quantity of matter in
it, as expressed in terms of weight--lies below an altitude of about
3½ miles above sea level, while about seven-eighths lies below the
ten-mile level. Above about five miles the atmosphere is too rare to
support life. The highest clouds seldom occur higher than ten miles.
Storms hardly ever reach that height. In short, the phenomena of life
and the phenomena of weather are confined to a layer of air so shallow,
in proportion to the dimensions of our globe, that on the surface of an
orange it would be represented by a sheet of thin paper.

The actual height of the atmosphere is not even approximately known.
There are theoretical reasons for believing that even at a height of
thousands of miles above the earth there are molecules of atmospheric
gases still under the control of the earth’s gravity, while at such
levels yet other atmospheric molecules are constantly escaping into
outer space. At an altitude of fifty miles the atmosphere is less
than 1/75,000 as dense as at sea level--i. e., more than seventy-five
times as attenuated as the best “vacuum” obtainable with an ordinary
mechanical air pump. At 300 miles it is computed to be about one
two-millionth as dense as at sea level.

The loftiest atmospheric phenomenon that we can observe directly is
the aurora, which has been photographed up to heights of more than
300 miles. The altitude of the aurora is determined by simultaneous
observations made at two or more points, and the same is true
of shooting stars and their trails, which seem to be especially
numerous between the levels of sixty and ninety miles. The so-called
“noctilucent clouds,” which shone by reflected sunlight throughout the
night for some years after the great eruption of Krakatoa and were
supposed to consist of fine dust from that volcano, were probably about
fifty miles above the earth. From the duration of twilight we infer
that above about forty-five miles the air is so tenuous that it cannot
reflect sunlight to the earth. Clouds furnish information concerning
the movements of the air at various levels up to ten miles or more.
Observations on mountains contribute further to our knowledge of the
atmosphere above the ordinary levels of habitation.

Of all methods of exploring the atmosphere in a vertical direction,
the most fruitful is the use of kites and balloons. In recent years
investigations of this character have become so extensive and so highly
specialized that they are regarded as forming a separate department
of meteorology, known as Aerology. It is by virtue of developments in
this field that meteorology has become “a science of three dimensions.”
Formerly meteorologists could do but little more than study the
_bottom_ of the weather, so to speak; but now they observe it and
chart it at all levels. The weather forecaster has daily reports of
conditions aloft to aid his predictions both for dwellers on _terra
firma_ and for the aeronaut; while the accumulated data of upper-air
observations are throwing new light on many difficult atmospheric
problems.

Scientific balloon ascents are no novelty. Some were made in the
eighteenth century, and many famous ones in the nineteenth, including
those of Biot, Gay-Lussac, Glaisher, Tissandier, and other daring
_savants_. The “record” height for such personal ascents was attained
in 1901, when Berson and Süring rose to 35,400 feet above Berlin.
Kites were sent up for meteorological purposes even before Benjamin
Franklin’s immortal experiment in 1752. Modern aerological methods
have, however, little in common with these pioneer undertakings.
Existing types of box kites, pilot balloons, sounding balloons, and
self-registering meteorological apparatus for upper-air research were
developed in the latter part of the nineteenth century, but their use
did not begin to bulk large in meteorology until about the beginning of
the present century. The epoch-making event in these undertakings was
the discovery of the _isothermal layer_.

It is a matter of common knowledge that the air is found to be colder
the higher one ascends in the atmosphere. Thus, even in equatorial
regions, the tops of high mountains are mantled in perpetual snow. The
rate of this temperature decrease averages about 1 degree Fahrenheit
per 300 feet. Previous to the year 1902 meteorologists supposed
that the atmosphere continued to grow steadily colder in an upward
direction indefinitely; but in that year a Frenchman, M. Teisserenc de
Bort, who had sent aloft hundreds of small unmanned balloons carrying
self-recording thermometers, announced that above a height of about
six and one-half miles the temperature ceased to fall. In fact, he
found that at about that level there was often a slight _increase_ of
temperature with increasing altitude for a certain distance upward, and
then a nearly uniform temperature as high as the balloons ascended.
This announcement was at first received with considerable skepticism,
but very soon similar observations were reported from other parts of
the world. A new “shell” of the atmosphere had been revealed--which,
as subsequent investigations proved, differs from the lower air in
other respects besides temperature--and it was at first named by
its discoverer the isothermal layer. He afterward substituted the
name _stratosphere_, now generally employed. In distinction from the
stratosphere, the part of the atmosphere lying below it is called the
_troposphere_.

The stratosphere has been explored in widely scattered parts of the
earth, and information concerning it is daily accumulating. Although
it extends over the whole world, the altitude at which it begins is by
no means uniform. The altitude is greater in summer than in winter; it
varies with the barometric pressure at the earth’s surface; and it is
decidedly greater over the equator than over the poles. The last fact
leads to an interesting paradox. Since over the equatorial regions
the temperature keeps on falling with ascent to a greater height than
in other latitudes, it is here that the lowest temperatures in the
atmosphere are found. A sounding balloon sent up from Batavia, Java,
in November, 1913, recorded 113° below zero Fahr., the lowest air
temperature ever observed. In middle latitudes the temperature of the
stratosphere averages something like 68° below zero Fahr.

The temperature of this interesting upper atmosphere varies a good
deal, both vertically and horizontally, but never shows the steady
vertical variation that characterizes the lower air. The stratosphere
contains no clouds (except occasional dust clouds), and has a
circulation quite distinct from that of the troposphere, the exact
nature of which, however, has not yet been determined.

The sounding balloon, already mentioned, is one of the four principal
types of aerial vehicle used in the study of the atmosphere, the
others being the pilot balloon, the captive balloon, and the kite.
The sounding balloon, or _ballon-sonde_, is a small free balloon
that carries no human aeronaut, but instead a set of superhuman
meteorological instruments, which register the temperature, the
barometric pressure, and sometimes the humidity continuously and
automatically through the whole course of their journey. The record
is traced on a revolving drum or disk, usually coated with lampblack.
In its commonest form the balloon is made of india-rubber, and when
launched is inflated to less than its full capacity with hydrogen. As
it rises to regions of diminished air pressure it gradually expands,
and it finally bursts at an elevation determined approximately in
advance. A sort of parachute, or sometimes an auxiliary balloon,
insures a gentle fall to the ground. Attached to the apparatus
there is generally a ticket offering the finder a reward for its
return, and giving instructions as to packing and shipping. Sooner
or later it generally comes back. In fact, the large percentage of
records recovered, even in sparsely settled countries, is not the
least remarkable feature of this novel method of research. Thus, of
seventy-two balloons sent up by a Franco-Swedish expedition in Lapland,
forty-one were eventually recovered with their instruments. One of
these fell into a lake and was found after three years.

No instruments are carried by the pilot balloon, which merely serves
to show, by its observed drift, the speed and direction of the air
currents at different levels. The pilot balloon is sighted, while in
flight, through a special form of theodolite, or, preferably, two
theodolites some distance apart. Several ingenious methods have been
devised for computing and plotting its actual course through the air.
Such balloons, apart from their use in scientific research, have become
one of the principal adjuncts of aeronautical undertakings all over
the world, and are also used by artillerists to enable them to make
proper allowance for the deflective effect of the wind on the flight of
projectiles. Hundreds of thousands of pilot balloons were sent aloft
for military purposes during the world war.

Meteorological instruments are sent up attached to kites or captive
balloons whenever--as in connection with weather forecasting--the
observations must be obtained more promptly than would be possible with
the aid of sounding balloons, but such devices can attain only moderate
altitudes. Kites have been raised to about four and one-half miles
above sea level, as compared with nearly twenty-two miles reached by a
sounding balloon and twenty-four miles by a pilot balloon. The average
height of sounding-balloon ascents is about ten miles. As already
stated, balloonists have risen to 6.7 miles. This is a little higher
than the best aeroplane record.

The use of the aeroplane for making meteorological observations is
still quite limited, but will inevitably increase. One other device
gives promise of yielding valuable aerological information, on account
of its ability to rise to extraordinary altitudes. This is a special
form of rocket, recently invented by Prof. R. H. Goddard, which is
propelled by several successive discharges of an explosive in the
course of its upward flight, and with which the inventor thinks it will
be possible to explore the whole vertical extent of the atmosphere.
Meteorological apparatus for use with the Goddard rocket has been
planned by Mr. S. P. Fergusson of the Weather Bureau.

The atmosphere presses down upon the earth with a weight that, at sea
level, amounts to about 14.7 pounds to the square inch, on an average.
This pressure is, at any point, exerted equally in all directions; it
acts, for example, on the whole surface of the human body, and this
means that a man of average size lives under a burden of some seventeen
tons of air. He is not incommoded because the pressure from without is
balanced by that of the air that permeates his body.

The pressure of the atmosphere decreases upward at nearly the same
rate as its density. Thus on mountains and plateaus it is considerably
less than in lowlands. At no place is the pressure invariable, nor
is there a constant relation between pressure and altitude, but,
knowing approximately the average atmospheric pressure over the
earth’s surface, and knowing also the area of the latter, we can
compute in round numbers the total weight of the atmosphere--about
5,000,000,000,000,000 tons. This is about 1/1,200,000 of the entire
weight of the earth.



CHAPTER II

THE RESOURCES OF THE ATMOSPHERE


In the economic stress of our times much is heard about “natural
resources.” This phrase suggests to most people’s minds the store of
minerals, fuels, and oil locked up in the ground; the waters available
for drinking, washing, irrigation, power production, and navigation;
the forests and other natural growths of useful vegetation; and the
soil in which we raise our crops. A moment’s reflection, however,
will show that this is a one-sided enumeration. The resources of the
_atmosphere_ are as essential to humanity as those of the land and the
waters, if not more so.

The coal that is dug out of the earth consists mainly of carbon,
which, in bygone ages, was extracted by plants from the air. Moreover,
it would be of no use to us if we did not have the oxygen of the air
in which to burn it. Neither could we smelt metallic ores without
oxygen. All our forests and all our crops draw far more of their solid
substance from the air than from the soil. Fuel and water are valuable
sources of power, but so is the moving air that drives sailing ships
and windmills, and the atmospheric pressure that helps to operate
suction pumps. It is the moisture of the air that feeds our streams
and, directly or indirectly, waters all plants that grow upon the
land. Lastly, it is the atmospheric oxygen that we breathe that keeps
us from very speedily becoming incapable of using any of the other
resources of Nature.

Air and water together contain, in their oxygen, nitrogen, hydrogen,
and carbon, all the major constituents of our foods in unlimited
abundance. It is tantalizing to think of the slow and roundabout way
in which these things are wrought into edible shape--and the prices
we have to pay for them. No less tantalizing, when coal is scarce and
costly, is the thought that every vagrant breeze is laden with the
carbon dioxide from which the chemistry of living plants so readily
extracts the chief element of fuels. The total carbon dioxide of the
atmosphere amounts to something like 2,200,000,000,000 tons, equivalent
to 600,000,000,000 tons of carbon.

We have spoken of the utility of the air as a source of power. It is,
perhaps, even more useful as providing an easy means of storing and
transmitting power. The engineer stores up energy in a mass of air
by compressing it. When the air subsequently expands it gives up its
energy, and, in so doing, may be made to perform a variety of useful
tasks. By a somewhat analogous process energy is applied to creating a
vacuum, in order that the ordinary pressure of the atmosphere may be
made available for doing a particular piece of work. The suction pump,
the siphon, and the vacuum cleaner furnish examples of this process;
and so do such familiar operations as sucking beverages through a straw
and filling a medicine dropper.

From crude types of bellows, with which, from remote antiquity, air
was compressed for the purpose of blowing fires, have been developed
a host of wonder-working appliances of the present day, such as the
air brake, the pneumatic tube, the compressed-air locomotive, diving
apparatus, the caisson, certain kinds of refrigerating machinery, and
a long list of pneumatic tools. To cap the climax of ingenuity in this
field, methods involving both the compression and the expansion of air
have been discovered whereby this invisible, elusive substance may
be changed to a visible liquid and a visible solid; a process having
extremely valuable applications, as we shall presently see.

Compressed air, as a means of transmitting power, rivals such
mechanical devices as gearing, belting, and rope drives, when it is
applied near the compressor; or it may be conducted for many miles in
pipes, thus competing with the electric current; or, finally, it may
be transported in tanks to the place where it is to be used, a process
analogous to the use of the electric storage battery. Compressed air
has, moreover, certain advantages over other methods of transmitting
power for a number of special purposes. Thus for use in coal mines it
is safer than electricity because it is free from the danger of sparks.
There are a great many cases in which the air itself is used in the
process to which the power is applied, as in different kinds of air
blast, from the simple bellows to the blowers of blast furnaces; also
in aerating apparatus, oil and fuel burners, spraying, cleansing, etc.

A familiar form of air compressor is the hand pump used for inflating
bicycle tires. This simple device illustrates two important facts;
first, that a considerable amount of energy must be used to overcome
the expansive force of the air, and, second, that part of the energy
applied to the pump produces heat. That the heat thus produced and
dissipated in the surrounding air represents a loss of energy is
apparent; but energy is wasted in another way that is, perhaps, not so
evident. When a gas is heated its expansive force is increased. Hence,
on account of the heating of the air in the tire, the pump has to do
more work to accomplish a given amount of compression than it would
need to do if the air remained cool.

In order to avoid this loss, the air compressors used for industrial
purposes are provided with some sort of device for keeping the air cool
during compression. This is accomplished by a spray of water inside
the compressor cylinder, or, more commonly, by inclosing the cylinder
in a water jacket. In producing high pressures, the air is compressed
by degrees in two or more cylinders, and cooled between the successive
stages. Lastly, before compressed air is applied to driving tools or
machinery, it is often reheated to increase its pressure. For most
industrial purposes the pressure of compressed air does not exceed 75
pounds to the square inch (5 “atmospheres”). For charging the tanks
of compressed-air locomotives, for liquefying gases, and a few other
purposes, much higher pressures are used. In laboratory experiments air
has been compressed to the enormous pressure of 60,000 pounds to the
square inch, or 4,000 atmospheres. At a pressure of 14,000 pounds to
the square inch compressed air has been successfully used for blasting
in mines in place of ordinary explosives.

The use of pneumatic tools began in the sixties of the last century,
when pneumatic drills were employed with conspicuous success in the
construction of the Mont Cenis and Hoosac tunnels. Such tools are now
indispensable adjuncts not only of tunneling and mining, but also of
nearly every department of metal-working and wood-working, and have
contributed incalculably to the welfare of mankind.

Imagine a workman with an ordinary hammer driving such a tool as
a chisel, punch, or calking iron, and estimate the amount of work
accomplished in the course of a day spent in this wearisome labor.
Then consider how such operations are performed with the help of that
versatile substance, air. The pneumatic hammer consists of a piston
working in a cylinder, to which compressed air is conveyed from a
compressor by means of a flexible hose. The hammer is so designed that
the air causes the piston to work back and forth with great rapidity.
A chisel, rammer, or other percussion tool is loosely fitted in the
nose of the hammer, so that the piston will strike it a blow at each
forward motion. The workman has nothing to do but hold the tools in
place. With a common hammer or mallet a workman will strike from twenty
to a hundred blows a minute, according to the nature of the work. The
speed of the pneumatic hammer ranges from 1,000 to 20,000 blows per
minute, so that its sound is a continuous buzz. Such hammers are used
for calking, chipping, riveting, and a great number of other purposes.

In another large class of pneumatic tools work is done by rotation
instead of percussion. The piston is replaced by a motor, which turns
an auger, drill, or other tool for such operations as boring, screwing,
reaming, etc.

The use of pneumatic tubes for transporting letters, parcels, and
the like, although suggested as early as 1667, has been in practical
operation only since 1854, when a tube 220 yards long was built in
London to convey telegraphic dispatches. The articles to be transported
are placed in a carrier fitting closely inside the tube and propelled
either by introducing air under pressure behind it or by exhausting
the air in front of it. Scores of miles of such tubes laid underground
are now in operation in London, Paris, Berlin, New York, and other
large cities for carrying mail matter. In the United States the
pneumatic cash carrier, used in stores, is the commonest application of
“pneumatic dispatch,” as this system of transportation is called.

The use of compressed air instead of a brush for applying paint,
varnish, and whitewash is a further illustration of the versatile
possibilities of air as a means of transmitting power.

When an inclosed body of air or other gas is subjected to pressure,
its volume is diminished and its density is increased. It is natural
to inquire what will happen if the external pressure be increased
indefinitely. Will the inclosed substance eventually cease to be
gaseous and become a solid or a liquid? The answer to this question,
furnished about half a century ago through the researches of Thomas
Andrews, is that no amount of pressure will liquefy a gas unless
its temperature is below a certain point. This point, known as the
_critical temperature_, is widely different for different substances.
For most of the atmospheric gases it is exceedingly low. Thus oxygen
must be cooled to 118° below zero Centigrade (180° below zero
Fahrenheit) before it will liquefy under any pressure, and the critical
temperature of nitrogen is still lower. Efforts to liquefy the gases
of the atmosphere were unsuccessful for a long time on account of the
difficulty of attaining such low temperatures.

Nowadays the problem is so completely solved that the manufacture
of liquid air is a commonplace commercial enterprise, and millions
of gallons are produced every year. Liquid air is the principal
commercial source of pure oxygen, nitrogen, and other gases found in
the atmosphere. It is also used as a refrigerating substance in various
industrial and scientific processes, and new uses are being found for
it from year to year.

Like many other latter-day miracles, compared with which the alleged
feats of necromancy seem tame and puerile, the liquefaction of air is
founded on quite simple principles. The earliest commercial process
was invented, in its main features, by Linde in 1895, and the newer
processes are merely modifications of this one.

Experiments of the English physicists Joule and Thomson showed that
when a gas under pressure is forced through a small orifice, beyond
which it expands, it undergoes a certain amount of cooling. This fall
in temperature, known as the “Joule-Thomson effect,” is generally quite
small, but Linde devised a means of multiplying it in his “regenerative
cooling process.” The air to be liquefied is first compressed to,
say, 100 atmospheres, cooled as much as possible by water, and passed
through a long spiral tube. At the end of the spiral it escapes through
a small nozzle, and is thus somewhat further cooled by the effect above
mentioned. This cooled air then passes back around the spiral tube, and
causes still more cooling of the air in the latter. The escaping air is
again compressed and goes through the same process as before. Thus its
temperature grows constantly lower, until finally the stream issuing
from the nozzle is a liquid instead of a gas. The liquid collects in a
reservoir, from which it can be drawn off when desired.

The liquid air thus obtained has a temperature of about 315° below
zero Fahrenheit. It is generally drawn into a vessel called, from the
name of the inventor, the Dewar flask, which is open at the top, but
otherwise insulated from the temperature of the surrounding air by
having a double wall, with a vacuum between the walls. The familiar
thermos bottle is constructed on the same principle. In such a vessel
liquid air can be kept for hours and even days, and it is thus
available for use in many interesting laboratory experiments.

Liquid air looks much like water, except for its slight bluish color.
It boils--i. e., changes back to ordinary air--at a temperature
only slightly above that at which it is produced, and this boiling,
of course, goes on rapidly at the surface of the liquid, owing to
absorption of heat from the air above. Liquid air is lighter than
water, upon which it consequently will float. A cubic foot of liquid
air is the equivalent of about 800 cubic feet of ordinary air at 60°
Fahrenheit and atmospheric pressure.

The curious effects of liquid air, only a few of which can be mentioned
here, are not irrelevant to the subject of atmospheric resources,
since they aid in various ways in carrying out important scientific
researches. Almost all liquids are solidified and almost all solids are
hardened and stiffened by immersion in liquid air. Alcohol is promptly
frozen in it, and at the same time gives out so much heat that the
liquid air boils violently and the congealing alcohol overflows the
vessel in a little avalanche of snow. India rubber becomes as brittle
as glass. Meats become so hard that when struck by a hammer they
ring like steel. Chemical action is enormously reduced by exposure to
the low temperature of liquid air, and so is the electric resistance
of metals. One might suppose that such a temperature would be fatal
to all forms of life, but this is not the case. A goldfish, frozen
solid in liquid air, revives and swims vigorously a few seconds after
being replaced in water. Bacteria survive hours of exposure to the
temperature of liquid air, while the seeds of higher plants, even after
several days of similar treatment, sprout the same as other seeds.

Most of the atmospheric gases have not only been liquefied, but
also frozen solid. An important exception is helium, which has been
liquefied only at a temperature of 452° below zero Fahrenheit. The
remarkable feat of liquefying helium was accomplished in 1908 by the
Dutch physicist Kamerlingh Onnes, who subsequently, in his attempts
to solidify this substance, attained the unprecedented temperature of
less than 2 (Centigrade) degrees above “absolute zero,” or 456° below
zero Fahrenheit, by the rapid evaporation of the liquid under greatly
reduced pressure.

[Illustration]

[Illustration: EXPLORING THE UPPER AIR. Left: Beginning of a
pilot-balloon flight. Right: Sending up a sounding-balloon. Note the
parachute, which wafts the basket of instruments gently to the ground
after the balloon bursts. (_Photographs from U. S. Weather Bureau._)]

Although, when air is liquefied, the oxygen and nitrogen are condensed
simultaneously, the latter has a lower boiling point than the former
and therefore passes off more rapidly when the liquid is allowed to
evaporate. This fact makes it possible to separate the two substances,
by the process known as “fractional distillation,” and hence liquid air
plants have been established for the special purpose of manufacturing
oxygen and nitrogen, for both of which there is a large and growing
commercial demand. Scores of millions of cubic feet of oxygen are used
every year in the wonderfully efficient process of welding metals
with the oxyacetylene blowpipe, the flame of which has a temperature
of about 6,000° Fahrenheit. Most of the supply now comes from liquid
air. An equally large amount is used in a recently introduced method of
cutting metal. The object to be cut is first heated to incandescence,
after which a jet of oxygen is played upon it. The metal actually burns
away in the stream, and a clean cut is made like that of a saw. It is
interesting to reflect, when we fill our lungs with oxygen in order to
keep our bodily machinery in operation, that the same atmospheric gas
is applied to the building of motor cars, bicycles, safes, boilers, and
battleships. Cartridges made of lampblack, dipped for a few moments
in liquid oxygen and then primed with a fulminate cap, constitute an
explosive as powerful as dynamite and much cheaper to produce. A small
percentage of oxygen added to the air supplied to blast furnaces has
been found to effect a great saving of fuel used in the furnace.

[Illustration: METEOROGRAPH FOR USE WITH SOUNDING BALLOON. (Fergusson
pattern. U. S. Weather Bureau, 1919.) The aluminum case, surrounded by
hoops of rattan to protect the apparatus when it falls to the ground at
the end of the flight, contains a set of very light self-registering
meteorological instruments. (_Photograph from U. S. Weather Bureau._)]

[Illustration: KITE METEOROGRAPH. (U. S. Weather Bureau Pattern.)
The four pens record the barometric pressure, temperature, humidity,
and wind-force on a sheet of paper wound around the large cylinder,
which is turned by clockwork. Note the fan wheel inside the tube, for
measuring the force of the wind. The apparatus is made chiefly of
aluminum and is inclosed in an outer case of aluminum when sent aloft
attached to the kite. (_Photograph from U. S. Weather Bureau._)]

The most important industrial demand for nitrogen is for use in
“fixation” processes--i. e., for making nitrogen compounds to be used
as fertilizers, explosives, etc. Before describing these processes,
it may be of interest to mention that some of the “rare” gases of the
atmosphere are now obtained on a commercial scale as by-products of
the manufacture of oxygen and nitrogen from liquid air. Thus neon, on
account of its exceedingly small resistance to the passage of electric
discharges, is a promising substance for filling glow lamps; especially
as means have been found of correcting the glaring red color of the
light which characterized the original neon lamps. Argon is likewise
used for filling electric lamps.

The idea of using the unlimited store of atmospheric nitrogen for the
benefit of agriculture and the manufacturing industries has been very
prominently before the public in recent years, and gained special
notoriety during the late war, when great efforts were being made
to increase the supply of nitrogenous materials suitable for use in
explosives. Nitrogenous matters in the soil are indispensable to
the growth of plants, and as long ago as 1898 Sir William Crookes,
in an address before the British Association for the Advancement of
Science, alarmed the world by pointing out the possibility of a general
famine owing to the prospective exhaustion of Chilean nitrates and
other sources of nitrogenous fertilizers. Nitrogen also enters on an
immense scale into the composition of many industrial products besides
explosives. No wonder popular writers have dwelt upon the fact that
the atmosphere contains far more nitrogen than mankind needs for every
possible purpose--actually something like 20,000,000 tons over every
square mile of the earth’s surface.

A widespread misunderstanding, however, prevails as to the problem
involved in utilizing this supply of nitrogen. Free (i. e., uncombined)
nitrogen is of no use as a fertilizer, and it cannot be readily used in
the arts. The process of extracting it from the atmosphere is an easy
one, thanks to the liquid air industry. The real difficulty is to make
this inert gas enter into chemical combination with other substances,
forming useful compounds such as ammonia and nitrates; in other words,
to “fix” it.

As we have stated on another page, lightning discharges cause nitrogen
and oxygen to combine in the atmosphere, and perhaps also combine
nitrogen and hydrogen to form ammonia. There is one other natural
process by which atmospheric nitrogen is fixed. Certain species of
bacteria are able to extract this gas from the atmosphere and combine
it with other materials. Some of these bacteria are independent
organisms, while others form colonies of parasites growing on the roots
of higher plants, chiefly members of the pea family. In the latter case
the bacteria use the nitrogen of the air and carbohydrates drawn from
the roots on which they grow to form nitrogenous compounds, which are,
in part, transmitted to the host plant.

Unfortunately these natural processes do not suffice to maintain
agricultural soils in a high state of fertility. Mineral deposits of
combined nitrogen are practically limited to the nitrate fields of
Chile, from which more than two million tons of nitrate of soda are
exported annually; but this supply cannot last more than a few decades.
Combined nitrogen in the form of ammonia is supplied on a large and
rapidly growing scale from by-product coke ovens, and another perennial
source of nitrogenous matter is found in animal and vegetable refuse of
all kinds, including fish scrap and slaughter-house refuse, garbage,
sewage, manure, etc. Since, however, the demands of agriculture and
the manufacturing industries greatly exceed the total amount of
combined nitrogen obtainable from all these sources, the ingenuity of
inventors has been spurred to the task of fixing atmospheric nitrogen
by artificial methods, and several such methods have now been put in
operation commercially. Their combined product at present constitutes
nearly one-third of the total nitrogen supply of the world.

It is not proposed here to describe these methods in detail, but
it may be mentioned that one of them, known as the “arc process,”
imitates the action of lightning in combining the nitrogen and oxygen
that occur naturally in the air, while the others utilize nitrogen
that has been previously separated from the air by the liquid air
process. The arc process requires, for commercial success, a large
supply of cheap electrical power, and it is at present almost confined
to Norway and Sweden, where electricity is obtained from waterfalls.
In this process air is blown through a huge electric flame, spread
out by a powerful electromagnet. The air yields nitric oxide, which
is combined with water to form nitric and nitrous acids, and these
substances are combined with others to form marketable products. The
most widely used fixation process, and the one which the United States
Government proposed to employ in the large plants that were in course
of construction in this country at the close of the war, is known as
the “cyanamide process.” This process requires, as a part of its raw
materials, large supplies of limestone and coke, from which calcium
carbide is made in an electrical furnace. The calcium carbide, at red
heat, absorbs nitrogen, forming an intermediate product from which, by
further processes, are made ammonia and nitric acid. A third method of
fixing atmospheric nitrogen, which has been applied on a vast scale
in Germany and is now coming into use in other countries, is commonly
called the “Haber process.” In this process nitrogen is combined with
hydrogen, obtained from water, to form ammonia, the combination being
facilitated by the presence of what chemists call a “catalyzer,” i. e.,
a substance that enables other substances to combine without itself
undergoing any change. Several different catalyzers have been used in
the Haber process.

Two or three other methods of nitrogen fixation are beginning to assume
commercial importance.

While the power of the wind holds an important place among the
resources of the atmosphere, it cannot be said that the utilization
of this resource has undergone developments in modern times at all
comparable with the striking inventions and discoveries we have just
been recording, if we except the use of the wind in aeronautics.
Atmospheric resources used by aeronauts will be discussed in subsequent
chapters.

The chief use made of the wind to-day, as in ages past, is to propel
sailing ships, and its use for this purpose is, of course, of less
importance, in a relative sense, than it was before the introduction of
steam. The importance of windmills has also greatly declined. This fact
was strikingly brought out some years ago when the United States Bureau
of Statistics collected, through American consuls abroad, detailed
information concerning the use of the windmills in foreign countries.
In most parts of Europe windmills are rapidly disappearing. In Holland,
for example, the traditional home of the windmill, the perpetual task
of draining the polders is now performed by steam pumps, and the total
number of windmills is estimated to be only about one-tenth what it was
centuries ago. Our own country is probably the only one in which the
use of windmills is increasing. The modern American windmill, with its
disklike assemblage of numerous light sails, and ingenious contrivances
for veering, reefing, etc., is a much more efficient contrivance
than the old-fashioned windmill; but its utility, like that of other
windmills, is limited by the irregular force of the winds.

For years the hope has been entertained that the windmill would
eventually become a common means of generating electricity, but this
hope has not yet been realized, though isolated installations of this
character are in successful use.



CHAPTER III

THE ATMOSPHERE AS A HIGHWAY


Within the last few years the atmosphere has assumed a new and
tremendous importance in human affairs as a medium that affords
facilities for travel and transportation far superior, in many
respects, to those offered by the land or the water. The aerial
highways are now open for business and pleasure. This is a fact that
the majority of people find it difficult to realize. The navigation of
the air on a general scale has so long been looked upon as a dream of
the future that we cannot readily adjust our minds to the reality.

The story of the slow steps by which this momentous fact has been
brought to pass is far too long to be told here. What we purpose to
do in the present chapter is to sketch the multifarious uses to which
man is now applying the aeronautical knowledge and skill that he has
acquired. At the same time we shall anticipate, to some extent, the
developments of the near future; for the lines of progress are so
clearly marked out that it is possible to do this without giving too
much rein to the imagination.

In a subsequent chapter, dealing with Aeronautical Meteorology, we
shall touch briefly upon the mechanical principles that underlie aerial
navigation, by way of preface to a more detailed description of the
conditions of wind and weather encountered by aircraft, and of the
services that the meteorologist is rendering to the aeronaut.

The history of aeronautics may be divided into two periods, with the
year 1914 as the dividing line between them. Before the great war
the many brilliant minds that were trying to solve the problems of
aerial navigation received comparatively little help or encouragement
from humanity at large. The airship and the aeroplane were both
accomplished facts, but most people looked upon them as ticklish
contrivances of very little practical value. From the year 1909 onward
aviation occupied an immense share of public attention; liberal prizes
for aerial feats were offered; new records for speed, altitude, and
endurance were made from day to day; but to the public, and perhaps to
most of the aviators themselves, all this meant merely that a new and
thrilling sport had been created, rather than a new art of boundless
utility. Very few business men felt inclined to invest money in the
development of aircraft, and the governments of the leading nations,
with a single exception, were incredibly blind to the importance of
building air fleets for use in war. The exception was Germany, which
not only gave strong support to Count Zeppelin in the building of his
dirigibles, but developed military aviation to such an extent that she
entered the war with about 800 aeroplanes and a thousand trained pilots.

With the outbreak of the war the budding art burst into vigorous bloom.
Unlimited funds were now available for experimenting and building.
Thousands of flyers invaded the air, and the battle zone was a testing
ground on a vast scale, where one improvement was hardly introduced
before it was replaced by another. Some of the best engineering talent
of the world was diverted from many and various fields to the one task
of supplying the demands of the military aeronauts for more speed, more
power, more reliable motors, better materials and appliances. Thus the
war not only perfected aeronautics--especially aviation--as an art,
but practically created it as an industry. At the close of hostilities
the world found itself in possession of a vast fleet of aircraft, a
multitude of aircraft factories, and a great army of trained aeronauts.
For a time people asked--and perhaps some still ask--“What shall we do
with them?”

There are many answers to this question, and new ones are coming to
light every day. In the aggregate they mean that a new era has dawned
in human affairs--the era in which the sky has been annexed to the
world in which man lives. Henceforth we shall have more elbow room. We
shall no longer be imprisoned in Flatland, but set free in Spaceland.
It is impossible to foresee all the implications of this fact, but
those that are already apparent suffice to fill us with enthusiasm.

Some of the most vexed problems of the present day will soon be
solved by aerial navigation. Take that of our overcrowded cities.
Everybody knows how first the trolley car and then the automobile
helped to relieve the congestion of towns by making it feasible for
people to live many miles from the scenes of their daily work, but
at the same time seriously swelled the traffic of the streets in
business quarters. Aircraft will bring far greater improvements in
this respect, without corresponding disadvantages. In a few years it
will probably be no inconvenience to live fifty or a hundred miles
from one’s place of business. Aeroplanes, built for carrying several
passengers in perfect comfort, _already_ fly at speeds of from 120
to 150 miles an hour, and are almost independent of weather. Much
greater speeds will doubtless be common in the future. Automobiles,
all running on the same level, have almost reached the limit of space
available in our busiest streets, and, under such conditions, they
have nearly lost the advantage of speed they once possessed over the
obsolete horse-drawn vehicle. There can never be such crowding in the
air. When a great volume of aerial traffic is concentrated toward the
centers of towns, people will fly their vehicles at various prescribed
levels, and probably “park” them on many-storied landing stages. New
methods of landing will undoubtedly be invented. The device known as
the “helicopter,” which has made progress toward the practical stage
during the past year, points out the possibilities in this direction.
In the helicopter the propeller blades revolve around a vertical shaft,
thus permitting the vehicle to rise or descend vertically. A prize
of $100,000 has recently been offered by M. Michelin, the well-known
French patron of aviation, for the perfection of this device, which may
soon revolutionize the design of flying machines.

Mr. Holt Thomas, the Englishman whose foresight and enthusiasm have
done so much to hasten the arrival of practical commercial aeronautics,
believes that in the near future the main airways of the world will be
served by airships rather than by aeroplanes. For long journeys the
airship has the advantage that it can carry an ample supply of fuel
without encroaching too much upon the space available for passengers
and cargo. It is, therefore, especially suitable for transoceanic
journeys. Hitherto airships, when not in flight, have been housed
in enormous hangars, involving heavy cost of installation and their
landing has required the services of hundreds of men--an operation that
will probably seem laughable in its crudity to the next generation.
The airship of the future will probably never go into a hangar at
all except for occasional overhauling, as an ordinary ship goes into
drydock. Hence only a few of these costly structures will be needed.
While in service the airship will, on reaching an air port, moor
herself at the bow to a great steel tower, and swing with the wind as a
marine vessel swings at her anchor. At the top of the tower there will
be a landing stage for passengers and freight, connected by lifts with
the ground below. From the main air ports, thus equipped, will radiate
minor air routes, served by aeroplanes, and, in some cases, by flying
boats.

Such landing places for airships were predicted by Kipling in his “With
the Night Mail”--but the author’s vista was of the year 2000! We are
not traveling so slowly as that. Consider what it means that the world
heard with bated breath of Blériot’s flight over the English Channel in
1909; and just ten years later men had flown over the Atlantic Ocean.

We have been writing of the future; but we need not look ahead for
illustrations of the practical value of aerial navigation. Useful feats
already accomplished are so astonishing in their variety that they
make one cautious about assigning a limit to the possible applications
of the new art. It has happened, for example, that a man who had
booked passage on a trans-Pacific steamer missed his boat at Seattle;
whereupon he hired an aeroplane, at a cost of $75, and overtook the
steamer on her way down Puget Sound, thus saving some weeks of delay
in waiting for the next one. Another man, who produces honey on a large
scale, found that spray-poisoned orchards were playing havoc with
his bees. He traveled in an aeroplane over the surrounding country,
selecting stands for his hives at safe distances from such orchards,
and he estimates that this precaution saved him $10,000 in a single
year. In August, 1919, a flying boat deposited a bag of mail on the
White Star liner _Adriatic_ two hours after the ship had left New York.

Several aerial mail routes are now in operation on both sides of the
Atlantic. The first regular service of this character in America
was begun May 15, 1918, between New York and Washington, and during
the first year carried 7,720,840 letters, with few accidents and no
fatalities. The first year of service cost the Government $137,900,
and the sale of aeroplane mail stamps during the same period yielded a
revenue of $159,700. Out of 1,261 possible trips on this route, 1,206
were undertaken, and only fifty-five were abandoned on account of
unfavorable weather. During 1919 the Post Office Department not only
established other aerial routes, but relegated the aerial mail service
to the ranks of the commonplace by reducing the postage on letters
carried by aeroplane to the ordinary first-class rate of two cents an
ounce.

In Europe lines of fast aeroplanes carrying mails, passengers, and
freight daily over regular routes are becoming part of the established
order of things. The operators of a line between London and Paris,
which was inaugurated in November, 1919, are now planning to establish
an hourly service. Some of these lines have been equipped with wireless
telephony, so that the pilots can keep in constant communication
with numerous stations of the company along the route, and also
with one another. They are thus able to obtain, among other things,
current information about the prevalence of fog or other atmospheric
conditions at points ahead of them. Presumably the passengers who
patronize the aeroplane express will also, eventually, enjoy the use
of the wireless telephone _en route_. In connection with the new air
routes suitable landing grounds, for regular or emergency use, are
being laid out at short intervals; the ideal aimed at, for the present,
being the so-called “ten-mile chain”; i. e., a series of emergency
landing grounds about ten miles apart. From ordinary flying levels a
pilot on such a route can always glide to one of these grounds in case
his motor fails. The landing grounds will be utilized, under certain
restrictions, for grazing cattle and for agricultural purposes, to
help cover the cost of rental and maintenance. During 1919 the British
Government established a chain of landing grounds in Africa, all the
way from Cairo to the Cape.

One of the developments of the war was the use of aeroplanes for
photographic mapping. The aeroplane flies over a long tract of ground,
and the camera, exposed vertically, takes pictures automatically
at fixed intervals. The pictures thus taken are carefully joined
together in a single strip. A second tract, parallel with the first,
is photographed in the same manner, and so on, until the whole area
has been covered. Eventually all the pictures are assembled to form
a so-called “mosaic.” This process is highly successful for mapping
a flat country, but presents difficulties when there are hills and
mountains. Some sort of stereoscopic process will probably be
perfected for depicting accurately differences in level and producing
a “contoured” map. Although aeronautical mapping does not yet replace
old-fashioned methods, it already has several obvious uses. It is
especially suitable for the revision of existing maps. Thus the plan
of a city can be quickly brought up to date by this process. In the
United States the Geological Survey has been engaged for many years in
producing large-scale topographic maps of all parts of the country.
This work proceeds slowly, and some of the maps are ten or fifteen
years old. The contours and other natural features on such a map are
still correct, but changes in the region due to the work of man are
often extensive. Revision of these features can easily be made by the
method above described.

For the preliminary mapping of a new country, by photography or by
hand, the aeroplane offers the means of saving an immense amount of
time and effort. The surveyor no longer needs to cut tracks through
the jungle or scale mountains. No region is very difficult of access
to the aviator. The summit of Mount Everest, the highest mountain
in the world, is actually a mile lower than the greatest altitude
attained by an aeroplane. Aviation has become an important feature of
exploring expeditions. Captain Amundsen, the polar explorer, qualified
as an air pilot before he embarked on his drift across the North
Polar basin, and took aeroplanes with him on that journey. In India
the Survey Department has organized a regular aerial photographic and
reconnoissance service, and has lately photographed the high waters
of the River Sutlej in order to obtain data for a big electrification
project. Photographs of the Nile country have also been made for
hydrological purposes. British aviators in Mesopotamia have mapped the
flood boundaries of the Tigris and provided data for estimating crop
areas. In the Philippines an engineer recently made a long aeroplane
flight to determine which of three general routes was most suitable
for a new railway. Many months of time and thousands of dollars were
thus saved, as it was only necessary to send out one party of locating
engineers instead of three after the selection had been made.

Recently the aerial surveyor has become the rival of the hydrographer
in mapping shoals, channels, submerged rocks, and other features
beneath the water. If the water is clear and suitable atmospheric
conditions prevail, objects submerged to a considerable depth may be
distinctly seen from an aeroplane flying far above the surface. It
was on account of this fact that Allied aviators were able to spot
submerged German submarines during the World War. The camera, equipped
with proper plates and ray filters, can pierce the water even better
than the eye. Thus objects have been photographed at a depth of more
than 50 feet. British aviators charted the harbor of Rahbeg, on the
coast of Arabia, by the process in 1917. In this country the leading
exponent of underwater photography is Dr. Willis T. Lee, of the United
States Geological Survey, who has taken scores of photographs showing
submerged features of the waters adjacent to Chesapeake Bay. It is
likely that rivers like the Mississippi, with ever shifting sand bars,
will soon be made safe by monthly or weekly mapping from the air. In
earthquake regions, such as southern Italy and Japan, the changing
coast lines, shallows and harbors can easily be photographed after
each new quake, thus keeping navigation open and protecting the lives
of mariners.

Another application of this process of sighting submerged objects from
the air is the aerial fish patrol. The plan of using aircraft to locate
schools of fish appears to have been first suggested by Professor
Joubin, of the Oceanographic Institute of Monaco, and it has been
carried out with much success in both Europe and America. Its promoters
hope that it will eventually revolutionize the fishing industry and add
greatly to the world’s food supply. In the year 1919 seaplanes from the
North Island Air Station at San Diego, California, made regular flights
at an altitude of about 500 feet over the adjacent waters as an adjunct
to the important fisheries in that vicinity. When a school of fish was
detected, the aviator dropped low enough to ascertain the species,
and if it proved to be of a commercial kind, such as the sardine, the
news was flashed by wireless to the fishing fleet. The ocean in the
neighborhood of San Diego was divided into numbered squares, shown on
charts, and locations were reported by number. In 1920 a daily patrol
was maintained by Navy seaplanes over the waters of Chesapeake Bay in
behalf of the menhaden fishery. According to an official report, “the
experiments fully demonstrated the commercial value of planes in this
fishery.” It is believed that aircraft might be used with equal success
in connection with the whaling industry.

The United States Forest Service has made considerable use of Army
aeroplanes and aviators in patrolling the great forests of the West,
where a constant lookout for fires must be kept throughout the summer.
There are about 28,000 forest fires in this country every year, and
the average area burned over amounts to more than 8,000,000 acres,
entailing an average annual loss of $10,000,000 worth of timber.
Observations are maintained on mountain peaks and towers, but the
aerial watchman commands a much greater range of vision and can readily
detect fires in places such as deep canyons where they are, in many
cases, hidden from the existing lookout points. When a big fire is
in progress, the aviator can quickly ascertain its extent and report
the information by wireless to the fire-fighting forces. In case the
fire is difficult of access on account of the absence of roads, the
fire fighters can be transported to the spot in aeroplanes. It has
even been proposed to fight forest fires by dropping bombs filled
with fire-extinguishing chemicals. At one time it was thought that
aeroplanes might largely replace fixed lookout stations, but experience
shows that both systems of observation are desirable. Many foresters
favor the use of small dirigible airships in place of aeroplanes, owing
to their ability to fly very low, when desired, land in any small
clearing, discharge passengers by rope-ladder while hovering over a
selected spot, and transport relatively large loads of men and supplies.

Such are a few of the valuable peace-time uses that have already been
found for the aerial vehicles that owed their production chiefly
to the late war and for the host of pilots trained during the same
conflict. Undoubtedly the immediate future holds far more interesting
developments in store.

One important practical aspect of aeronautics remains to be mentioned,
and that is the question of safety. In their early days the steamboat
and the steam railway were both risky contrivances. It is recorded
that at one time steamboats were barred from the Thames on account of
their dangers. Undoubtedly the tradition of frequent boiler explosions
lingered in people’s minds long after it had ceased to be a substantial
fact. Aerial navigation--and particularly aviation--has now passed
beyond the pioneer stage, but it still bears the dubious reputation
that it acquired when it was in its infancy. Aerial travel, under
standardized conditions, is no longer unsafe. There are good reasons
for regarding it already as safer than automobiling. According to a
report of the British Department of Civil Aviation, there were 21,000
commercial flights in Great Britain during the six months from May 1
to October 31, 1919, and 52,000 passengers were carried. The total
mileage covered was 303,000. Not a single passenger was killed during
this period, and only ten were injured. There were two fatalities among
pilots and six pilots were injured.

Commander Read, who made the first transatlantic flight, writes on this
subject:

“There are some pilots with whom I would refuse to risk my life.
But, given a modern machine with the proper attention paid it, and a
skillful but conservative flyer, it is as safe a means of rapid transit
as an automobile traveling at less than half the speed. Nowadays there
is scarcely ever an accident in an aeroplane of standard type due to
the fault of material; they are all due to the inexperience or to the
dare-devil stunting proclivities of the pilot--the pilot who ‘takes
chances.’”

Aeronautics is now more than an art. It is a rapidly expanding branch
of applied science. Aeronautical engineering has become one of the
recognized professions. Some of the leading government laboratories
of the world, including the National Physical Laboratory in Great
Britain and the United States Bureau of Standards, are devoting their
attention to aeronautical research. There are also many unofficial
“aerodynamical” laboratories for studying, with the aid of wind tunnels
and other apparatus, the many problems pertaining to the physics of
flight and the principles of aeroplane designing.

Aeronautical questions have begun to figure conspicuously in
jurisprudence. Legislators, as somebody has said, are busy making
vertical laws to supplement the old-fashioned horizontal ones. In
international law, especially, aerial navigation has given rise
to thorny problems and it is already the subject of elaborate
international agreements.

The physiological effects of flight and altitude have added a new
chapter to the science of medicine. Seasickness has been the crux of
the ship’s doctor; will “air sickness” prove equally baffling? What are
the therapeutic possibilities of flying? Will physicians advise their
patients to seek a “change of air” vertically instead of horizontally?

The atmosphere, once monopolized by the birds, has become the abode
of man. That is one excellent reason why everybody should acquire a
knowledge of meteorology--the science of the air.



CHAPTER IV

DUST AND SMOKE IN THE ATMOSPHERE


When the moralist reminds us that we are children of the dust and
predestined to a dusty end, there is a grain of comfort in the
discovery that modern science regards dust as one of the most important
things in the whole economy of nature. No longer does dust seem an
appropriate symbol of insignificance and humility when one surveys the
bulk of serious literature that has been written about it, considers
the caliber of the men who have devoted the better part of their lives
to the study of it, or inspects the great array of ingenious apparatus
that has been devised for its investigation.

The dust of which we have to speak in the present chapter embraces all
small particles of solid matter found anywhere, or at any time, in the
earth’s atmosphere. Particular kinds of dust have, of course, their
special names. Soot, the visible part of smoke, is a form of dust that
has played a very conspicuous part in human affairs; hence the separate
mention of smoke in the heading of this chapter.

While there are many agencies that help to charge the atmosphere with
dust, the most important of them all is the wind. Let us see what
happens when the wind blows over the surface of a dusty road, for
example. If the air flowed in a smooth horizontal stream over such
a surface, its friction would drag the dust along on the ground,
but would not lift it. Such surface drifting, due to the horizontal
component of the wind’s motion, does, of course, occur, and its effects
are strikingly visible in the shifting dunes that often form over a
broad surface of sand or snow. All winds near the earth’s surface are,
however, full of waves and eddies, and in many cases, as over a stretch
of strongly heated soil, there are strong updrafts, sometimes extending
to a great height in the atmosphere. All kinds of dust are heavier
than air, and, contrary to popular belief, never truly “float” in the
atmosphere. Dust may enter the atmosphere at high levels, through the
disintegration of meteors, or it may be spouted up by volcanoes, but
dust blown up from the earth’s surface rises only because the air is
rising with it; and, in still air, all dust sinks more or less rapidly
toward the ground. The rate of its fall depends upon its specific
gravity, and upon the size and shape of the dust particles. Other
things being equal, the finest particles fall most slowly. Exceedingly
fine dust, even without upward air movements to support it, requires
months or even years to fall to the ground from the higher levels of
the atmosphere.

Upward movements in the air suffice to carry millions of tons of dust
aloft every year, and horizontal air currents carry the same dust
far and wide over the earth. The transportation of soil by the wind
leads to some results of remarkable interest, practical as well as
scientific. In the first place, far-reaching changes in topography are
brought about by this process. Thus in China vast areas are covered to
a depth of hundreds or even thousands of feet with a fine yellowish
earth, called “loess,” which is believed to have been blown thither by
the winds from the deserts of Central Asia. Less extensive deposits of
this wind-borne material are found in many other parts of the world,
including the Mississippi Valley. Another effect of wind transportation
is the mixing of soils. There is a constant interchange of soil
material between different regions, so that the composition of the
soil on a particular farm, for instance, is not the same now that it
was a few years ago or that it will be a few years hence. Lastly, the
presence of dust in the atmosphere, whether derived from the soil or
otherwise, has various interesting and important effects upon the heat
and light we receive from the sun and modifies, in numerous ways, the
conditions of human life upon our planet.

Several cases in which enormous quantities of solid matter have been
carried to great distances by the wind have formed the subject of
elaborate investigations on the part of meteorologists. Thus, during
the three days, March 8-10, 1901, heavy dust storms occurred in the
deserts of southern Algeria, and the sequel of these storms was
carefully studied by Hellmann and Meinardus. A widespread cyclonic
storm, central over Tunis at the time, sucked up the dust, which was
carried northward by the winds at high altitudes. Deposits from this
dust cloud occurred over an area extending as far as 2,500 miles from
the place of origin. Reports collected from hundreds of observers
indicated that 1,800,000 tons of dust fell over the continent of
Europe, and one-third of this fell north of the Alps. As much more is
believed to have fallen over the Mediterranean, while on the African
coast itself the deposit is supposed to have amounted to 150,000,000
tons. In March, 1918, a shower of dust discolored falling snow at
various places in the United States over an area of at least 100,000
square miles, extending in an east-west direction from Dubuque, Iowa,
to Chelsea, Vt. Reports of this shower were collected by Messrs. E. R.
Miller and A. N. Winchell, who estimate that the amount of dust could
not have been less than a million tons, and may have been several
hundred million. The dust is believed to have been blown up from the
arid regions of the far southwestern United States and to have been
transported a thousand miles or more.

Off the west coast of Africa, between the Canaries and the Cape Verde
Islands, haze due to dust blown up from the Sahara Desert is frequently
encountered by vessels, especially during the first four months of the
year. This haze probably gave rise to the ancient legend of a Sea of
Darkness--the _Mare Tenebrosum_--one of the mysterious terrors of the
ocean reported by the navigators who first sailed toward the New World.

Extensive deposits of atmospheric dust have attracted attention from
the earliest times. Ehrenberg, in 1849, collected records of 349 such
cases, and published a map showing their distribution, which embraces
the greater part of the world. Atmospheric dust is always brought down
in greater or less quantities by rain. When it consists of fine powdery
sand, the rain sometimes acquires a brownish or reddish tinge, staining
objects on which it falls and constituting the “showers of blood” that
have been regarded as prodigies from remote antiquity. Homer describes
such a shower, and many similar occurrences are recorded by the Roman
historians. Italy, owing to its proximity to the African coast, is
often visited by these showers, which still strike superstitious terror
into the hearts of the peasantry.

The millions of meteors that enter the earth’s atmosphere every day
contribute their quota of dust, though the total amount is small
compared with that of the material lifted from the earth. Fine
ferruginous particles are often seen on the snowy summits of high
mountains and the polar ice fields, and both their appearance and their
composition indicate that they are derived from meteors.

Forest fires, burning peat beds, and other conflagrations on a large
scale discharge quantities of dust into the atmosphere. Cinders from
the great Chicago fire spread over a large part of the globe. They are
said to have reached the Azores some forty days after the beginning
of the catastrophe. In Europe, the once common practice of burning
the moors to prepare them for cultivation gave rise to huge volumes
of smoke, which was carried by the wind hundreds and even thousands
of miles. The stronghold of this old custom--which still survives to
some extent--was East Friesland, in northwestern Germany, and the
characteristic haze to which it gave rise, known as “moor smoke”
(German, _Moorrauch_), was sometimes observed as far away as Spain,
Italy, and Greece.

The famous “dark days” that figure in both ancient and modern history,
though in a few cases probably due to eclipses of the sun, have
generally been the result of an abnormal accumulation of smoke or
dust in the air; sometimes arising from volcanic eruptions, but more
often from burning forests, moors, or prairies. Forest fires are the
principal cause of dark days in the United States. Probably the most
celebrated of such days was May 19, 1780, when, in consequence of
great forest fires along Lake Champlain and down to the vicinity of
Ticonderoga, darkness like that of night prevailed in New England.
All but the most necessary business was suspended, the schools were
dismissed, and the greater part of the population flocked to church to
prepare for the end of the world, which was believed to be at hand.
The great Idaho fire of August, 1910, was responsible for dark days
over a larger area than in any other case on record in this country.
Artificial light was required in the daytime over a broad belt,
extending from Idaho to northern Vermont, but smoke was observed far
beyond this area. The British ship _Dunfermline_ reported that on the
Pacific Ocean, 500 miles west of San Francisco, the smell of smoke was
noticed and haze prevailed for ten days. When smoke in the air forms
a rather thin layer, through which the sunlight penetrates feebly,
we sometimes get an effect similar to the golden glow of sunset, a
yellow or coppery tinge being cast over the landscape. Such was the
cause of the “yellow day” still remembered in New England--September
6, 1881--attributed to the burning of the immense peat bogs of the
Labrador barrens.

Another occasional cause of atmospheric dustiness is the eruption of
volcanoes, especially those of an explosive character, which carry fine
dust to heights at which it cannot be washed out of the atmosphere by
rain. The remarkable dry fog of 1783--the most famous in history--which
covered the greater part of Europe and North America for three or
four months--was undoubtedly due to the violent eruptions of that
year in Iceland and Japan. Its connection with the Iceland eruption
was suggested even by contemporary writers. The outbreak of Krakatoa,
in the East Indies, in 1883, spread a veil of dust over the greater
part of the globe. For two or three years its presence in the air was
the cause of striking optical phenomena, including gorgeous sunset
glows. The story is told of an American fire brigade which, deceived
by one of these brilliant sunsets, set out to extinguish what was
mistaken for a great fire in a neighboring village. A large species of
corona around the sun, known as “Bishop’s ring,” because it was first
observed by the Rev. Sereno Bishop of Honolulu, appeared shortly after
the eruption and reached its maximum intensity the following year.
This was due to the diffraction of light by the exceedingly fine dust
from the volcano, and the same phenomenon has been seen after other
great explosive eruptions; e. g., that of Mont Pelée, in 1902. Some
authorities believe that the finest particles of dust from the Krakatoa
eruption were carried to an altitude of over fifty miles above the
earth, and remained suspended at very high levels for several years,
constituting the strange “noctilucent clouds,” seen on summer nights
from 1885 onward. These clouds glowed with a silvery luster, attributed
to reflected sunlight.

A persistent veil of volcanic dust in the upper air is thought to
exercise marked effects upon terrestrial temperatures, and prolonged
periods of intense vulcanism have been regarded as the cause, or one
of the causes, of the recurrent ice ages of which geology furnishes
the record. This explanation of ice ages was advanced by P. and F.
Sarasin, in 1901, and was first put upon a scientific basis by Dr. W.
J. Humphreys in 1913; but the idea that volcanic dust might be the
cause of cold seasons was suggested by Benjamin Franklin as early as
1784. Franklin’s speculations on this subject were prompted by the cold
winter of 1783-1784, which followed the extraordinary fog of 1783,
already mentioned. Humphreys has published a list of all the great
volcanic outbreaks recorded since 1750, and has shown that each of them
registered itself in the temperatures of the earth and also, since
accurate measurements began to be made of solar radiation, in these
instrumental records. Thus, the intensely cold winters of 1783-1785
followed the tremendous eruptions of Asama, Japan, and Skaptar Jökull,
Iceland, in 1783; the famous “year without a summer” (1816) was the
sequel of the gigantic outbreak of Tomboro, in the Sunda Islands, in
1815, which is said to have hurled thirty-six cubic miles of solid
matter into the atmosphere; and definite periods of low temperatures
and reduced sunshine were observed after the eruptions of Mont Pelée,
in 1902, and Mount Katmai, Alaska, in 1912.

The effect of a volcanic dust veil in lowering temperatures on earth
is attributed chiefly to the fact that, while the fine grains of dust
are able to reflect back into space the short waves of radiation coming
from the sun, they do not bar the passage of the long heat waves
radiated outward from the earth. According to Humphreys’s calculations,
such a veil is about thirtyfold more effective in shutting solar
radiation out than in keeping terrestrial radiation in. This process is
just the reverse of the familiar effect of the greenhouse; where the
glass lets in the short waves of solar radiation but does not readily
let out the long waves of earth radiation.

A small contingent of atmospheric dust consists of common salt (sodium
chloride) due to the evaporation of spray from the ocean. This
substance is frequently found in rain, as well as in samples of air,
not only near the seashore, but even in the interior of continents and
on high mountains. According to Du Bois the amount of sodium chloride
annually deposited on the dunes of Holland is at least 6,000,000
kilograms (more than 6,600 tons).

One of the striking phenomena of arid regions is the dust whirlwind;
exemplified in the “devils” of India and South Africa, the “twisters”
of Texas, etc. E. E. Free, in his treatise on “The Movement of Soil
Material by the Wind” (U. S. Bureau of Soils, Bulletin 68), says of
these whirls:

“They may be seen nearly every hot day, sometimes running rapidly over
the surface; sometimes remaining nearly, if not quite, stationary,
but never losing their rapid rotation. They usually last only a few
minutes, but occasionally persist much longer. One observed by Pictet
lasted for over five hours. They are largest and last longest on the
flat, bare plains of the desert, and are usually seen in a calm or
when only a light breeze is blowing, although their occurrence in
windy weather is not unknown. These whirls have been noticed by many
travelers in desert and steppe regions and have been carefully observed
by Baddeley in India, and by Pictet in Egypt. They are frequent in
China and on the pampas of South America, and occasionally occur during
the dry season even in the humid regions. One of the most interesting
phenomena in connection with the dust whirls is the occurrence of
systems of several whirls, each revolving rapidly about its own center
and also moving about a common center in a more or less perfect circle
a few rods in diameter.”

The little whirls often seen on dusty roads are a miniature variety of
the same phenomenon.

One very important class of dust particles in the atmosphere consists
of organic matter, living or dead, including the pollen of plants and
the countless myriads of microorganisms, as well as a variety of other
products of the animal and vegetable kingdoms. An abundance of pollen
in the air accounts for the occasional fall of yellow rain, described
as “sulphur rain,” “golden showers,” etc. The promptness with which
a piece of stale bread becomes moldy in a damp atmosphere is one of
many proofs of the omnipresence in the atmosphere of the microscopic
spores of fungi, ready to propagate their species with amazing rapidity
as soon as they light upon a suitable nutrient medium. Last, but not
least, bacteria, the most minute of all known organisms--so small that
thousands or millions of them clustered together would make a mass not
larger than the head of a pin--swarm in the air, as they do in water,
the soil, and the bodies of animals. Fortunately, while certain species
of bacteria carry disease and death with them, the great majority are
harmless to mankind.

[Illustration: AITKEN’S DUST COUNTER]

A great many different methods are in use for determining the total
amount of solid matter present in a given volume of air, counting the
number of particles, or gathering samples for microscopic examination.
Thus a known volume of air may be drawn through a filter of cotton
wool or bubbled through distilled water, and the dust detained by the
cotton or deposited in the water may be weighed. In certain types of
apparatus the air is drawn or forced against a plate or tube coated
with glycerin, oil, varnish, gelatin, or other adhesive surface, to
which the dust remains attached. Several devices depend for their
operation upon the fact that when a volume of confined air is cooled
by expansion a point is eventually reached at which the water vapor
present condenses to form a fog, each droplet of which is supposed
to have a single particle of dust as its “nucleus.” This is the
principle involved in the well-known Aitken dust counter, which has
been so extensively used in different parts of the world, and has
furnished most of the impressive statistics of air dustiness found
in textbooks and reference books. Thus, from indications supplied by
this instrument, it is stated that a cubic inch of town air contains
50,000,000 particles of dust; that a room, near the ceiling, was found
to contain 88,000,000 particles per cubic inch; and that a cigarette
smoker sends 4,000,000,000 particles into the air at every puff. Recent
authorities are inclined to look upon these figures as misleading,
for the reason that the nuclei counted with Aitken’s instrument are
probably so infinitesimal in size that they hardly deserve to be called
dust; indeed there is good reason to believe that an indefinitely
large proportion of them may actually be molecules of gases.

The effects of dust, both inorganic and organic, upon the health of
humanity will be considered in another chapter. Certain kinds of
dust are of economic importance on account of their inflammable and
explosive character when mixed with the right proportions of air.
Thus the cereal dusts made in the handling and working up of grain
into food products occasionally give rise to serious accidents. These
occur in cereal, flour, and feed mills, grain elevators, starch and
glucose factories, and on farms in connection with the use of threshing
machines. During a period of ten years, 1906-1916, cereal dust
explosions resulted in the loss of eighty lives and the destruction
of property to a value of $2,000,000 in the United States. A study
of this subject has been made by the United States Department of
Agriculture, and various recommendations have been published with a
view to preventing the occurrence of sparks in the neighborhood of
these dangerous dusts. Coal dust in mines likewise causes numerous
explosions. Preventive measures include wetting the dust, moistening
the air, and powdering the walls, roof and floor of the mine with a
nonexplosive rock dust, which has the effect of stifling an incipient
fire or explosion.

The last species of dust that we have to consider in this chapter is
one that constitutes a literal blot on civilization, since the noblest
cities and monuments of mankind are defaced with it. Neither are
the evils of this kind of dust wholly æsthetic, for it is extremely
injurious to health and enormously expensive. After enduring coal
smoke as a necessary evil for generations, civilised humanity has
now embarked upon a vigorous campaign for its elimination, and very
encouraging results have already been achieved in many parts of the
world. The war against smoke is carried on by numerous societies in
Europe and America; a multitude of laws and ordinances (not all of them
effective) have been enacted on the subject; it has been the occasion
of international conferences and expositions; and its literature has
grown so copious that a partial bibliography of the subject, published
a few years ago by the Mellon Institute, of Pittsburgh, fills 164 pages.

The smoking of chimneys is costly, in the first place, because it is
due to imperfect combustion and the waste of part of the heating value
of the fuel, and, in the second place, on account of the damage wrought
by the deposit of the soot. Thus a smoky atmosphere entails big laundry
and dry-cleaning bills, frequent repainting of houses, injury to metal
work, damage to goods in shops, and excessive artificial lighting in
the daytime. Throughout the United States it is said that smoke causes
an annual waste and damage amounting to five hundred million dollars.
In Pittsburgh alone--before the reform produced by vigorous legislative
and scientific measures, following an exhaustive investigation by the
Mellon Institute of Industrial Research--the cost of the smoke nuisance
was estimated at nearly ten million dollars a year. Means of mitigating
this evil include the introduction of improved appliances for burning
soft coal, and the use of other kinds of fuel. The electrification of
the railway lines entering cities is an important measure of relief.
It is estimated that more than one-third of the smoke found in certain
American cities comes from locomotives.

Systematic measurements of the amount of solid matter contributed
to the atmosphere by smoke have been made at various places in this
country and abroad, and yield startling figures. Measures of the
“sootfall” in Pittsburgh, before the evil there was mitigated, showed
an annual average deposit amounting to 1,031 tons per square mile.
London’s average is 248 tons per square mile for the whole city and
426 tons in the central districts. In the heart of Glasgow the annual
sootfall is 820 tons per square mile.

In Great Britain measurements and analyses of soot and the study of
its effects have been carried out on a large scale for a number of
years by the Advisory Committee on Atmospheric Pollution, attached
to the Meteorological Office. The Committee has installed “pollution
gauges,” of uniform type, at about twenty-five places in England and
Scotland. The soot that falls into these gauges is collected once
a month, weighed and analyzed. This organization also makes direct
measurements of the purity of the air, and has acquired a unique body
of observations that can be used to test the success of efforts made
to abate the smoke nuisance, besides providing interesting comparisons
between the incidence of respiratory diseases and the amount of solid
matter in the air.



CHAPTER V

WEATHER AND WEATHER INSTRUMENTS


The fact that a vast proportion of the conversations in which human
beings engage begin with remarks about the weather has often been
noted, but perhaps never fully explained. Meteorologists sometimes
adduce this fact as evidence that weather is a subject of overshadowing
importance. This bit of reasoning will not, however, bear critical
analysis. It carries with it the implication that people talk about
weather because weather is uppermost in their thoughts. How often is
such the case? Brown, meeting Jones, remarks that it is a fine day. Are
we to infer that Brown was meditating upon the agreeable state of the
atmosphere before he vouchsafed this not altogether novel observation?
Hardly. There is about one chance in a thousand that weather was in his
mind at all.

It is a plausible thesis that people talk so much about weather
because, at an earlier period in the history of mankind, this subject
_was_ of supreme importance. Perhaps it is a custom handed down from
our remote ancestors, whose occupations were nearly all carried on
out-of-doors and who enjoyed but a precarious shelter from the elements
in their rude habitations. In India, as the period of the monsoon rains
approaches, anxiety about the timely arrival and the abundance of
these showers eclipses all other thoughts in the mind of the peasant,
because a severe drought at this season means a famine. When our
forefathers lived by hunting, fishing, and crude systems of grazing and
agriculture, they were, no doubt, equally solicitous about atmospheric
conditions that directly affected their food supply. In those days
comments on the weather were by no means empty formulas. Men rejoiced
together that the day was fine, because it was a circumstance upon
which their dinner depended; and the prehistoric equivalent of “What
beastly weather!” was probably accompanied by a significant tightening
of the belt.

Certain it is that in very early times people gave a great deal of
attention to the weather and acquired a fund of wisdom on the subject
which, along with a certain amount of superstitious unwisdom, has come
down to us in the shape of weather proverbs. Many of these proverbs
undoubtedly originated before the dawn of history, for they are found
in substantially the same form among widely scattered races of mankind.
Various popular weather prognostics familiar at the present day are
mentioned in such ancient documents as the Vedas, the Bible, and the
cuneiform tablets from the library of Assurbanipal.

Speculations about the weather occupy much space in the writings of
the Greek philosophers, and a formal treatise on meteorology, written
by Aristotle (fourth century B. C.), remained the standard work on
this subject for two thousand years. More or less systematic weather
records were kept by the Greeks long before the Christian era, and they
produced a number of almanacs, in the shape of marble tablets, showing
the average winds and weather for particular dates throughout the
year. A copious collection of the weather indications found in both
Greek and Roman almanacs, dating back to the fifth century B. C., has
been made by Dr. Gustav Hellmann.

Some of the meteorological instruments used today have a very
respectable antiquity. Ancient statistics of the rainfall of India,
recently brought to light, show that some sort of rain gauge must
have been in use in that country in the fourth century before our
era. Measurements of rainfall were made in Palestine in the first
century A. D. The only other meteorological instrument dating back to
classical antiquity, so far as known, is the weather vane. The Tower
of the Winds, at Athens, built about a century before the Christian
era, originally bore at its summit a vane in the shape of a bronze
Triton, holding in his hand a wand, which was designed to point at
one or another of the eight symbolical figures of the principal winds
surrounding the octagonal tower, thus showing which way the wind was
blowing at the time. The Roman writer Varro has left us a description
of a vane that could be read indoors by means of a dial on the ceiling.

Instrumental weather observations did not become the rule, however,
until the end of the seventeenth century, when the use of thermometers,
hygrometers, barometers, and rain gauges began in Italy and spread
rapidly to other countries. The origin of each of these instruments is
commonly ascribed to a particular inventor--the thermometer to Galileo,
the barometer to Torricelli, etc.--but the truth is that the idea of
the instrument was, in each case, a slow growth, to which many minds
contributed. Thus a form of thermoscope--a device for showing but
not for measuring the expansion and contraction of air with changes
of temperature--was described by Philo of Byzantium in the third
century B. C. Galileo supplied such an instrument with a scale, but
without fixed points, thus converting it into a crude thermometer, but
it was not until half a century later that the Grand Duke Ferdinand
II of Tuscany introduced the idea of filling the thermometer with
alcohol, in place of air, and sealing it so that it was not affected
by changes in barometric pressure. The thermometric scale now used in
English-speaking countries, which bears the name of Fahrenheit, appears
to have been devised by the Danish astronomer Ole Römer, from whom
Fahrenheit borrowed it. In short, any _brief_ account of the invention
of the principal meteorological instruments necessarily ignores the
just claims of many inventors; to say nothing of the fact that what is
written on the subject to-day is likely to be refuted to-morrow by the
discovery of some forgotten book or manuscript.

We are on safer ground in saying that the plan of _measuring_ the
weather, instead of merely observing it, became general early in
the eighteenth century; and that about the middle of the nineteenth
century the further improvement was introduced of making meteorological
instruments trace their own records, so that the human observer was, to
a great extent, dispensed with. Self-registering instruments are now
the rule at important meteorological observatories and stations, though
they do not, even yet, record all the elements of weather, and at a
host of minor stations none of them have yet replaced the eye of the
observer.

Now let us see what things go to make up the weather, and how these
things are observed by the modern meteorologist.

The pressure of the atmosphere, if not exactly a part of the weather,
is so intimately associated with it that we cannot exclude it from our
list of weather phenomena. Atmospheric pressure is measured with the
_barometer_, and the importance of this instrument as a key to weather
changes is fully recognized--and indeed overrated--by the layman, who
sometimes calls it the “weather glass.”

[Illustration: MERCURIAL BAROMETER (Fortin type)]

Until recently all British and American barometers were read in
inches and all others in millimeters. Since atmospheric pressure is
a force, the practice of measuring it in units of length is rather
like measuring time in bushels or potatoes in hours. The inconsistency
is serious from a scientific point of view, because it divorces
barometric measurements from other physical measurements, in which
pressures are measured in units that have nothing to do with length;
viz., dynes per square centimeter. Accordingly, some of the leading
meteorological services of the world have lately adopted a new unit
of barometric pressure, known as the _bar_, which is equivalent to
1,000,000 dynes per square centimeter. It is subdivided according to
the ordinary metric notation, and its most commonly used subdivision is
the _millibar_, equivalent to 0.03 inch on the old-fashioned barometer
scale, under standard conditions.

[Illustration: ANEROID BAROMETER, GRADUATED IN MILLIBARS AND INCHES

For the benefit of sailors a curve is shown indicating the _mean annual
pressure_ in different latitudes along the meridian of 30° W. (Courtesy
of the British Meteorological Office.)]

The mercurial barometer is so delicate and cumbersome that for many
practical purposes it is replaced by the more convenient though less
accurate _aneroid barometer_. A self-recording barometer (usually an
aneroid) is called a _barograph_. In its ordinary form, this instrument
carries a pen, which traces a continuous record of the barometric
pressure on a strip of paper wound around a cylinder turned by
clockwork. Generally the instrument runs for a week before the paper
has to be changed. The barograph is a very instructive instrument,
because it shows, not only the pressure, but also the _changes_ of
pressure--i. e., just how fast the barometer is rising or falling, or,
as meteorologists say, the “barometric tendency.” The way in which
barometric changes are related to weather will appear in a later part
of this book.

The mercurial barometer consists of a glass tube, sealed at its upper
end and having at its lower end a “cistern,” which is open to the air.
The tube is filled with mercury at its open end, and then inverted over
the cistern, and the mercury descends until the weight of the portion
standing above the level of the mercury in the cistern just balances
the pressure of the air on an area equal to the cross section of the
tube. The height of the mercurial column is read from a graduated
scale attached to the tube. Certain corrections are applied to the
reading, in order to eliminate variations due to temperature, etc.,
and, if to be entered on a weather map, the reading is reduced to
sea-level value. In the aneroid barometer, a thin-walled metal box,
exhausted of air, undergoes changes of shape in response to changes
in atmospheric pressure. The movements of the box are communicated by
levers to a pointer moving around a dial (or to the recording pen, in
the barograph).

Since the pressure of the atmosphere diminishes with increasing
altitude at a fairly definite rate, the barometer is used for measuring
heights. Sometimes it is graduated directly, for this purpose, in feet
or meters, and it is then called an _altimeter_.

Among the meteorological elements that unmistakably pertain to weather
the most important is the _temperature_ of the air. The thermometer,
with which temperature is measured, is, in its common form and in its
essential features, too familiar to require description here; but we
may remark that, as in the case of the barometer, several methods of
graduating this instrument have been used. Besides numerous obsolete
systems, there are three different thermometric scales--the Fahrenheit,
the Centigrade, and the Absolute. The first is still the prevailing
one in English-speaking countries, and the second prevails in all
other countries. The Absolute scale, long familiar to physicists, has
recently come into somewhat limited use in meteorology. It starts at
the “absolute zero”--the temperature of a body totally devoid of heat.
This temperature has been nearly attained in laboratory experiments
with liquid helium. One advantage that the Absolute scale possesses
over the others is that it has no below-zero readings. Such readings
are a source of occasional errors when temperature is recorded on the
Fahrenheit or the Centigrade scale.

The freezing point of water is 32° Fahrenheit = 0° Centigrade = 273°
Absolute. The boiling point of water, at sea level, is 212° Fahrenheit
= 100° Centigrade = 373° Absolute.

While the layman is well acquainted with the thermometer, he sometimes
fails to understand certain differences between the scientific and
unscientific methods of using this instrument for weather-measuring
purposes. On a hot summer day he is, perhaps, inclined to feel
aggrieved because the official record of temperature does not
adequately express the state of his feelings, to say nothing of being
at odds with the impressive instrument displayed at the corner drug
store. Hence the following explanation is in order:

It is the function of the official thermometer to indicate the true
temperature of the _air_. A thermometer exposed to direct sunshine
records its own temperature--i. e., the temperature of the glass and
mercury--and nothing else. A thermometer “in the shade”--under a tree,
for example--comes nearer to showing the true air temperature; but it
is exposed to radiation from surrounding objects and its readings will
vary with the nature and location of these objects. The meteorological
thermometer is nearly always installed in a kind of latticed screen,
or shelter. It is thus largely protected from radiation, while the air
circulates freely around it. Only when thermometers are exposed under
such standard conditions is it possible to obtain comparable readings
of the temperature at different places, so that, for instance, maps may
be drawn showing the distribution of this element over a country. The
best location for the thermometer screen is a few feet above sod. Many
thermometers of the United States Weather Bureau are installed on the
roofs of tall buildings; not because this is an ideal location, but
because no better is available in the heart of a large city, where, for
practical reasons, the office has to be placed. In many small towns the
site of the station is such that the thermometer screen (or “instrument
shelter,” as it is called in the Weather Bureau) can be placed close
to the ground, and at the same time get ample ventilation and be free
from the radiation of buildings. In certain large cities the Bureau
maintains a branch station in a park or in the suburbs, where a
satisfactory exposure for all instruments can be secured.

The artificial temperature of a city street is too local and indefinite
a thing to be inscribed on weather maps, utilized by the forecaster,
or embodied in climatic statistics. As a concession, however, to the
demand of the “man in the street” for a record of conditions prevailing
in his own sphere, the Weather Bureau has installed in several cities
little pavilions in which working meteorological instruments are
displayed for the benefit of the public. The thermometers in these
so-called “kiosks”--which are modeled, with improvements, after the
weather pavilions found at European health resorts--always read several
degrees higher in hot weather than the thermometer at the regular
Weather Bureau station in the same vicinity. Such records are erratic,
at best, and present indications are that the kiosks will eventually be
abolished.

[Illustration: MAXIMUM AND MINIMUM THERMOMETERS]

Besides the ordinary thermometer, there are instruments that answer the
questions “How hot was it to-day?” and “How cold was it last night?”
These are known, respectively, as the _maximum_ and the _minimum
thermometer_. They hang almost horizontally in the screen. The former
has a constriction just above the bulb, which prevents the mercury
from retreating after it has reached the highest reading for the day.
It can be reset by whirling it on a pivot. The minimum thermometer
is filled with spirit instead of mercury. A little index inside the
column is carried toward the bulb by the surface of the alcohol as the
temperature falls. When the temperature rises the index remains behind,
marking the lowest point reached. The highest and lowest temperature
of the day, as well as the temperature at any moment of the day,
can be read from the _thermograph_, or self-registering thermometer.
In the commonest type of thermograph changes of temperature alter
the curvature of a flexible metal tube filled with spirit, and the
movements of the free end of the tube are communicated by levers to a
recording pen.

On an average day, in our climates, the air is coldest about sunrise.
The appearance of the sun checks the atmospheric cooling due to the
loss of heat from the earth that has been going on through the night,
and the air begins to warm up. As long as the amount of incoming heat
from the sun is greater than the amount of outgoing heat from the
earth, the temperature will continue to rise. After noon, when the
sun is highest, the supply of solar heat diminishes, but it is still
greater, for a time, than the heat loss from the earth, and for this
reason the temperature, as a rule, keeps on rising until some time
toward the middle of the afternoon, when the maximum temperature of the
day occurs.

_Humidity_ is an element of weather that is more often talked about
than understood. Atmospheric humidity is the state of the atmosphere
with respect to the amount of moisture it contains in a gaseous form,
not in the form of a liquid. This gaseous moisture is called _water
vapor_, and it is not directly perceptible to the senses, as liquid
water is. As we have explained elsewhere, the capacity of the air for
water vapor increases with the temperature. The actual amount present
at any time, per unit volume, is called the _absolute humidity_, and
the ratio of this amount to the maximum amount the air can hold at the
same temperature is called the _relative humidity_. The latter is
generally expressed in percentage. When the air is charged to its full
capacity with aqueous vapor its relative humidity is 100 per cent.

The relative humidity usually varies greatly through the day, being
generally lowest when the temperature is highest, and _vice versa_. It
is an element of much practical interest, because it is one of the main
factors in determining the drying power of the air, the other important
one being wind. The air feels dry when evaporation proceeds rapidly
from our skin, either on account of low relative humidity, brisk
air movement, or both. People are hardly conscious of high relative
humidity except when, in hot weather, it retards the evaporation of
perspiration, and the latter collects in liquid form on the skin.

[Illustration: THERMOGRAPH]

Relative humidity does not owe its importance in human affairs solely
to its physiological effects, for it plays a prominent part in
numerous industries--textile, metallurgical, chemical, leather, food,
and all those employing drying processes. In the spinning of cotton
and wool, for example, the humidity of the workroom greatly affects
the weight of the material, the size of the yarn, and the length
and flexibility of the fibers. Humidity must likewise be taken into
account in such diverse industries as manufacturing candy, bread, high
explosives and photographic films, drying macaroni and tobacco, and
operating blast furnaces. There are engineers who specialize in the
business of installing “humidifying” and “dehumidifying” systems in
workshops, and also, for hygienic purposes, in schoolhouses and other
public buildings.

The absolute humidity, the relative humidity and the _dew point_ (the
temperature to which the air must be brought to start condensation
of its moisture) are all determined by means of instruments called
_hygrometers_. The hair hygrometer depends for its action upon the
fact that a hair, freed from oil, not only absorbs moisture from
the atmosphere, but elongates when damp and contracts when dry. The
instrument, which includes a single human hair or a bundle of such
hairs, is so designed that these changes move an index over a graduated
scale. This and other types of hygrometer can be arranged to record
their own readings continuously, constituting a _hygrograph_.

The form of hygrometer most commonly met with at meteorological
stations is called a _psychrometer_. This usually consists of a pair
of mercurial thermometers, one of which, known as the “wet-bulb
thermometer,” has its bulb wrapped in thin muslin. The other, called
the “dry-bulb,” is an ordinary thermometer. The muslin is moistened,
either just before making a reading, or continuously with a wick. In
the former case the thermometer is generally whirled several times
before the reading is taken. Unless the air is saturated, the wet bulb
is cooled by evaporation, and the difference between the readings of
the two instruments enables the observer, with the aid of suitable
tables, to obtain the absolute and relative humidity and the dew
point. The most accurate results are obtained from the _aspiration
psychrometer_, of Assmann, in which air is drawn past the bulb of the
thermometer by a small fan, driven by clockwork.

Deposits of liquid and frozen water from the atmosphere, in their
various forms, are known collectively as “precipitation,” and in
the aggregate they constitute a feature of the weather hardly less
important than temperature. Indeed an average rainstorm or snowstorm is
a more obtrusive event than any other equally common manifestation of
the weather; while an excess of precipitation or a prolonged lack of
it, constituting a _drought_, may be as serious in its consequences as
a “hot wave” or a “freeze.”

Precipitation--familiarly called “rainfall”--is much more extensively
measured than any other meteorological element, for there are,
throughout the world, a vast number of places at which this is the only
feature of the weather that is regularly observed. In Europe alone
there are about 19,000 “rainfall stations.” Rainfall is measured in
depth; viz., in inches or millimeters. A moderate shower of several
hours’ duration will yield an inch or two of rain, while in extreme
cases several inches may fall in an hour. Snow is sometimes measured
as such--i. e., the actual depth that falls, or, more commonly, the
amount lying on the ground from day to day--but in order that records
of snowfall may be combined with those of rainfall for the purpose of
determining the total precipitation, the snowfall must be reduced to
its “water equivalent,” either by melting the snow before measurement
or by estimating this equivalent or by weighing the snow caught in a
receiver of known area and computing the corresponding depth of water.

There are many kinds of _rain gauge_. As a rule the gauge has a
funnel-shaped receiver with a small opening through which the water
flows into the lower part of the gauge; loss of the accumulated water
by evaporation is thus checked. There is usually some device for
magnifying the depth of rainfall in order to facilitate measurement.
In American gauges the rain flows into an inner tube having one-tenth
the horizontal area of the receiver, and its depth is thus magnified
ten times. A measurement is made by thrusting a graduated wooden stick
to the bottom of the tube and noting the height to which the stick is
wetted.

[Illustration: TIPPING-BUCKET RAIN GAUGE]

Of devices for obtaining an automatic record of rainfall, the _tipping
bucket_ (or, as the British call it, the “tilting bucket”) is probably
the most serviceable, and it is the one most widely used in this
country. This instrument is as simple as it is ingenious. The “bucket”
is a little metal trough, pivoted in the middle, so that it can tilt
back and forth, seesaw-fashion. It is divided into two compartments by
a central partition. Rain falling into the funnel-shaped receiver at
the top of the gauge flows into whichever compartment of the bucket is
uppermost, until the weight of the water causes the bucket to tip,
thus emptying one compartment and presenting the other to the incoming
stream. When the second compartment is filled, the bucket tips in the
opposite direction. The parts of the gauge are of such dimensions that
each tip of the bucket corresponds to 0.01 inch of rainfall. The gauge
is connected electrically with registering apparatus indoors, so that
every tip of the bucket is recorded. The registration sheet shows the
time of occurrence as well as the amount of rainfall.

The two most important things about the wind that are observed and
recorded by meteorologists are its direction and its force. It is
the universal custom to regard as “the direction of the wind” the
direction _from_ which, rather than toward which, it blows. Moreover,
it is only the horizontal direction of the wind that is ordinarily
observed, though many winds have a considerable upward or downward
slant, and, locally, a wind may even blow straight up or straight
down. The direction of the wind may be observed in several makeshift
ways, such as by watching the drift of smoke from chimneys, or, as
sailors do, holding up a wet finger to the breeze. Instrumentally and
scientifically it is observed with a special type of _vane_, much more
accurate in its indications than the weather vanes and weather cocks of
ornamental and symbolical architecture. The nonscientific vane, once
set in motion, is likely to be carried too far by its own momentum,
and may even spin completely around under a sudden impulse. In the
scientific vane this tendency is restrained by means of a spread tail;
the pressure of the wind on the diverging blades serving to hold the
vane in the correct position. The vane, like most other meteorological
instruments, is self-recording at all important meteorological
stations. The type used by the Weather Bureau registers the direction
of the wind every minute.

The force of the wind is obtained from an _anemometer_. Most
anemometers do not, however, show this directly, but are designed to
measure the speed or so-called “velocity” of the wind, from which its
force may be computed. The speed is observed in miles per hour or
meters per second. In considering some of the possible effects of wind
it is well to bear in mind that its force increases as the square of
the velocity. This means, for example, that a wind of 20 miles an hour
is four times as strong, and one of 30 miles an hour nine times as
strong as a wind of 10 miles an hour.

One of the external features of a weather station that invariably
attracts the attention of the passer-by is an instrument consisting
of four hemispherical cups revolving horizontally in the wind. This
scientific whirligig is the _Robinson cup anemometer_, which, in spite
of its shortcomings, is the most widely used instrument of its class
throughout the world. As generally constructed, the cups are supposed
to turn 500 times for a mile of wind movement. Actually the relation
between the speed of the cups and the speed of the wind is somewhat
variable, and at high velocities the indications of the instrument are
seriously erroneous. The Robinson anemometer has a dial from which
direct readings can be made, but at large stations it is connected
electrically with a registering device in the observer’s office, which
makes a mark for each mile of wind and shows how the speed of the wind
varies through the day.

There are many other types of anemometer, and some of them tell a much
more detailed story of the wind’s variations than does the Robinson
instrument. On the other hand, thousands of weather observers
dispense with anemometers altogether and merely estimate the strength
of the wind from its effects. This applies to nearly all observers
at sea, and, in Europe, to the vast majority of observers on land.
Such estimates are recorded on a scale ranging from zero, for a calm,
generally up to ten or twelve for the strongest winds ever experienced.
Several different scales are in use. The best known is the Beaufort
Scale, devised by Admiral Sir F. Beaufort, in 1805. The following
table of the Beaufort Scale, as adapted for use on land, is from the
“Observer’s Handbook” of the British Meteorological Office:

  --------+---------------+---------------------------------+-----------
  Beaufort|  Explanatory  | Specification of Beaufort Scale | Equivalent
   number |    titles     |    for use on land based on     | speed in
          |               |    observations made at land    | miles per
          |               |            stations             |  hour at
          |               |                                 |  33 feet
  --------+---------------+---------------------------------+-----------
     0    | Calm          | Calm; smoke rises vertically    |        0
     1    | Light air     | Direction of wind shown by      |
          |               |   smoke drift, but not by wind  |
          |               |   vanes                         |        2
     2    | Slight breeze | Wind felt on face; leaves       |
          |               |   rustle; ordinary vane moved   |
          |               |   by wind                       |        5
     3    | Gentle breeze | Leaves and small twigs in       |
          |               |   constant motion wind extends  |
          |               |   light flag                    |       10
     4    | Moderate      | Raises dust and loose paper;    |
          |   breeze      |   small branches are moved      |       15
     5    | Fresh breeze  | Small trees in leaf begin to    |
          |               |   sway; crested wavelets form   |
          |               |   on inland waters              |       21
     6    | Strong breeze | Large branches in motion;       |
          |               |   whistling heard in telegraph  |
          |               |   wires; umbrellas used with    |
          |               |   difficulty                    |       27
     7    | High wind     | Whole trees in motion;          |
          |               |   inconvenience felt when       |
          |               |   walking against wind          |       35
     8    | Gale          | Breaks twigs off trees;         |
          |               |   generally impedes progress    |       42
     9    | Strong gale   | Slight structural damage occurs |
          |               |   (chimney pots and slates      |
          |               |   removed)                      |       50
    10    | Whole gale    | Seldom experienced inland;      |
          |               |   trees uprooted; considerable  |
          |               |   structural damage occurs      |       59
    11    | Storm         | Very rarely experienced;        |
          |               |   accompanied by widespread     |
          |               |   damage                        |       68
    12    | Hurricane     |                                 | Above 75
  --------+---------------+---------------------------------+-----------

The clouds receive more attention at some weather stations than at
others. A routine observation consists of noting the kinds of clouds
visible, the direction or directions from which they are moving,
and the degree of cloudiness--i. e., the extent to which the sky is
clouded, stated in tenths, from O = cloudless, to 10 = completely
overcast. At many of the more important stations the movements of
clouds are observed with a _nephoscope_. The reflecting nephoscope,
used in this country, consists of a black mirror in which the image of
the moving cloud is watched, the direction of its motion being read off
from the graduated circular frame of the mirror. There is also a device
for measuring the apparent speed of the cloud. From this the actual
speed can be calculated if the height of the cloud is known. There are
other nephoscopes, such as Besson’s in which the cloud’s movements are
watched directly, and not by reflection.

[Illustration: BESSON’S COMB NEPHOSCOPE]

The importance of sunshine among the elements of weather and climate
is evidenced by the fact that at least two States of the Union,
South Dakota and California, contend for the title of “the Sunshine
State”--which does not properly belong to either of them. Arizona is
the sunniest State of all, and the whole Southwest is sunnier than
South Dakota.

Devices for registering the duration of sunshine are called _sunshine
recorders_. One type (the Campbell-Stokes) works on the burning-glass
principle; in others the sun’s rays trace a record on photographic
paper. The instrument used by the Weather Bureau consists of an air
thermometer having a bulb at each end, one bulb being coated with
lampblack. There is a small column of mercury between the two inclosed
masses of air. The thermometer is inclosed in a sheath of glass, from
which the air is exhausted. When the sun shines on this instrument,
the air in the black bulb warms and expands, and the mercury is
forced toward the other bulb until it comes in contact with a pair of
electrodes, thus closing an electrical circuit. While the circuit is
closed, the registering apparatus connected with the instrument makes
a step-shaped mark once every minute. When the sun stops shining, the
mercury drops back, the circuit is broken, and the recording pen merely
traces a straight line.

[Illustration: ELECTRICAL SUNSHINE RECORDER]

At the larger stations of the United States Weather Bureau the
direction and speed of the wind, the rainfall and the duration of
sunshine are all recorded on a single sheet of paper, wound around a
large cylinder, which is turned by clockwork. The paper is ruled with
lines to denote the hours and minutes of the day, and a fresh sheet is
put on the cylinder every day at noon. This complex registering device,
sometimes called in book language a _meteorograph_, but colloquially
referred to by weather men as the “triple register,” is entitled to
high rank among labor-saving machines; for, with hardly any attention,
except for a few minutes at noon, it does the work of a staff of
trained meteorologists on duty day and night.

[Illustration: BRITISH TYPE OF SUNSHINE RECORDER

(_Campbell-Stokes Pattern_)]

We have now enumerated the elements of weather most commonly observed
at meteorological stations, and the principal types of meteorological
instruments, with special reference to those used in the United States.
In nearly every civilized country there are certain stations at which
regular observations are maintained of a number of phenomena not
mentioned in the foregoing paragraphs, such as the intensity of solar
radiation (measured with the _pyrheliometer_), evaporation (measured
with _atmometers_ or _evaporimeters_), and the temperature of the
soil; and the number of stations is rapidly growing at which the winds
and weather far aloft in the atmosphere are observed by means of kites
and balloons. Meteorologists of the Old World use a great many types
of apparatus that are rarely seen in this country, and some of our
instruments are but little known abroad.



CHAPTER VI

CLOUDLAND


One of the things a tea kettle is good for is to provide, by means of
the little cloud seen at its nozzle and erroneously called “steam,” an
example of what happens when the invisible gas that is truly steam,
or water vapor, is cooled below its dew point in the free air. This
cloud has, however, been the starting point of a vast number of halfway
explanations. A generation or so ago physicists were content to say
that aqueous vapor turns to drops of water in the air merely on account
of being cooled. The question of how the drops get their start, or why
the moisture forms drops at all, does not seem to have troubled them.

One way in which air or any other gas is cooled is by expanding against
pressure. Some of the energy in the gas, originally manifesting itself
as heat, is applied to the work of expansion, and thus ceases to be
heat. Hence the temperature of the gas falls. Conversely, if a mass
of gas is compressed, the mere process of compression raises its
temperature. The heat produced in pumping up a bicycle tire is the
classic example of the latter fact. Heating by compression and cooling
by expansion are called, respectively, “dynamic heating” and “dynamic
cooling.” The processes thus described are of the utmost importance in
meteorology.

If air of average humidity is admitted to the receiver of an air pump
in the usual way and suddenly expanded by partial exhaustion, a cloud
of moisture is seen to form in the receiver. This moisture is condensed
and made visible by the dynamic cooling of the air. If, however,
after the receiver is exhausted air of the same humidity as before is
admitted through a filter of cotton wool, and is then similarly cooled
by expansion, no cloud will form. Evidently the filter has removed from
the air something that is essential to the process of condensation.

Perhaps it will occur to the reader that, in some obscure way, the
filter has prevented water vapor itself from entering the receiver.
There are several methods by which we can ascertain whether such is the
case. One of the simplest is to admit a little smoke to the receiver
before expanding the filtered air. In this case the cloud _does_ form,
showing that moisture is present, and also showing that smoke, though a
perfectly dry substance, aids the formation of the water drops.

Such experiments have led to the conclusion, now universally admitted,
that when water drops form in the atmosphere they always form around
“nuclei” of something that is not water. These nuclei are often
referred to as “dust particles,” but it is recognized that a vast
proportion of them are very much more minute than the dust that worries
housewives. They are largely beyond the power of the microscope, and
some of them, indeed, appear to be of molecular size, consisting of
molecules of hygroscopic gases, such as the oxides of sulphur and of
nitrogen.

Another important fact about water drops in the atmosphere has come
to light within the last half century. Since water is much heavier
than air, meteorologists of an earlier generation were puzzled by the
fact that the drops in clouds apparently float, instead of falling
to the ground. In the attempt to account for this supposed floating,
bygone authorities assumed that the drops were hollow “vesicles,” like
little bubbles. This assumption was eventually disproved by the optical
phenomena exhibited by the drops, as well as on other physical grounds.
Moreover, it is now known that a cloud never really floats, though the
rate at which its constituent particles fall with respect to the air is
generally very small, on account of the resistance they encounter. Thus
a very slight upward current usually suffices to maintain the altitude
of a cloud, or even to increase it. The speed with which a drop falls
increases with its size. Hence large drops may fall rapidly from great
heights all the way to the ground, constituting rain; but in a great
many cases such drops evaporate on falling into warmer air below the
cloud level, and thus the lower surface of the visible cloud remains at
about a constant height.

The drops in clouds and fog have often been measured, either by noting
their optical effects or by microscopic examination. Many are found to
be from 0.0006 to 0.0008 inch in diameter. The speed with which such
drops fall through still air can be calculated. A drop 0.0008 inch
in diameter falls at the rate of about half an inch a second, or 150
feet an hour. Even if a cloud consisting of such drops preserved its
integrity for an hour or more while sinking, its descent at this slow
rate would hardly be perceptible from the ground.

Some clouds consist of ice needles or tiny snowflakes. Apparently these
icy particles are produced directly in solid form, without passing
through the liquid stage. It is supposed that, like drops, they
require nuclei on which to condense, but this matter has not been fully
investigated. Another point that awaits elucidation is the fact that
the clouds that form in air much below the freezing point sometimes
consist of water and sometimes of ice. The common fleecy clouds of our
winter skies are composed of water drops, and such clouds also occur
in the polar regions. Dr. G. C. Simpson, when serving as meteorologist
of Scott’s antarctic expedition, observed fog consisting of liquid
water at a temperature of 29° below zero, Fahrenheit. Why such greatly
“undercooled” drops should sometimes occur in the atmosphere when at
other times, with higher temperatures, atmospheric moisture takes the
form of ice is not at all clear.

There are several ways in which the free air may be cooled to the
point at which condensation occurs. The commonest is dynamic cooling,
due to the rise of a mass of moist air and its expansion under the
reduced pressure that prevails at the higher levels. This process is
beautifully illustrated in the formation of the roundish masses of
fleecy cloud known as _cumulus_, on a warm summer day. Each of these
clouds marks the summit of a column of air that is rising after having
been heated at the surface of the earth. When the process goes on
very actively, the cloud may tower up to enormous heights, forming a
thundercloud. Some clouds are formed by the mixing of air of different
temperatures. Fog, which is merely cloud at the surface of the earth,
is often formed by the cooling of the air in contact with cold land or
water. The persistent fogs of the Newfoundland Banks are due to the
passage of warm moist air from the Gulf Stream region over the cold
Labrador Current. On the other hand, a cold wind blowing over warm
water will also often produce a fog by lowering the temperature of
the moist air overlying the water. A common cause of land fog is the
cooling of the air adjacent to the ground in consequence of nocturnal
radiation. The moister the air, the more readily fog forms, and hence
the frequent formation of fog by night along rivers and over marshes
and damp valleys.

Town fogs, such as the famous “London particular” and the fogs of
Lyons, usually consist partly of smoke. Dense fogs of this sort occur
when the conditions of the atmosphere are such as to cause the smoke
to hang low over the city, instead of being dispersed. These fogs
constitute a serious economic problem. Thus it is estimated that they
cost the people of London upwards of half a million dollars a year, due
to extra lighting, damage to vehicles, loss of business, etc. Since
marine fog is also a source of enormous loss, through causing delays
and accidents, and since fog along air routes is the greatest of all
obstacles to successful aerial navigation, it is no wonder that much
ingenuity has been devoted to the attempt to disperse fog artificially.
Electric discharges have been successfully used for this purpose on a
small scale.

The depth of a fog may be anything from inches to miles. Measurements
made by the United States Coast Guard during the international ice
patrol of the North Atlantic show that the fogs on the Newfoundland
Banks are very commonly so shallow that the mastheads of vessels rise
above them, though in some cases they were found, from observations
with kites, to be from 2,500 to 3,000 feet thick. Observations on the
mountains of the California coast show that the upper level of fog
in that region rarely exceeds 4,000 feet. On the other hand, aviators
flying between London and Paris have encountered fog more than 10,000
feet deep.

The United States Weather Bureau classifies a fog as “dense” if it
hides objects at a distance of 1,000 feet; otherwise it is described as
“light.” British meteorologists record fogs on a scale of five degrees.

During the ice patrol of the _Seneca_ in 1915 samples of foggy air were
examined for the purpose of calculating the amount of water and the
number of drops they contained per unit volume, as well as the size
of the drops. A block of dense fog 3 feet wide, 6 feet high, and 100
feet long was found to contain less than one-seventh of a glassful of
water, distributed in 60,000,000,000 drops. During the densest fog of
the voyage the diameter of the fog particles averaged 0.0004 inch; just
about the limit of visibility with the naked eye.

In spite of the extremely attenuated state of the water in fogs, as
indicated by these figures, the moisture they deposit on terrestrial
objects is great enough to be of considerable agricultural importance
in some parts of the world. Thus along the coast of Peru, where the
rainfall is negligible (though not, as often stated, nonexistent),
a wet fog known as the “garúa” suffices to maintain a luxuriant
vegetation during several months of each year.

There are frozen fogs as well as frozen clouds. The “frost smoke”
that rises over the Norwegian fjords and over ice-free spots in the
polar seas is generally composed of icy particles or snowflakes. An
ice fog that sometimes forms in mountain valleys in the western United
States is known as the “pogonip”--a name derived from the Shoshonean
language. This fog often appears very suddenly, even in the brightest
weather. The minute needles of ice of which it consists are said to be
extremely injurious to the lungs. There are tales of a whole tribe of
Indians perishing from its effects. Whatever truth there may be in such
stories, it is greatly dreaded by both the Indians and the whites. The
mountains of Nevada appear to be the favorite home of the pogonip.

What meteorologists call “dry fog” is a haze of dust or smoke,
sometimes very dense. We have already described the prevalence of
this turbid state of the atmosphere following volcanic eruptions, the
burning of forests and moors, and desert dust storms. Under the head
of dry fog many writers include a sort of heat haze, which does not
necessarily involve the suspension of either solid or liquid matter in
the air, but is due to the mixing of local air currents of different
densities, especially when evaporation is proceeding rapidly from moist
ground under strong sunshine. The _callina_ of Spain and the _qobar_ of
the upper Nile region are probably due partly to this cause, and partly
to dust.

[Illustration: ALTO-CUMULUS CLOUDS. These clouds always occur in
roundish fleecy masses or in elongated fleecy rolls, with blue sky
between. A score of different types have been distinguished and named
by certain cloud specialists. (_Photographed by A. J. Weed._)]

[Illustration: CUMULUS. Cloud photography has become a special branch
of photographic art, entailing not only the use of appropriate lenses
and plates, but also of ray filters, or other special devices for
sharpening the contrast between the cloud and the blue sky. Mr.
Ellerman’s pictures, made on Mt. Wilson and elsewhere in California,
stand in the front rank. (_Photographed by F. Ellerman._)]

One more species of fog requires mention here, viz., the dirty,
foul-smelling “painter” of the Peruvian coast, which deposits on
vessels lying in the harbor of Callao and elsewhere a slimy brown
substance known as “Peruvian paint.” This substance comes from the
ocean and is probably due to the decomposition of marine organisms. The
“painter” prevails during the months December to April. According to a
plausible hypothesis a change in the temperature of the water at that
season, resulting from a periodical shift of ocean currents, kills
vast quantities of plankton, the decay of which would give rise to the
phenomena observed.

[Illustration: MAMMATO-CUMULUS. A rather rare cloud form, associated
with thunderstorms and tornadoes. It is known in Scotland as the
“pocky” (i. e. baggy) cloud and in parts of England as “rain balls.”
(_Photographed by L. C. Twyford._)]

[Illustration: CUMULO-NIMBUS. This is the thundercloud. (_Courtesy of
the Naval Air Service._)]

Clouds, though they are nothing more than masses of fog situated at
some distance from the earth, are susceptible of a classification,
according to shape and texture, that is not applicable to fog. Among
the billions of human beings who, in all ages, have amused themselves
by discovering pictures in the clouds it would be remarkable if a
good many had not, from time to time, conceived the idea of reducing
these pictures to a few general types. According to a note published a
few years ago in the “Quarterly Journal” of the Royal Meteorological
Society, there is some reason to believe that an elaborate
classification of the clouds was in use among the ancient Hindus. A
passage quoted from an Indian work of the fourth century B. C. says:

“Three are the clouds that continuously rain for seven days; eighty are
they that pour minute drops; and sixty are they that appear with the
sunshine.”

In the occidental world, however, we have no record of any attempt
to classify the clouds prior to the year 1801, when the following
classification was proposed by the French naturalist Lamarck:

  Nuages en balayures (cloud sweepings),
  Nuages en barres (clouds in bars),
  Nuages pommelés (dappled clouds),
  Nuages groupés (grouped or piled clouds),
  Nuages en voile (veil clouds),
  Nuages attroupés ou moutonnés (clouds in flocks).

In 1803 the English meteorologist Luke Howard published the system of
classification that, with some additions and modifications, is now
in general use. This system is based upon three fundamental forms;
viz., fibrous or feathery clouds (_cirrus_), clouds with rounded
tops (_cumulus_), and clouds arranged in horizontal sheets or layers
(_stratus_). Intermediate forms are described by compounding the
names of the primary types; e. g., _cirro-cumulus_, _cirro-stratus_,
etc. The rain cloud is called _nimbus_. Howard’s classification was
quickly adopted in all countries. His definitions were translated into
German by no less a personage than Goethe, who, in his enthusiasm over
Howard’s achievement, wrote a poem about it, and also a separate poem
about each of the principal types of cloud!

The Latin names that Howard gave to the clouds made his system
immediately available for international use; and in nearly all of the
many systems of cloud nomenclature that have since been proposed the
excellent plan of using Latin names has been preserved. Very soon,
however, after Howard’s classification appeared, a list of proposed
English equivalents of his names was published in the “Encyclopædia
Britannica”--which, nevertheless, did not change its name to “British
Encyclopædia”--for the benefit of the unlettered majority, supposed
to be incapable of using a few Latin terms that were, in fact,
shorter and no more difficult to pronounce than their suggested
English substitutes! A piquant sequel to this episode is that these
superfluous English cloud names, “curl cloud,” “stackencloud,” “fall
cloud,” “sondercloud,” “wane cloud,” and “twain cloud,” still survive
in the dictionaries--and nowhere else. They are practically unknown to
meteorologists, and were never adopted generally by the laity.

Of course some English names, which have been evolved and not
deliberately invented, are applied to certain types of cloud in
English-speaking countries; but the Latin names, comprised in the
International Cloud Classification, should be learned by everybody.
This classification, which has been adopted by the International
Meteorological Committee and is used by all official weather services,
is a little more detailed than Howard’s, upon which it is based; and
there is a tendency to add new terms to it from time to time.

There are ten principal types of cloud in the International
Classification, and the name of each type has an official abbreviation
(a great convenience for those who record the clouds from day to
day). The following definitions, translated from the French text of
the “International Cloud Atlas,” have been published by the British
Meteorological Office:


1. CIRRUS (CI.)--_Detached clouds of delicate appearance, fibrous
(threadlike) structure and featherlike form, generally white in color._

Cirrus clouds take the most varied shapes, such as isolated tufts
of hair--i. e., thin filaments on a blue sky--branched filaments in
feathery form, straight or curved filaments ending in tufts (called
_cirrus uncinus_), and others. Occasionally cirrus clouds are arranged
in bands, which traverse part of the sky as arcs of great circles,
and as an effect of perspective appear to converge at a point on the
horizon, and at the opposite point also, if they are sufficiently
extended. Cirro-stratus and cirro-cumulus also are sometimes similarly
arranged in long bands. [Certain forms of cirrus are called “mares’
tails.” The long bands crossing the sky, as just described, are known
as “polar bands” or “Noah’s ark.”]


2. CIRRO-STRATUS (CI.-ST.)--_A thin sheet of whitish cloud; sometimes
covering the sky completely and merely giving it a milky appearance;
it is then called cirro-nebula, or cirrus haze; at other times
presenting more or less distinctly a fibrous structure, like a tangled
web._

This sheet often produces halos around the sun or moon.


3. CIRRO-CUMULUS (CI-CU.) (_Mackerel sky_)--_Small rounded masses or
white flakes without shadows, or showing very slight shadow; arranged
in groups and often in lines._


4. ALTO-STRATUS (A.-ST.)--_A dense sheet of a gray or bluish color,
sometimes forming a compact mass of dull gray color and fibrous
structure._

At other times the sheet is thin, like the denser forms of
cirro-stratus, and through it the sun and moon may be seen dimly
gleaming as through ground glass. This form exhibits all stages of
transition between alto-stratus and cirro-stratus, but, according
to measurements, its normal altitude is about one-half that of
cirro-stratus.


5. ALTO-CUMULUS (A.-CU.)--_Larger rounded masses, white or grayish,
partially shaded, arranged in groups or lines, and often so crowded
together in the middle region that the cloudlets join._

[Illustration: A CLOUD BANNER OVER MT. ASSINIBOINE, CANADIAN ROCKIES.
(_Photographed by Dr. C. D. Walcott._)]

[Illustration: CIRRUS (with a few patches of lower clouds in the
foreground). This is cirrus, but not of the “mare’s tail” variety.
There are many distinct types of cirrus, which have sometimes been
given separate names. (_Photographed at the Observatory of Trappes,
France._)]

The separate masses are generally larger and more compact (resembling
strato-cumulus) in the middle region of the group, but the denseness
of the layer varies and sometimes is so attenuated that the individual
masses assume the appearance of sheets or thin flakes of considerable
extent with hardly any shading. At the margin of the group they form
smaller cloudlets resembling those of cirro-cumulus. The cloudlets
often group themselves in parallel lines, arranged in one or more
directions.


6. STRATO-CUMULUS (ST.-CU.)--_Large lumpy masses or rolls of dull gray
cloud, frequently covering the whole sky, especially in winter._

Generally strato-cumulus presents the appearance of a gray layer broken
up into irregular masses and having on the margin smaller masses
grouped in flocks, like alto-cumulus. Sometimes this cloud form has the
characteristic appearance of great rolls of cloud arranged in parallel
lines close together (“roll cumulus”). The rolls themselves are dense
and dark, but in the intervening spaces the cloud is much lighter and
blue sky may sometimes be seen through them. Strato-cumulus may be
distinguished from nimbus by its lumpy or rolling appearance, and by
the fact that it does not tend to bring rain.

[Illustration: Copyright O. P. Anderson

A LENTICULAR CLOUD OVER MT. RAINIER]


7. NIMBUS (NB.)--_A dense layer of dark, shapeless cloud with ragged
edges from which steady rain or snow usually falls. If there are
openings in the cloud an upper layer of cirro-stratus may almost
invariably be seen through them._

If a layer of nimbus separates in strong wind into ragged cloud, or
if small detached clouds are seen drifting underneath a large nimbus
(the “scud” of sailors), either may be specified as _fracto-nimbus_
(FR.-NB.).


8. CUMULUS (CU.) (_Wool-pack cloud_)--_Thick cloud of which the upper
surface is dome-shaped and exhibits protuberances, while the base is
generally horizontal._

These clouds appear to be formed by ascensional movement of air in the
daytime, which is almost always observable. When the cloud and the
sun are on opposite sides of the observer, the surfaces facing the
observer are more brilliant than the margins of the protuberances.
When, on the contrary, it is on the same side of the observer as the
sun, it appears dark with bright edges. When the light falls sideways,
as is usually the case, cumulus clouds show deep shadows. True cumulus
has well-defined upper and lower margins; but one may sometimes see
ragged clouds, like cumulus torn by strong wind, of which the detached
portions are continually changing; to this form of cloud the name
fracto-cumulus may be given.


9. CUMULO-NIMBUS (CU.-NB.)(_The thundercloud_)--_Great masses of cloud
rising in the form of mountains or towers or anvils, generally having a
veil or screen of fibrous texture (“false cirrus”) at the top, and at
its base a cloud mass similar to nimbus._

From the base local showers of rain or snow, occasionally of hail or
graupel, usually fall. Sometimes the upper margins have the compact
shape of cumulus, or form massive heaps round which floats delicate
“false cirrus.” At other times the margins themselves are fringed with
filaments similar to cirrus clouds. This last form is particularly
common with spring showers. The front of a thunderstorm of wide extent
is frequently in the form of a large low arch above a region of
uniformly lighter sky.


10. STRATUS (ST.)--_A uniform layer of cloud, like fog, but not lying
on the ground._

The cloud layer of stratus is always very low. If it is divided
into ragged masses in a wind or by mountain tops, it may be called
_fracto-stratus_. The complete absence of detail of structure
differentiates stratus from other aggregated forms of cloud.


We have given the foregoing official definitions and descriptions
in full in order to aid the reader as much as possible, so far as
verbal information goes, in learning to call the common clouds by
their names. Good pictures are, of course, an essential part of this
process, and apart from those that illustrate the present text, many
collections of such pictures are easy of access. Some may be obtained
free or at nominal cost from the Weather Bureau in Washington and from
the Meteorological Office in London. The “International Cloud Atlas”
(second edition, Paris, 1910) is now out of print, but may be consulted
in libraries.

Of the clouds above enumerated, cirrus, cirro-cumulus, and
cirro-stratus are the highest, and are always ice clouds. They
consist in some cases of separate, minute crystals--a fine dust of
ice--producing, according to the forms of the crystals, one or another
of the various forms of halo around the sun and moon; while in other
cases the crystals are aggregated in small snowflakes, so that the
cloud is a real snowstorm in midair. The altitude of these clouds
generally ranges from 4 to 8 miles. In the equatorial region their
height is often 10 miles or more. The other main types of cloud are
composed wholly or chiefly of water. Alto-cumulus and alto-stratus are
clouds of medium altitude; strato-cumulus and nimbus are low clouds
(generally not more than a mile high); while stratus, the lowest cloud
of all, grades into fog, which commonly rests on the earth. Since
cumulus and cumulo-nimbus are produced by the condensation of moisture
from rising air currents, the height of their bases varies widely
with the temperature and humidity of the lower air; the average height
is rather less than a mile. Their vertical extent, however, is much
greater than that of the other cloud types. Cumulo-nimbus sometimes
towers to a height of 4 or 5 miles above its base, and it is then
commonly crowned with ice clouds, including a filmy “scarf cloud”
draping the summit, and spreading wisps of so-called “false cirrus,”
drawn out horizontally by the upper winds.

Besides the ten main classes of clouds, a few distinct minor varieties
are recognized by all meteorologists. Among these is the “lenticular
cloud”; an isolated small cloud, which frequently shows iridescence,
and the shape of which has been compared to that of a lens or an
almond. This cloud may remain stationary, or nearly so, but it really
marks the position of a billow in a stream of air, the moisture
condensing at one edge of the cloud and dissolving at the other.
Another distinctive and rather rare form of cloud, seen chiefly in
connection with thunderstorms, is _mammato-cumulus_, likewise known
as “pocky cloud,” “festoon cloud,” “rain balls,” etc. It consists of
numerous sacklike or udderlike protuberances, convex downward.

When a stream of moist air is forced to ascend in passing over a
mountain its moisture is often condensed by the process of dynamic
cooling, already explained, and a “cloud cap” is seen over the summit.
In local weather lore such caps are generally regarded as a sign
of rain. These clouds attached to mountains were called “parasitic
clouds,” by writers of a century ago, who proposed some naïve
explanations of them. Occasionally a “cloud banner” streams far to the
leeward of the mountain. One of the most famous and striking of cloud
caps is the “tablecloth” that spreads over Table Mountain, near Cape
Town, when a moist wind blows in from the sea. Sometimes the local
topography causes the wind that has swept up over a mountain to form a
second “standing” wave to the leeward of the summit, and this may also
be marked by a cloud, which, like the cloud cap, presents a delusive
appearance of permanence, while it is, in reality, in constant process
of formation on the windward side and dissipation on the leeward.
The two clouds thus formed, one over the summit and the other to the
leeward, are often seen at Table Mountain, and are further exemplified
in the celebrated “helm and bar” of Crossfell, in the English Lake
District.

In the case of a wind blowing athwart a ridge or mountain range, a bank
of cloud may extend along the whole crest, as in the “foehn wall” that
appears along Alpine heights when the foehn wind is blowing.

Some day meteorology will be taught in art schools, for the same reason
that anatomy now is. When that blissful day arrives painters will
probably show us skies less at odds with nature than those that deface
the work of artists of all degrees of celebrity, including the “old
masters.”



CHAPTER VII

PRECIPITATION


Meteorologists have been in much perplexity over the naming and
classification of the various deposits of atmospheric moisture known
collectively as “precipitation.” The subject is one to which a good
deal of attention has been paid in recent years, but it must be
admitted that, even at the present time, the terminology of this group
of atmospheric phenomena is not yet satisfactorily settled, either in
English or in any other language.

When, for example, a record of weather occurrences states that hail
has fallen, this statement, unequivocal as it may seem to the layman,
often raises a question in the mind of the meteorologist. For centuries
people talked and wrote about hail before it occurred to men of science
to inquire whether one and the same thing was always described under
this name. The pursuit of this inquiry led to disconcerting results,
one of them being the discovery that we do not now know, in many cases,
what bygone weather observers meant when they made the entry “hail” in
their records.

There are at least three different kinds of icy lumps and pellets that
fall from the sky, and they have all been called hail. What science now
regards as true hail occurs only in connection with thunderstorms, and
therefore chiefly in warm weather. It consists of balls or irregular
lumps, each of which, on examination, is found to have an opaque
snowlike center, surrounded by ice, which is often in alternately clear
and opaque layers. The second class of icy particles takes the form of
miniature snowballs, about the size of large shot or small peas. It
falls in cold weather, often in conjunction with ordinary snow. Because
it readily crumbles, English-speaking meteorologists have called it
“soft hail”; but this name is inappropriate for the two cogent reasons
that, though friable, it is not soft, and that it is not hail; hence
this term is now giving way to the German name “graupel” (in which _au_
= _ow_ in “growl”). Lastly, little pellets or angular particles of
clear ice sometimes fall in cold weather. These frozen drops, though
fairly common, have, until recently, enjoyed the distinction of being
anonymous, so far as the scientific world was concerned, while the
general public called them various things, including “hail.” A few
British authorities have tentatively styled this form of precipitation
“ice rain,” a name which has, however, been otherwise applied. Finally,
in the year 1916, the United States Weather Bureau took the bull by the
horns and decreed that such ice particles should be called “sleet.”

Although this decision of the Weather Bureau was arrived at only after
an exhaustive overhauling of literature and much correspondence with
philologists, scientific men, engineers, and others, it remains to be
seen whether it will eventually prevail throughout the English-speaking
world. In England “sleet” nearly always means a mixture of snow and
rain. On the other hand, a great many Americans have been in the habit
of applying this term to the coating of smooth ice, due to rain in
cold weather, which often breaks down the branches of trees, lays low
miles of wires, and incidentally produces one of the most beautiful
spectacles of American winters.

This leads us to another difficulty. The icy coating just mentioned
has, for some years, been called “glazed frost” by the British
Meteorological Office, and the United States Weather Bureau now calls
it “glaze.” It has likewise been called, even in scientific books,
“silver thaw”; and an instance of its occurrence on a large scale is
termed, both popularly and scientifically, an “ice storm.”

To pursue this lamentable record of cross-purposes just a little
further, it may be added that the expression “silver thaw,” besides
being one of the aliases of glaze, or glazed frost, has been applied
in various official British publications, until recently, to a very
different rough or feathery deposit of ice from fog, now called by both
the Meteorological Office and the Weather Bureau “rime.”

Needless to say, when the scientific authorities are unable to agree
about these terms, our dictionaries are sadly at sea in regard to them;
so, altogether, the task of writing a chapter on precipitation is beset
with verbal difficulties that would not be encountered in writing on
many far more recondite subjects.

Fortunately the name of the most important kind of
precipitation--rain--is reasonably free from ambiguity. To be sure,
opinions may differ as to whether a “Scotch mist” is a rain or a wet
fog--and if one happened to have insured a lawn fête against rain at
Lloyds’ the uncertainty on this point might lead to litigation--but,
generally speaking, “it rains or it does not rain,” as we are told in
the books on logic.

“Rain,” says Dr. Hellmann, “is the most widespread, most frequent,
and most copious form in which the aqueous vapor of the atmosphere
condenses. The area of its distribution embraces the whole surface
of the earth, with the exception of the interior of Antarctica
and probably of northern Greenland. The English and the Norwegian
expeditions found no rain even at the edge of Antarctica. As the land
rises inland to an altitude of about 2,800 meters about the South
Pole, it may safely be assumed that only snow and no rain falls in the
heart of Antarctica. At the North Pole, which lies in the midst of the
sea, it probably rains at times; while on the high plateau of northern
Greenland probably snow alone falls. As to its frequency, there are
arid regions in which the average annual number of days with rain is
less than one, while this number probably rises to 280 in some tropical
districts. With the exceptions of the polar regions already mentioned,
there are probably no regions where it never rains.”

In its intensity rain varies all the way from the finest drizzle or the
sprinkle of occasional drops up to the torrential downpours often known
as “cloud-bursts.” Before citing instances of heavy rains, it may be
well to remind the reader that an inch of rainfall is equivalent to 101
tons of water per acre, or 64,640 tons per square mile. In the county
of Norfolk, England, in August, 1912, a single day’s rainfall brought
down 670,720,000 tons of water--more than twice the volume of water
contained in England’s largest lake, Windermere. Doubtless this record,
for showers of similar extent and duration, has often been surpassed
in other countries, including our own, and _a fortiori_ within the
tropics.

The most remarkable example of a heavy brief shower was recorded at
Porto Bello, on the Isthmus of Panama, May 1, 1908, when a fall of
2.47 inches _in three minutes_ was registered by a self-recording rain
gauge. An average heavy rainstorm in the eastern United States yields
about this amount in twenty-four hours. At Baguio, in the Philippines,
forty-six inches of rain fell from noon of July 14, 1911, to noon of
the following day--probably a “world record” for twenty-four-hour
rainfall. The corresponding record for the United States is 22.22
inches at Altapass, N. C., July 15-16, 1916.

Statistics of what the meteorologist calls “excessive rainfall”--i.
e., abnormally heavy rain during brief periods--have been collected
over the greater part of the world for much more weighty reasons than
to satisfy curiosity as to which showers were “record breakers.” Such
data are indispensable to engineers in connection with the building of
sewers, reservoirs, and dams, and in flood-protection work. Sewers must
be made large enough to carry off the “storm water” from the heaviest
showers that ever occur in the locality; while, on the other hand,
in the absence of rainfall statistics, much money might be wasted in
making them larger than necessary. A great flood raises questions as
to the intensity of the rainfall that caused it, and the frequency
with which similar rains may be expected to occur in the drainage area
concerned. The unprecedented floods in the Ohio Valley and adjacent
regions in March, 1913, which caused losses amounting to $200,000,000,
led to an exhaustive study of the records of storm rainfall in the
eastern United States, carried out by the engineers of the Miami
Conservancy District. Their report on this subject (published at
Dayton, Ohio, in 1917) is probably the most elaborate discussion of the
kind hitherto prepared for any part of the world. Of the 2,641 storms
investigated, seventy-eight, which covered areas of 500 square miles or
more, were found to have had a rainfall amounting to at least 20 per
cent of the normal rainfall for a year.

The distribution of rainfall over the earth (using the word “rainfall”
in the broad sense to include snow reduced to its water equivalent)
is most conveniently described in terms of mean annual values. This
element is very unevenly divided between different parts of the globe
and between the different regions of every large country. The raininess
or dryness of a climate is determined especially by the prevailing
movement of moisture-bearing winds and the relief of the land, while a
second important control is the location of a region with respect to
storm tracks. The rainiest regions are found on the windward slopes of
mountain ranges not far from the ocean, where the moist winds, forced
by the mountains to ascend rapidly, cool dynamically and shed their
moisture. Thus the southern slopes of the eastern Himalaya receive an
enormous rainfall from the southwest monsoon, blowing from the Indian
Ocean, and an abundant rainfall also prevails on the south slopes of
the high mountains of eastern Tibet, while northern Tibet, in the lee
of the mountains, is a desert.

For more than half a century the little hill station of Cherrapunji,
in Assam, at an altitude of 4,100 feet, has been credited with having
the heaviest rainfall in the world. According to the latest official
record, its average annual precipitation is 426 inches. Recently it
has come to light that Cherrapunji has a serious rival in the Hawaiian
Islands. A fragmentary record totaling 90 months, between 1911 and
1921, kept on Mount Waialeale (altitude 5,075 feet), in the island of
Kauai, showed an average of 455 inches per annum, and there is reason
to believe that a longer record will give an even higher average
for this place. The spot in question, which has been described as
“Uncle Sam’s dampest corner,” is so difficult of access that it can
be reached only after a three-day trip on foot over mountain trails.
Hence the United States Geological Survey has installed here a huge
rain gauge--said to be the largest in the world--capable of holding an
entire year’s rainfall, so that measurements need be made only once a
year.

The heaviest average annual rainfall in the United States (not
including Alaska) is about 130 inches in Tillamook County, Ore. Over
most of our Atlantic seaboard States the rainfall ranges from forty
to fifty inches. Extensive tracts in southern California and western
Nevada have a rainfall of five inches or less. Any region with an
annual rainfall of less than ten inches is normally a desert, though
irrigation or “dry farming” methods may enable its inhabitants to
practice agriculture.

The process of rain formation is not well understood. As we have seen,
the existence of nuclei in the air serves to explain why, when the
conditions of temperature and humidity are right, moisture condenses in
the tiny droplets that constitute clouds. The difficulty is to explain
how, at certain times, quantities of drops are formed of a size large
enough to carry them rapidly to the earth. The number of nuclei is so
great that, as Humphreys has pointed out, even if all the water vapor
in a volume of humid air was condensed upon them, the size of each drop
would remain very small. He has suggested that, in a column of rising
air, the small drops formed at the base of a cloud filter out most of
the nuclei, so that at greater heights there are relatively few of the
latter, which can, therefore, gather sufficient water about them to
form drops of “falling” size.

The speed with which drops fall through the air, which is only a
fraction of an inch per second for the average cloud droplet, increases
rapidly with the size of the drop up to a certain point, but for the
drops that reach the earth as rain the speed of fall tends to become
approximately uniform. Several investigators have measured the size
of raindrops. One method of doing this is to allow the drops to fall
into a shallow layer of fine, uncompacted flour. Each drop forms a
little pellet of dough, which is found, by experiments with previously
measured drops (produced for the purpose and dropped from various
heights), to correspond very closely with the size of the drop.
These pellets dry and harden, and can then be carefully measured,
photographed, etc. Hundreds of samples of raindrops were thus measured
by Mr. W. A. Bentley of Jericho, Vt., and the measurements were
tabulated with reference to the kinds of clouds from which they fell,
the distribution of large and small drops in the different parts of
a storm, and other circumstances attending their fall. Drops of very
different dimensions are found to fall at one time. The commonest sizes
recorded by Bentley were from one-thirtieth to one-eighth of an inch in
diameter; but many drops too minute to form casts were estimated to be
less than a hundredth of an inch in diameter, while the largest drops
observed had a diameter of a quarter and even a third of an inch. This
range of size corresponds to a range in the rate of fall from about
five feet a second for the smallest drops up to about twenty-five feet
a second for the largest. The maximum size of raindrops is limited by
the fact that very large drops are broken up in their fall through the
air. Theoretically, the limiting size is somewhat less than the largest
sizes found by Bentley.

[Illustration: CASTS OF RAINDROPS FROM A THUNDERSHOWER

_Collected and Photographed by W. A. Bentley_]

While rain is often the final product of snow that melts before it
reaches the ground, snow is probably never formed from raindrops, but
always condensed directly from water vapor. The finest snow consists
of separate ice crystals, while snowflakes of larger sizes are always
made up of several crystals partly melted together. The flakes on rare
occasions attain a diameter of three or four inches, and larger sizes
have been reported.

For ages mankind has admired the diversity of beautiful forms exhibited
by snow crystals. Drawings of such crystals, and also of the frost
tracery on window-panes, were made as early as the sixteenth century,
by the learned Swedish historian Olaus Magnus, and many collections of
similar drawings have been published since his time; but nowadays the
combination of the camera and the microscope gives us a far greater
wealth of information concerning these interesting objects. Bentley,
whose study of raindrops we have just mentioned, has made and published
photomicrographs of hundreds of different forms of snow and ice
crystals, and several collections have been published in Europe.

One of the facts revealed by the camera is that the perfectly regular
forms of these crystals seen in drawings are comparatively uncommon in
snow as it reaches the ground. A snow crystal is so fragile that it
is easily mutilated by the wind and by contact with other crystals.
In very calm weather and at the beginning of a snowstorm many single
and perfect crystals are wafted gently to the ground, and their beauty
is revealed when they fall on dark objects, especially if they are
examined with a magnifying glass. In spite of their immense variety
in detail, all perfect snow crystals and other ice crystals have six
sides or principal rays. When secondary rays form they are parallel
with the adjacent primary rays. There are two principal forms of ice
crystal--the tabular and the columnar. Sometimes the two forms are
combined; a column or rod of hexagonal section will have at one or both
ends a hexagonal plate. Both the size and the shape of snow crystals
depend to some extent upon the temperature of the air. The smallest
crystals form in the coldest weather. Star-shaped crystals are most
abundant when the temperature is not far below freezing, while at lower
temperatures there is a preponderance of hexagonal plates.

The cohesive character of moist snow, which is utilized by the younger
generation in the making of snowballs and snow men, enables this
substance to assume naturally a variety of striking forms. Thus a
strip of snow lying along a window ledge or the branch of a tree, will
sometimes slip down in the middle and hang in festoon-shape, supported
only at the ends, constituting a “snow garland.” Over a level or
gently sloping surface of snow the wind occasionally rolls muff-shaped
snowballs, which are known as “snow rollers.” Thousands of them are
sometimes formed at once, and the largest may grow to the size of
barrels. Huge overhanging caps of snow formed on tree stumps, posts,
and the like have been aptly named “snow mushrooms” by Mr. Vaughan
Cornish, who has described those that occur in great numbers in the
Selkirk Mountains of western Canada.

[Illustration: AN ICE STORM AT PHILADELPHIA, DECEMBER, 1914. The
branches are broken by heavy deposits of glaze. The photograph in the
upper left corner, by Dr. David Fairchild, shows a glaze-incased twig.
(_Photographs from U. S. Weather Bureau._)]

Perhaps the strangest of all the shapes assumed by snow is seen in the
greatest perfection in the high Andes of tropical Argentina and Chile.
Here are found innumerable pinnacles of snow or glacier ice, averaging
from four to seven feet in height, though sometimes much higher. Viewed
from a distance, they bear an uncanny resemblance to throngs of
white-robed human beings, and they have thus acquired the Spanish name
of _nieve de los penitentes_ (“snow of the penitents”). In the abridged
form _nieve penitente_ this name is now applied to more or less similar
formations in other mountainous regions. Fine examples are found in the
Himalaya, and one of the Himalayan peaks has been named Mount Nieves
Penitentes. The origin of these pinnacles has been the subject of
much discussion. Sunshine and wind both appear to take part in their
formation. Some remarkable “snow honeycombs,” approaching the form of
_nieve penitente_, are produced in hot, dry summer weather among the
glacier fields of Mount Rainier.

[Illustration: THE NIEVE PENITENTE IN THE ARGENTINE ANDES

(_Photograph by Dr. Juan Keidel._)]

Snow has its economic aspects, comparable in importance to those of
rain. The problem of snow-removal crops up every winter in our American
cities, and is not always solved with brilliant success. In the larger
cities of Europe snow is removed by spreading salt on the streets to
reduce the snow to slush, which is then washed into the sewers with
water, but this method does not seem to be generally applicable to the
heavier snowfall of this country. The snow-removal conference held by
a number of municipal engineers in Philadelphia in 1914 brought this
difficult phase of street cleaning prominently before the engineering
world, and it has been actively discussed in recent years in the
technical journals. Snow presents a formidable problem in the operation
of many railway lines, the solution of which takes the form of snow
sleds, fences, plows of various types, flangers, gasoline torches for
melting snow in switches, etc.

Economically, snow is perhaps most important in its effects on water
supply, and this is true especially of mountain snowfields, the melting
of which feeds adjacent streams. There are great areas in our Western
States where the water required for irrigation is obtained almost
entirely from melting snow. The mountain slopes constitute natural
reservoirs, from which the moisture that falls in the winter as snow
is gradually fed through the spring and summer to the surrounding
country. In these regions extensive “snow surveys” are sometimes made
in the early spring, in order to ascertain the total amount of water
available. Professor J. E. Church, of the University of Nevada, was
one of the pioneers of this idea, and both he and the experts of the
Weather Bureau have developed ingenious apparatus and methods for
making rapid estimates of the snowfall and its equivalent volume of
water lying over a given area. The snow surveyor travels over the
watershed, often on skis or snowshoes, cutting sections of snow with
a cylindrical “snow sampler” and weighing them with a small spring
balance. The Weather Bureau also maintains in the Western mountains a
number of special stations at which daily measurements of snowfall are
made for the benefit of irrigation projects. The use of “snow bins”
and other forms of gauge for holding an entire winter’s snowfall--thus
obviating the necessity of frequent measurement--has not proved very
satisfactory in this country. An analogous device, known in French as
a _totalisateur_, is, however, very extensively used in the Swiss and
Italian Alps.

The heaviest snowfall in the United States occurs in the high Sierra
Nevada of California and in the Cascade Range of Washington and
Oregon. At places in both of these regions more than sixty-five feet of
snow has fallen in a single winter. The snow sometimes lies twenty-five
feet deep on the ground, burying one-story houses to the eaves.

A fall of snow under a cloudless sky is fairly common in the polar
regions and is sometimes observed in calm and very cold weather in
the temperate zones. Rain from a cloudless sky is a more doubtful
phenomenon, of which only a few observations are recorded, most of
them of early date. If such rain occurs, it may come from clouds that
have passed beyond the horizon before the raindrops reach the earth.
Probably the older reports of this phenomenon really relate to dew,
which was once believed to fall from the sky.

Of the three haillike forms of precipitation that we have mentioned
above, true hail is much the most important, on account of the large
size sometimes attained by hailstones and the damage that they are
consequently able to cause. The maximum possible size of a hailstone
cannot be positively stated, but stones larger than a man’s fist
and weighing over a pound have several times been reported on good
authority. During a hailstorm in Natal, on April 17, 1874, stones fell
that weighed a pound and a half and passed through a corrugated iron
roof as if it had been made of paper. Hailstones fourteen inches in
circumference fell in New South Wales in February, 1847. At Cazorla,
Spain, on June 15, 1829, houses were crushed under blocks of ice, some
of which are said to have weighed four and one-half pounds. In October,
1844, a hailstorm at Cette, France, wrecked houses and sank vessels.
In the state of Bihar, India, October 5, 1893, hail covered the ground
to a depth of four to six feet; six persons were buried beneath it and
perished, and hundreds of cattle were killed. In the Moradabad district
of India, May 1, 1888, about 250 people were killed by hail. The
velocity attainable by falling hailstones is perhaps most strikingly
shown by the fact that, even when falling obliquely, they have been
known to pierce a pane of glass with a clear round hole, like a bullet
hole, leaving the rest of the pane intact.

Hail appears to be formed in the violent updraft of air at the front of
a thunderstorm. In this turbulent region the hailstone, first frozen
at a high level, probably makes several journeys alternately up and
down, as it encounters stronger or weaker rising currents; at one time
gathering a coating of snow aloft, and at another a coat of ice from
the rain below, until finally, on account of its large size or on
account of a weakening of the upward blast, it falls to the ground. A
record of these ups and downs in the life of a hailstone is seen in the
concentric layers of clear and snowlike ice of which it is composed.

Although, from immemorial usage, we still speak conventionally of
the “falling of the dew,” it has now been known for more than a
century--especially since the publication of Wells’s “Essay of Dew” in
1814--that dew does not fall. The cooling of air below the dew point of
its water vapor by contact with any cold object results in a deposit
of visible moisture, which is liquid or frozen, according to whether
the temperature is above or below the freezing point, respectively.
This process is not exclusively nocturnal. It is observed by day in
the familiar “sweating” of ice pitchers and also in the appearance
of moisture on pavements, stone walls, and the like, in places shaded
from the sun. At night the rapid cooling of the earth by radiation,
especially (but not, as often stated, exclusively) under a clear sky
and in still weather, favors this condensation of moisture, in the
liquid form, as dew, or in the frozen form, as hoarfrost. The deposit
occurs most copiously on objects that lose heat rapidly by radiation
and gain it but slowly by conduction. Water vapor exhaled from the
tissues of plants and from the soil undoubtedly contributes its quota
to the moisture available for condensation, but this hardly seems to
be a reason for asserting, as some writers have done, that dew comes
mainly from the earth rather than the air.

Hoarfrost is often described as “frozen dew.” This expression is
misleading, for, although dew-drops are sometimes frozen into little
globules of ice, hoarfrost is more often condensed directly from
atmospheric water vapor in the shape of ice crystals.

“Glaze” and “rime”--to use the latest official designations of the
two kinds of ice coating formed from water in the atmosphere--differ
greatly in appearance, as a rule, though transition forms are sometimes
found. Glaze is produced by the falling of rain on surfaces whose
temperature is below freezing, and is typically smooth and transparent.
Rime is a rough deposit formed from fog, the drops of which are
“undercooled”--i. e., are below the freezing point--and turn to ice on
coming in contact with solid objects. The most remarkable examples of
rime are seen on mountains and in the polar regions. It occurs on the
branches and leaves of trees, and on the corners and edges of upright
objects, rather than on horizontal surfaces. In drifting fog it grows
most rapidly if not entirely on the windward side of objects--i. e., it
builds up against the wind. On Ben Nevis it has been observed to grow
at a rate of more than an inch an hour. Trees, posts, telegraph poles,
and the like are thus eventually changed to shapeless masses of rough
or feathery ice.



CHAPTER VIII

WINDS AND STORMS


The study of the movements of the atmosphere constitutes a rather
formidable branch of science known as _dynamic meteorology_.
This subject has engaged the attention of a number of able
physicists--though far too few--and has begun to assume the character
of an exact science, but is still fruitful of unverified hypotheses.

We shall have only a little to say here about the theories and
hypotheses relating to atmospheric circulation. They are at present, to
a notable degree, in process of revision. Important modifications in
them have resulted from the revelations of upper-air research, as well
as from progress in other fields of inquiry. There are, however, a few
fundamental matters that we must not ignore. We shall start with the
solar heat that keeps the atmospheric machinery in motion.

Of the heat that comes to us from the sun, it is estimated that more
than one-third is reflected by clouds and the earth, or scattered by
dust and air molecules, and thus passes back into space without having
had any effect in heating the atmosphere. Part of the remainder heats
the atmosphere directly, and the rest indirectly, after first heating
the underlying land and water. In both cases, certain atmospheric
gases--notably water vapor--absorb a great deal more heat than others.

The first step in the production of a wind is a difference in
temperature between two parts of the earth’s surface, and hence of the
overlying air. Such contrasts of temperature always exist, both locally
and on a large scale. The high sun of the equatorial regions heats the
earth much more strongly than the low sun of high latitudes; a water
surface has a more equable temperature than an adjacent land surface;
a stretch of bare earth is warmer by day and colder by night than a
neighboring tract covered with vegetation; and so on. Differences in
atmospheric temperature produce differences in pressure, which gravity
tends to adjust by setting up a circulation.

The exact manner in which this circulation is begun and maintained is
not yet perfectly clear, and current ideas on the subject are difficult
to put into brief language. Meteorological writers now lay less stress
than formerly upon the lateral spreading, at high levels, of air that
has been heated and expanded at the earth’s surface, and the inward
flowing of the lower air toward the heated area. There is, we know, an
initial impulse that tends to drive air from a region of high pressure
toward a region of low pressure; but the actual movement of the air is
another matter. The “life history” of an air current is found to be a
very devious affair.

The important fact, for practical purposes, is that air does not flow
in a straight line from the place where the pressure is high to that
where it is low. As soon as it begins to flow it curves from the
straight path, in accordance with Ferrel’s Law, which is thus stated:

“In whatever direction a body moves on the surface of the earth, there
is a force arising from the earth’s rotation that deflects it to the
right in the northern hemisphere, and to the left in the southern
hemisphere.”

This law applies to all bodies moving freely over the earth, and not
merely to the winds.

At the earth’s surface, if the atmospheric pressure is measured
simultaneously at various places by means of barometers, we can get a
clear picture of the horizontal distribution of pressure by drawing on
a map lines, called _isobars_, through places at which the pressure
is identical. The isobars reveal the presence of extensive areas over
which the pressure is above the average and of others over which it
is below the average. If, at the same time, we chart the flow of air
by indicating the direction of the winds at various points, we shall
notice that the air shows a strong tendency to travel _around_ these
areas; and if we could observe its course a thousand feet or so above
the earth we should find the tendency even more pronounced at that
level.

Another important law, springing in part from Ferrel’s Law and
describing the movements of the air around areas of high and low
pressure, is called Buys Ballot’s Law. One way of stating this law is
as follows:

“If you stand with your back to the wind, in the northern hemisphere,
the barometer will be lower on your left than on your right. The
reverse is true in the southern hemisphere.”

The reader, whether he lives in the United States or any other
civilized country, will have no difficulty in securing documentary
evidence of the correctness of Buys Ballot’s Law in the shape of the
daily weather maps issued by the various meteorological services. On a
weather map showing conditions anywhere in the northern hemisphere it
will be found that the winds (which are indicated by little arrows),
though subject to a good many local variations, have a general tendency
to blow in the direction followed by the hands of a clock (“clockwise”)
around an area of high pressure, and in the opposite direction
(“counterclockwise”) around an area of low pressure. It will likewise
be noticed that, in general, the winds, instead of blowing along the
isobars, are strongly inclined inward in the case of a low-pressure
area and outward in the case of a high-pressure area. Lastly, if the
map indicates the force of the winds at different places, it will be
seen that winds are strongest where the isobars are close together and
weakest where they are far apart.

It is a matter of much interest to aeronauts that the force of the
wind generally increases with altitude, and that, in the lower flying
levels, the winds are little, if any, inclined to the isobars.

The spacing of the isobars is called the _barometric gradient_. One of
the conventional ways of expressing a gradient numerically is to state
the horizontal difference of pressure, in hundredths of an inch, for an
interval of fifteen nautical miles; but meteorologists as a rule merely
describe gradients as “steep,” “gentle,” “moderate,” etc., without
indicating their numerical values.

If the great difference in temperature between the equatorial and polar
regions were the only factor in the control of atmospheric circulation,
there would be a strong barometric gradient between these regions and
there would result a simple circulation of winds, blowing poleward
from the equator aloft and equatorward from the poles at the earth’s
surface. The former tendency of writers on meteorology and physical
geography was to regard such a circulation as a substantial fact,
though modified by the effects of the earth’s rotation and various
local causes. Thus the idea has prevailed of a wholesale, direct
exchange of air between the poles and the equator. Nowadays we can
hardly maintain this idea, because we see that, on account of the great
deflections they undergo, the main drifts of air are approximately
along parallels of latitude and not along meridians of longitude.
Within the tropics the general drift of the lower air is from the east
(and near the equator this drift prevails up to a great height); in
middle latitudes it is from the west; and in the circumpolar regions
it is again from the east. Air from the equator presumably does find
its way to high latitudes, and _vice versa_, but neither rapidly nor
directly.

Perhaps the dominant feature of the whole circulation is the banking up
of the air in so-called high-pressure belts at about latitude 30° North
and South. From these “belts”--which are really broken up into separate
areas of high pressure, and which shift north and south to a certain
extent with the seasons--blow the northeast and southeast _trade
winds_, in the full development of which, found only over the Atlantic
and the eastern Pacific, we have the most remarkable “permanent winds”
of the globe.

Between the trade wind belts lies the equatorial region of low
pressure, known as the “doldrums.” This is, in general, a region of
light and variable winds, heavy rains, and thunderstorms.

In the temperate zones of both hemispheres, on the poleward side of
the high-pressure belts above mentioned, the general drift of the
atmosphere near the earth’s surface is from west to east. In the south
temperate zone there is a very strong preponderance of west winds,
especially over the vast oceanic tract of the “roaring forties,” where
blow the boisterous “brave west winds,” well known to sailors. The
corresponding belt of the northern hemisphere, which includes all but
the southernmost part of the United States, is described as a region of
“prevailing westerly winds,” but it is also a region of storm tracks,
and hence, as local episodes in the general movement of the atmosphere
from west to east, there are constant shifts of the wind to all points
of the compass, for reasons that will presently be explained.

On the poleward borders of the two belts of “prevailing westerlies,” a
little outside the Arctic and Antarctic Circles, there are zones of low
pressure. That of the southern hemisphere is a continuous girdle around
the earth, and has the lowest pressures found anywhere in the world.
The corresponding subarctic zone, while fairly continuous in summer,
is broken up in winter by the formation of high-pressure areas over
Siberia and northern Canada.

The permanent ice sheets of Greenland and Antarctica are regions of
high pressure, with calm air in the interior and strong outblowing
winds at the borders. Furious blizzards prevail at the margin of
Antarctica.

The table on the opposite page will serve as a recapitulation of the
facts above stated.

  +-------------------------------------------------------------------+
  |                                                                   |
  |                              NORTH POLE                           |
  |                                                                   |
  | Calms and high pressure over the interior of Greenland.           |
  |   Out-blowing winds at the border.                                |
  |                                                                   |
  | More or less broken subarctic belt of low pressure.               |
  |                                                                   |
  | Prevailing westerlies (much interrupted by moving cyclones and    |
  |   anticyclones).                                                  |
  |                                                                   |
  | Belt of high pressure at about lat. 30° N. “Horse latitudes,” or  |
  |   “calms of Cancer.”                                              |
  |                                                                   |
  | Northeast trade winds.                                            |
  |                                                                   |
  | Equatorial belt of low pressure, calms, and variable winds. The   |
  |   “doldrums.”                                                     |
  |                                                                   |
  | Southeast trade winds.                                            |
  |                                                                   |
  | Belt of high pressure at about lat. 30° S. “Calms of Capricorn.”  |
  |                                                                   |
  | Prevailing westerlies, more constant than in the northern         |
  |   hemisphere. “Brave west winds.”                                 |
  |                                                                   |
  | Subantarctic belt of very low pressure.                           |
  |                                                                   |
  | Calms and high pressure over the interior of Antarctica. Violent  |
  |   outblowing winds at its border.                                 |
  |                                                                   |
  |                              SOUTH POLE                           |
  |                                                                   |
  +-------------------------------------------------------------------+
                 WIND AND PRESSURE BELTS OF THE GLOBE.

The great wind and pressure belts of the globe are much more constant
and sharply defined over the oceans than over the land, and it was upon
the high seas that mankind first distinguished them and gave them their
names. The northeast trade winds of the Atlantic wafted Columbus to the
New World and aroused the misgivings of his sailors, who wondered how
they should ever sail homeward against them. The high-pressure belt
north of these trade winds is a region of calms, which Maury called the
“calms of Cancer.” This region, or a part of it, is likewise known as
the “horse latitudes,” the story being that, in the old sailing days,
vessels laden with horses were often becalmed here so long that the
cargoes had to be thrown overboard. As a matter of course, the prosaic
modern etymologist declines to accept this origin of the name and has
proposed others less picturesque. The so-called equatorial calms, which
lie mostly a little north of the equator, are often nicknamed the
“doldrums,” or sometimes the “equatorial doldrums,” to distinguish them
from other regions of dolorous, baffling calms. The doldrums vary a
great deal in width and the masters of sailing ships try to cross them
where they are narrowest.

The name of the trade winds implies that, according to the old nautical
phrase, they “blow trade,” or constantly in one direction. Strictly
speaking, they vary considerably in direction, at any one spot, though
they are nearly always from an easterly quadrant, and they are even
more variable in force. The average speed of the Atlantic trades is
about eleven miles an hour. In view of the prospective requirements of
aeronauts, it is a fact of much interest that the trades are rather
shallow winds. Their vertical thickness has been found, by observations
with pilot balloons and otherwise, to range from less than a mile
to two or three miles. Some distance above the trades there are
winds blowing more or less in the opposite direction, known as the
_counter-trades_. Aircraft will probably use the trade winds in flying
from southern Europe to the Caribbean, and the countertrades on the
return voyage to Europe.

In contrast to the permanent or quasi-permanent winds just described,
there are certain important winds of the “periodic” type, which reverse
their directions in the course of the year or from day to night. Some
of these, also, first became generally known through the reports of
mariners. The ancient Greek navigators utilized the monsoons in trading
with India; while we owe to the voyages of William Dampier, in the
seventeenth century, one of the earliest and best descriptions of land
and sea breezes.

A monsoon is a wind that blows from a continent toward the sea in
winter, when the land is colder than the water, and in the opposite
direction in summer, when the reverse conditions of temperature
prevail. The pressure gradient is reversed with the seasons, and
the wind varies accordingly. The most striking example of monsoon
winds is found in southern Asia--where these winds are of special
economic importance because they control the rainfall of India--but
well-developed monsoons also occur in Australia and West Africa, over
the Caspian Sea, on the coast of Texas, and elsewhere.

An analogous reversal of gradients, due to the change of temperature
over the land from day to night, is of common occurrence on the shores
of large bodies of water, resulting in land and sea breezes (or land
and lake breezes). The breeze blows from the land to the water by night
and in the opposite direction by day. These breezes are generally best
developed and most regular within the tropics, and particularly on
shores adjacent to mountains. East Indian fishermen put out to sea with
the land breeze in the early morning and come home with the sea breeze
in the afternoon. The refreshing and health-giving character of the sea
breeze of tropical climates has earned it the sobriquet of “the doctor.”

Mountain and valley breezes furnish another example of diurnally
reversed winds. Relatively cold and heavy air drains down from the
upper slopes by night, constituting the mountain breeze. By day the air
in the valley is warmed and expanded, and as it is confined laterally
by the sides of the valley it flows up the slopes, constituting the
valley breeze. Long before meteorologists undertook to classify the
winds of the globe, these mountain air currents attracted attention and
acquired local names. Among the Alps, alone, we find scores of such
names. In many cases, too, the breezes have acquired a legendry of
their own. Thus the _pontias_, a cold, nocturnal wind that blows out of
a narrow valley opening upon the plains of the Rhône near the town of
Nyons, is said to have been brought thither in a glove by St. Cæsarius,
Archbishop of Aries, for the purpose of improving the fertility of the
valley! There is a quaint little book about the pontias, published
by Gabriel Boule in 1647, in which the author sets forth at length
the arguments for and against the miraculous origin of this wind. The
Italian lakes are especially rich in locally named mountain and valley
breezes.

Parenthetically it may be remarked that wind nomenclature in general
is a vast subject, owing to the habit that prevailed in prescientific
times, and still prevails to some extent among nonscientific people,
of giving individual names to the winds characteristic of different
localities. As a matter of curious interest, we set down here some of
these names (a small fraction of the total number):

_Khamsin_, _leveche_, _leste_, _levanter_, _pampero_, _zonda_,
_papagayo_, _buran_, _purga_, _brickfielder_, _southerly burster_,
_williwaw_, _willy-willy_, _pontias_, _vésine_, _solore_, _joran_,
_morget_, _rebat_, _vaudaire_, _breva_, _tivano_, _ora_, _Wisperwind_,
_Erlerwind_, _Rotenturmwind_, _vent du Mont Blanc_, _vent d’aloup_,
_autan_, _tramontana_, _gregale_, _imbat_, _kite and junk winds_,
_bad-i-sad-o-bist roz_ (the furious “wind of 120 days” of Persia and
Afghanistan).

The present writer has collected several hundred local wind names--and
is constantly adding to the list.

There are several other types of wind peculiar to mountains besides the
alternating mountain and valley breezes. Most of these are descending
winds, or “fall winds,” which may blow by day as well as by night. Thus
a strong daytime wind sometimes blows down from lofty snowfields and
glaciers. A _foehn_ (pronounced like “fern,” but without the _r_) is a
wind that has been robbed of most of its moisture through precipitation
on the windward slope of mountains and which is further dried, and also
strongly heated by compression, in descending the leeward slope. Winds
of this type are common in the Alps, where they were first described
and named, and their heat and aridity led to the belief that they came
by way of the upper atmosphere from the distant deserts of Africa. Now
that their origin is better understood, we find that foehns prevail in
many other mountainous countries throughout the world, including the
western United States, where they are called _chinooks_. When the foehn
blows in winter, it causes snow to disappear with amazing rapidity--not
only melting it, but speedily drying the ground--whence it has earned
the name of “snow eater” in America, and “_Schneefresser_,” which means
the same thing and a little more, in Switzerland.

The _bora_ of the Adriatic and the _mistral_ of southern France are
winds that blow from a cold, mountainous interior down to a warm
coastal region, where they arrive as relatively cold winds, in spite
of the dynamic heating they have undergone in their descent. The bora
is sometimes moderate (_borina_) and at other times a tremendous gale
(_boraccia_), while the mistral has been known to blow a railway train
from the track in the valley of the Rhône.

The _blizzard_ is a wind of which Americans once thought they had
almost a monopoly until Sir Douglas Mawson located the “home of the
blizzard” on the shores of the Antarctic continent. The true blizzard,
whether American or Antarctic, is a violent, intensely cold wind,
heavily charged with snow. Such winds are a characteristic feature of
the winter climate of our Middle Western States. Although the name of
this wind first became current as recently as the seventies of the last
century, nobody knows its origin. Nowadays the name is often loosely
applied to big snowstorms that are not really blizzardlike.

The dry “hot winds” that sometimes wither the crops of our western
plains are the American antithesis of the blizzard. These winds belong
to the great sirocco family--the name “sirocco” having become, in
recent scientific usage, a generic designation for extensive hot winds,
whether dry or moist, as distinguished from local hot winds, such as
the foehn.

The _harmattan_ of West Africa is a dry, dusty wind from the Sahara,
and one that feels relatively cool; perhaps on account of causing rapid
evaporation from the skin. The _simoom_ (with a final _m_), especially
the variety blowing in southern Asia, is perhaps the hottest and most
parching of all winds--judging from its effects on animal life.

The great majority of the winds above enumerated are merely minor
features of what are called _cyclonic_ and _anticyclonic_ wind systems.
Reverting to what has been said about weather maps and Buys Ballot’s
Law--if the reader will examine a series of maps for successive days,
he will notice that the areas of high pressure and low pressure are not
stationary, but show a more or less rapid displacement, the general
direction of which, in our latitudes, is from west to east. The fact
that there are great traveling vortices or swirls in the atmosphere,
which, in whatever regions they occur, partake of the general drift
of the air around the globe, has been known for about a century, and
constitutes the corner stone of practical meteorology. In the temperate
zone, where these swirls are sometimes of moderate force and sometimes
very stormy, they are the chief factor in controlling changes of
weather from day to day, and their observation is the basis of weather
forecasting. Within the tropics, where they are much less frequent
and are confined to a few restricted regions, they are always violent
storms.

An area of high pressure, with its attendant system of winds, is called
an _anticyclone_ or _high_. An area of low pressure, with its winds, is
called a _cyclone_--sometimes. The word “cyclone” was invented by Henry
Piddington in the year 1848. Nearly all the early studies of cyclones
were made chiefly for the benefit of mariners, and related to the
severe revolving storms encountered at sea. Hence the word “cyclone”
passed into the general vocabulary with a connotation of violence,
which, in everyday speech, it still retains. Perhaps the early
“cyclonologists” themselves hardly realized that a “gentle cyclone” was
not a contradiction in terms.

Meteorologists are still so much under the influence of the popular
idea of a “cyclone” that they hesitate to apply this term to a
disturbance of moderate force, except in a few special phrases (such
as “extra-tropical cyclone”), though the adjective “cyclonic” is
used freely without reference to the force of the wind. British
meteorologists speak mostly of “depressions,” while American
meteorologists speak of “lows.” The status of the latter term, as well
as that of the term “high,” is, however, paradoxical. Though both
words have been used for years, they are nearly always printed with
quotation marks around them, as if they had not yet been assimilated in
the vocabulary. The Weather Bureau has lately taken to printing these
words in capital letters. Neither of these practices will be followed
in the present book.

[Illustration: PHOTO BY PROF. ELLERMAN SHOWING CLOUDS OR FOG CASCADING
THROUGH LAST FORK CAÑON AND INTO THE SANTA ANITA CAÑON. (_Letter from
F. A. Carpenter, May 29, 1919._)]

Tropical cyclones are called “hurricanes” in the West Indies and the
South Pacific, “typhoons” off the east coast of Asia, “baguios” in
the Philippines, and “cyclones” in the Indian seas. They form in the
doldrums, and generally take a long, sweeping course, curving westward
and poleward, and sometimes passing into the temperate zones, where
they either die out or increase in size, diminish in violence, and
become similar to the storms originating in the higher latitudes. One
of the curious features they often exhibit within the tropics is the
calm center at the “eye of the storm,” to which Tennyson alludes when
he writes of the blast (unknown to meteorologists) that drove a ship

  Across the whirlwind’s heart of peace,
  And to and thro’ the counter-gale.

These cyclones are the worst of all storms found at sea, and also
exercise their destructive effects over islands and along continental
coasts. The greatest disasters attending them have been due to the
inundation of low-lying shores by the huge waves they generate, as
in the Galveston hurricanes of 1900 and 1915 and in the far worse
catastrophes that have occasionally visited the coast of India.
Hurricanes of the West Indies occur chiefly from August to October,
inclusive. The number varies from none to a dozen a year (with four as
an average).

[Illustration: Copyright, Ewing Galloway

A CLOUDBURST NEAR CEDAR BRAKES, UTAH

A snapshot taken from the edge of the cañon.]

Over the large land areas of the north temperate zone highs and lows
show a tendency to travel over typical tracks, the locations of which
vary a good deal with the season. One of the most remarkable facts
about the lows of North America is that, wherever they come from,
whether from the Canadian northwest, the western United States, or the
West Indies, they nearly always leave the continent by way of the Gulf
of St. Lawrence or the northeastern corner of this country. Our North
American lows travel at an average speed of 600 miles a day. Highs
travel somewhat more slowly; about 540 miles a day is the average in
this country.

A _tornado_ is a small vortex in the atmosphere, occurring generally in
the southeastern part of a cyclonic area, where, in some cases, several
separate tornadoes develop at the same time. The tornado, for some
reason that is not altogether clear, is far more common in the interior
of North America, east of the Rocky Mountains, than anywhere else in
the world, though true tornadoes do occur in other countries. The West
African storms bearing this name are merely thundersqualls, quite
different from the American tornado. The chief visible feature of a
tornado is the so-called funnel-shaped cloud (sometimes balloon-shaped
or, again, like a great coiling serpent), which is always in contact
with the ground when destruction is in progress. The passage of the
storm is attended by a loud roaring or rumbling. The path of a tornado
varies in width from a few rods to half a mile or (rarely) more. Within
its borders buildings are blown to pieces, trees are uprooted and human
beings only find safety underground; while even at a distance of a few
yards outside the path no damage is done. The tornado travels at an
average speed of about twenty-five miles an hour. Its speed of rotation
has been estimated, from the effects produced, to amount to 500 miles
an hour in some cases; a wind force far exceeding that of any other
type of storm.

_Waterspouts_, which occur on the ocean and other large bodies of
water, are similar in character to tornadoes, though much less violent.
They range in height from 100 to 1,000 yards, or more. One measured
recently from the British steamer _War Hermit_, near Cape Comorin, was
4,600 feet high to the base of the overlying cloud. The column tapered
from 500 feet wide at the junction with the cloud to 150 feet wide at
the sea. Spray was thrown up to a height of more than 800 feet over a
region 250 feet in diameter.

_Thunderstorms_ occur chiefly in warm climates and during the warm
season in temperate climates, but they are by no means unknown in the
polar regions. They are characterized by rapidly rising air currents,
which may be either incidental to the circulation of a low, or due
to local overheating of the lower atmosphere. In the former case
they are called “cyclonic thunderstorms,” and in the latter “heat
thunderstorms.” This is only a rough classification, however. Some
thunderstorms partake of the features of both these types, and, on the
other hand, additional classes are distinguished by many authorities.
It is a common occurrence for thunderstorm conditions, starting in
some small area, to travel across country at a speed of perhaps thirty
or forty miles an hour, at the same time spreading out fanwise until
the front of the storm is hundreds of miles in length. This front
constitutes a “line squall” (so called from the long line or apparent
arch of dark cloud that marks its location), and is attended by more
or less thunder and lightning, but is not necessarily a continuous
thunderstorm. The characteristic wind of a thunderstorm is the squall
that rushes out in front of the storm when close at hand. This blast
of wind, lightning, hail and torrential rain are all agencies of
destruction in severe thunderstorms.

[Illustration: IDEAL CROSS SECTION OF A TYPICAL THUNDERSTORM

_A_, ascending air; _D_, descending air; _C_, storm collar; _D’_, wind
gust; _H_, hail; _T_, thunderheads; _R_, primary rain; _R’_, secondary
rain. (W. J. Humphreys.)]

Concerning the winds of the globe in general and the remarkable
atmospheric interchanges that they involve, Sir Napier Shaw writes:

“Of the millions of tons of air which form the atmosphere nearly the
whole is moving. The regions of calm at the surface at any one time,
taken all together, do not form a large part of the earth’s surface,
and above the surface calm regions are still rarer. Let us remember
that the motion of the air is always ‘circulation’; air cannot move
forward or backward or upward or downward without displacing other air
in front of it and being replaced by other air behind it, though the
circulation may be quite local and limited in extent, as is frequently
the case when warm air rises or cold air sinks. In the course of
investigations into the life history of surface air currents in the
Meteorological Office we have traced air over long stretches of the
surface of the Atlantic. We have found, on one occasion, the shores
of Greenland to be fed with air that left the middle of the Atlantic
four days previously, while in the course of six days air traveled from
Spitsbergen to join the northeast trade wind off the west coast of
Africa. On another occasion the air that formed the wind off the south
of Ireland was traced back to the north of Africa, but that which blew
at the opening of the Channel two days later came from Hudson Bay, via
the Azores.”

Such are the ever-shifting currents of the ocean of air.



CHAPTER IX

ATMOSPHERIC ELECTRICITY


Every schoolboy has read how Benjamin Franklin, by means of his famous
kite experiment, demonstrated the electrical nature of lightning, and
how the same versatile genius invented the lightning rod. It is not
proposed to repeat familiar history here. Neither shall we discuss
the dubious statements frequently put forth that lightning rods were
known before Franklin’s time, nor consider how much credit is due
the many philosophers who, at earlier periods, suspected lightning
to be a manifestation of electricity. The facts and ideas concerning
atmospheric electricity that we have to present in this chapter were,
for the most part, quite unknown to Franklin and to many generations of
_savants_ after him, and some of them are just now finding their way
into the textbooks.

Science still recognizes the existence of two kinds of
electricity--positive and negative--which, by combining, neutralize
each other’s effects. According to current ideas, however, the more
active agent in electrical phenomena is negative electricity, which
is believed to consist of (or to provide electrical charges for)
exceedingly minute particles called _electrons_.

Only a few years ago the smallest thing that science had to deal
with was the atom, and the lightest of atoms is that of hydrogen.
The discovery of electrons marks a new step toward the infinitely
little. The mass of the electron--or, in more popular and less exact
terms, its weight--is about 1/1800 that of a hydrogen atom. As to its
size: Imagine a billiard ball magnified to the size of the earth. Its
constituent atoms would be the actual size of billiard balls, but the
electrons of which each atom is composed would still be too small to be
seen with the naked eye. Now imagine each of these billiard-ball atoms
further magnified to the size of a large church. The electrons would
then be about as big as one of the periods on this page.

When we say that a body has an electrical charge we mean that it has
an _excess_ of positive or negative electricity. An ordinary molecule
of an atmospheric gas contains (or perhaps actually consists of)
equal amounts of the two kinds of electricity, and is therefore not
charged. There are, however, various ways in which an electron may
be detached from such a molecule, leaving it positively charged; and
again it may receive an extra electron, and thus acquire a negative
charge. Under the former circumstances it becomes a _positive ion_,
and under the latter a _negative ion_. Ions play a very important role
as carriers of electricity, because they are impelled to move toward
oppositely charged bodies or particles and combine with them. A gas
containing ions is said to be _ionized_; and it is the ionization of
the atmosphere that makes it a conductor of electricity.

The number of ions in a given volume of air has been the subject
of a great many measurements, both at observatories on land and in
the course of scientific expeditions at sea. There are ingenious
instruments called “ion counters,” in which air is drawn at a measured
rate through the apparatus and its electrical effects are noted.
The number of positive ions found in a cubic centimeter of the lower
atmosphere varies from a few hundred to a thousand or more, while the
number of negative ions in the same space is generally one or two
hundred smaller. The ionization is about the same over the ocean as
over the land.

There are several ways in which the air may become ionized. The
different rays given off by radioactive substances (Alpha, Beta, and
Gamma rays) all have the power of driving off electrons from the
molecules of gases; i. e., ionizing them. Air is undoubtedly ionized by
radioactive matters in the soil (radium and thorium) and especially by
the gaseous “emanations” of these substances in the atmosphere, which
are also radioactive. It has, however, been a problem to account for
ionization over the ocean; because the amount of radioactive matter
in sea water is immeasurably small, while the amount of radioactive
emanation in sea air is, according to the latest observations, only
about 2.5 per cent of that occurring over the land.

The clue to this mystery seems to be found in a special kind of Gamma
rays coming from some region far above the surface of the earth. These
rays are called the “penetrating radiation,” because they not only are
able, like the Gamma rays due to radioactive substances on earth, to
pass through the walls of hermetically sealed metal vessels and ionize
the air inside, but they also have the power of passing through a great
extent of atmosphere without being absorbed. They are estimated to be
about ten times as “penetrating” as the Gamma rays coming from known
terrestrial substances. The best proof that a radiation of this sort
comes from above is that when closed metal vessels are carried up in
balloons, there is, above an altitude of about half a mile, a rapid
increase in the rate at which ions are produced within them. As to the
source of this radiation, one suggestion has been that it comes from
strongly radioactive cosmic dust in the upper atmosphere. A hypothesis
that seems more plausible at present attributes it to the bombardment
of the atmosphere by electrons shot off from the sun.

The knowledge of ions in the atmosphere is one of the recent
acquisitions of science. On the other hand, it has been known for
some generations that the earth has normally a negative charge as
compared with the air, or the air a positive charge as compared with
the earth. Thus between the earth and any point in the air (except,
as we now know, at great altitudes) there is a difference of what is
called “potential,” of such a character that negative electricity will
follow any conductor provided for it away from the earth. Variations
of potential with altitude have long been measured by means of
instruments called “collectors,” which gather, so to speak, a sample
of the electrical charge of the air at any point and enable it to
be compared with that of the earth. The difference of potential is
measured in volts per meter of vertical distance. Thus we get the
“potential gradient,” which averages about 150 volts per meter in
the lowest part of the atmosphere. It is subject to great irregular
variations--especially during thunderstorms--and also to somewhat
regular rises and falls during each day, and to an annual fluctuation,
being much greater in winter than in summer.

It has also been known for a good many years that the air is a
conductor of electricity--though a poor one--and, therefore, does not
insulate the earth. Dr. W. F. G. Swann has expressed the extremely
small conductive capacity of the air for electricity in the statement
that a column of it one inch long offers as much resistance to the
passage of an electrical current as a copper cable, of the same cross
section, thirty thousand million million miles long!

A fact more recently learned, from observations in balloons, is that
the potential gradient falls off rapidly at high levels, and becomes
practically zero at an altitude of about six miles. From this fact
it is concluded that the lower six miles of the atmosphere contains
a charge of positive electricity just equal to the negative charge
at the earth’s surface. In other words, the lower atmosphere is not
only positive with respect to the earth, but in an absolute sense it
contains an excess of positive electricity.

Thus we have a negatively charged earth surrounded by a layer of
positively charged air. Since air is a conductor, it is not easy to see
why the opposite charges of the earth and the atmosphere do not combine
and neutralize each other. An interchange is, in fact, always going on
between them; negative ions flow upward from the earth and positive
ions flow in the opposite direction. This “earth-air current” is,
however, exceedingly small. Moreover, the opposite charges of the earth
and air remain from year to year in spite of it.

How does the earth retain its negative charge and the air its positive
charge? No other question relating to atmospheric electricity has, in
recent years, been so much debated as this. Discussion centers, as a
rule, upon the negative charge of the earth; for there are certain
reasons for assuming that, when once this is explained, the positive
charge of the atmosphere will require no special explanation.

One hypothesis is that the earth is bombarded by positive and negative
corpuscles from the sun, and that the negative corpuscles have such
penetrating power that they are able to reach the earth, while the
positive corpuscles are caught by the atmosphere. Another hypothesis
(Swann’s) is that the same “penetrating radiation” that, as we have
seen, helps to ionize the lower atmosphere has the effect of driving
downward a stream of electrons detached from the air molecules, thus
maintaining a constant supply of negative electricity to the earth. The
question is not yet settled.

It is now time to turn from these somewhat abstruse matters to a
subject of universal interest; viz., lightning. As recently as a few
decades ago, though there was already a copious literature on the
subject of lightning, very little was really known about it. Even its
superficial features were strikingly misunderstood until the advent of
photographic methods of investigation. Thus until the middle of the
nineteenth century sharply zigzag lightning flashes were represented
in scientific books as they still are in conventional art. That
so-called zigzag lightning is really sinuous was first asserted by
James Nasmyth, in 1856, and his contention was soon afterward confirmed
by photography. The camera has revealed a large number of other
interesting things about lightning.

Everybody has noticed an appearance of flickering in lightning flashes
that are of sensible duration. Several early investigators, such as
Arago, Dove, and O. N. Rood, had reached the conclusion that such
flashes are multiple, consisting of several successive discharges
along an identical path. Rough measurements of the intervals of time
between these discharges were made with various forms of rotating disk.
Far more accurate information is now obtained on this subject by the
use of a camera mounted on a vertical axis and swung in a wide arc, at
a fixed rate, by clockwork. The perfection of this device is due, in
part, to A. Larsen, in America, but especially to Dr. B. Walter, of
Hamburg, whose achievements in the photography of lightning far surpass
those of any other investigator.

It is obvious that if a discharge of lightning is not instantaneous,
but has a sensible duration, the rotary movement of the camera,
arranged as just mentioned, will spread out the image of the flash,
on the photographic plate, into a more or less broad band or ribbon.
Most photographs of ribbonlike streaks of lightning made with ordinary
cameras are, in fact, due to accidental movements of the apparatus
during exposure--such as an involuntary start of the operator, in case
the camera is held in the hands--though a certain amount of spreading
of the image is sometimes caused by what photographers call “halation.”
Pictures taken with the revolving camera show that some flashes are
practically instantaneous while others may last as long as half a
second or more. Those of the latter class nearly always show several
parallel streams of light, more or less distinctly separated by darker
spaces. Each of these bright streams represents a separate discharge
along the common path. As the speed with which the camera turns is
known, it is possible to determine the intervals of time between the
discharges of a multiple flash. These intervals may vary from a few
thousandths to one or two-tenths of a second, while the duration of
each of the consecutive discharges is probably not more than two or
three hundred-thousandths of a second in most cases. Sometimes the path
of the lightning flash is shifted by the wind while the picture is
being taken. In one case this shift was estimated at 36 feet.

Photography is also applied to determining the distance of a lightning
flash and hence the dimensions of any of its features. For this purpose
a stereoscopic method is used, two cameras being mounted side by side
and exposed at the same time. Sometimes one of the cameras is made to
revolve, while the other remains stationary. The stationary camera
will then show the relative positions of the flashes occurring during
exposure, while the moving camera will indicate the times at which they
occurred.

Streaks of “black lightning” and black borders of the white flashes,
both often seen in photographs, are a trick of the camera and are due
to what is called the “Clayden effect.” Some kinds of plates are much
more susceptible to this effect than others. When a flash of lightning
has registered its impression on such a plate, and, before the shutter
is closed, another flash occurs, the general illumination of the field
by light reflected from clouds, etc., often “reverses” the original
image, and consequently it prints black.

“Sheet lightning” presents the appearance of a diffuse glow over the
sky. When lightning of this character is seen playing about the horizon
on summer evenings, in the absence of an audible thunderstorm, it is
often called “heat lightning.” Most sheet lightning is probably a mere
reflection of ordinary streak lightning below the horizon or hidden
by clouds. Some authorities believe, however, that diffuse, silent
discharges actually occur in the clouds. Balloonists claim to have
encountered such discharges near at hand. An analogous phenomenon is
the glowing of so-called “incandescent” or self-luminous clouds, to
which several observers have called attention. A remarkable phenomenon
of somewhat similar character has been reported by Dr. Knoche, late
director of the Chilean meteorological service, who states that it
occurs on a vast scale along the crest of the Andes during the warm
season. The mountains seem to act as gigantic lightning rods, giving
rise to more or less continuous diffuse discharges between themselves
and the clouds, with occasional outbursts simulating the beams of
a great searchlight. These displays are visible hundreds of miles
out at sea. Something akin to this so-called “Andes lightning” has
occasionally been reported from other mountainous regions, including
the mountains of Virginia and North Carolina.

“Beaded” lightning and “rocket” lightning are as rare as they are
interesting. The former resembles a string of glowing beads, while the
latter is a form of streak lightning that shoots up into the air at
about the apparent speed of a skyrocket.

“Ball lightning” takes the form of a fiery mass (not always globular),
which generally moves quite deliberately through the air or along the
ground, and in many cases disappears with a violent detonation. It
occurs inside of buildings, as well as out of doors.

In order that a discharge of electricity may break through the
resistance of the air along paths as long as those commonly observed,
enormous differences of potential must exist in the atmosphere during
thunderstorms. How such conditions arise has been the subject of
an immense amount of speculation. The explanation now generally
accepted was proposed in the year 1909 by the English physicist and
meteorologist, Dr. George Simpson. This hypothesis is based upon the
fact, well attested by laboratory experiments, that the breaking up
of drops of water involves a separation of positive from negative
electricity; in other words, the production of both positive and
negative ions. In this process the drops become positively charged;
i. e., they retain a greater number of positive than of negative
ions, the latter being set free in the air. About three times as many
negative as positive ions are thus released.

Now a thunderstorm is accompanied by strong upward movements of the
air; so strong that small drops cannot fall to the ground, while large
drops, which would be heavy enough to fall through such rising currents
if they could retain their integrity, are broken up by the blast of air
and carried aloft, where they tend to accumulate, recombine, and fall
again. This process may be repeated many times, so that the positive
charge of the drops is continually increasing, and at the same time
negative ions are being set free and carried by the ascending air to
the upper part of the clouds. Here they unite with the cloud particles
and give them a strong negative charge. Thus eventually there is formed
a heavily charged positive layer of cloud between a heavily charged
negative layer above and the negatively charged earth beneath. When
the differences of potential thus brought about become great enough,
disruptive discharges of electricity will occur, and these may be
either between the upper and lower layers of cloud or between the
clouds and the earth, or, sometimes, between two different clouds.

Probably much the most frequent lightning flashes are those that occur
within a single thundercloud and do not reach the earth. However, it
will often happen that the negatively charged upper layer of cloud is
either carried very high or drifted away by the wind, and then the
discharges that occur will be chiefly between cloud and earth. Such
conditions are likely to prevail in the case of cyclonic thunderstorms,
in which there is often great difference in the direction and force of
the winds at different levels. On the other hand, heat thunderstorms
usually occur when the general winds are light at all levels, and it
is probable that such storms are relatively free from cloud-to-earth
discharges. We seem to have here an explanation of the paradox that
tropical thunderstorms, which are nearly always of the noncyclonic
type, though notoriously violent, are generally harmless.

It must not be inferred from what has been said above that the mystery
of the lightning flash is now fully resolved. This is far from being
the case. It is not at all clear how an electrical discharge can break
down the resistance of the air along a path a mile or more in length,
as commonly happens in the thunderstorm. It was formerly stated, on
good authority, that the difference of potential required to produce
such a flash would amount to upward of 5,000,000,000 volts. Certain
facts have lately been adduced to show that such great differences
of potential need not be assumed. Moving-camera photographs of the
sparks produced by electric machines show that such sparks begin with
small brush discharges which gradually ionize the air and thus build
up a conductive path for the complete discharge. Something of this
kind may occur in the atmosphere. Streaks of air already strongly
ionized and more or less continuous sheets of rain would also help to
provide conductors for a discharge. If lightning does build up its
path somewhat gradually, the process might, in certain cases, be so
slow as to account for the deliberate movement of rocket lightning,
and also, perhaps, furnish a clue to the hitherto unsolved mystery of
ball lightning. Humphreys has tentatively suggested that all genuine
cases of ball lightning are “stalled thunderbolts”; i. e., lightning
discharges that have come to a halt, or nearly so, in their progress
through the air.

As to the visibility of lightning Humphreys says, in his “Physics of
the Air”:

“Just how a lightning discharge renders the atmosphere through which it
passes luminous is not definitely known. It must and does make the air
path very hot, but no one has yet succeeded, by any amount of ordinary
heating, in rendering either oxygen or nitrogen luminous. Hence it
seems well-nigh certain that the light of lightning flashes owes its
origin to something other than high temperature, probably to internal
atomic disturbances induced by the swiftly moving electrons of the
discharge, and to ionic recombination.”

A few attempts have been made to measure the strength of current in a
lightning discharge. Many substances become magnetized when an electric
discharge occurs in their vicinity, and it has been pointed out by F.
Pockels that when basalt rock is magnetized in this way the amount of
magnetism is an indication of the greatest strength of current to which
it has been exposed. Pockels examined specimens of basalt from the top
of Mount Cimone, in the Apennines, where lightning strokes are common,
and found many of them more or less magnetized. He also exposed blocks
of basalt close to a branch of a lightning rod in the same region.
He thus obtained values for the strength of current in lightning
discharges ranging from 11,000 to 20,000 amperes. Humphreys, from the
crushing effect of a lightning stroke upon a hollow lightning rod,
has computed the strength of current in the case examined to be about
90,000 amperes.

The effects of lightning are so various that it would take a book to
describe them all. Its audible effects are discussed in our chapter
on atmospheric acoustics. Its chemical effects consist chiefly in the
production of oxides of nitrogen, ozone, and probably ammonia from
the constituents of the atmosphere along the path of the discharge,
and these substances, either directly or after further combinations
in the atmosphere, contribute to the fertility of the soil. Lightning
sometimes bores holes several feet deep in sandy ground and fuses
the material along its path, forming the glassy tubes known as
_fulgurites_. Similar holes are bored in solid rock.

The destructive effects of lightning are due chiefly to the heat
generated by the passage of an electric current through a poor
conductor. When moisture is present in the object struck, its sudden
conversion into steam produces the explosive effects seen in the
shattering of trees, the ripping of clothes from the human body, etc.
There is almost no end to the curious pranks played by lightning--some
disastrous, some comical, and some benevolent, as when persons crippled
with rheumatism, after having been knocked down and temporarily stunned
by a stroke of lightning, have found themselves completely cured of
their malady! A well-known book by Camille Flammarion, translated
into English under the title “Thunder and Lightning,” is almost wholly
devoted to these eccentricities of the lightning stroke.

[Illustration: A LIGHTNING PRINT ON THE ARM OF A BOY STRUCK BY
LIGHTNING NEAR DUNS, SCOTLAND, IN 1883

_Drawn from a photograph taken a few hours after the accident__. From
the Lancet._]

There is a very common belief that lightning sometimes impresses a
photographic image of trees or other objects of the landscape upon the
human body. The ramifying pink marks, known as “lightning prints,”
often found on the skin of persons who have been struck by lightning,
are, however, in no sense photographic, but are merely the lesions due
to the passage through the tissues of a branching electric discharge.

A few practical suggestions in regard to danger from lightning are
offered by Humphreys, as follows:

“Generally it is safer to be indoors than out during a thunderstorm,
and greatly so if the house has a well-grounded metallic roof or
properly installed system of lightning rods. If outdoors it is far
better to be in a valley than on the ridge of a hill, and it is always
dangerous to take shelter under an isolated tree--the taller the tree,
other things being equal, the greater the danger. An exceptionally tall
tree is dangerous even in a forest. Some varieties of trees appear to
be more frequently struck, in proportion to their numbers and exposure,
than others, but no tree is immune. In general, however, the trees
most likely to be struck are those that have either an extensive root
system, like the locust, or deep tap roots, like the pine, for the very
obvious reason that they are the best grounded and therefore offer, on
the whole, the least electrical resistance.

“If one has to be outdoors and exposed to a violent thunderstorm, it
is advisable, so far as danger from lightning is concerned, to get
soaking wet, because wet clothes are much better conductors, and dry
ones poorer, than the human body. In extreme cases it might even be
advisable to lie flat on the wet ground. In case of severe shock,
resuscitation should be attempted through persistent artificial
respiration and prevention from chill.

“The contour of the land is an important factor in determining the
relative danger from lightning because the chance of a discharge
between cloud and earth, the only kind that is dangerous, varies
somewhat inversely as the distance between them. Hence thunderstorms
are more dangerous in mountainous regions, at least in the higher
portions, than over a level country. Clearly, too, for any given region
the lower the cloud the greater the danger. Hence a high degree of
humidity is favorable to a dangerous storm, partly because the clouds
will form at a lower level and partly because the precipitation, and
probably therefore the electricity generated, will be abundant. Hence,
too, a winter thunderstorm, because of its generally lower clouds, is
likely to be more dangerous than an equally heavy summer one.”

It is estimated that the total property loss due to lightning in the
United States is about $8,000,000 a year, and the number of persons
struck about 1,500, of whom one-third are killed. Nine-tenths of these
accidents occur in rural localities.

Lightning rods neither prevent lightning stroke nor do they, as is
sometimes alleged, attract lightning to buildings. They merely provide
good conductors along which a stroke of lightning may reach the earth
without doing damage, and, within very moderate limits, determine the
path of discharge. While there are many unsettled points regarding the
theory of lightning rods and details of construction, their general
utility is strikingly indicated by statistics showing the comparative
amount of damage done by lightning to rodded and unrodded buildings.
According to the United States Bureau of Standards, information
gathered in this country shows that “taking rods as they come in
the general run of installations, they reduce the fire hazard from
lightning by 80 to 90 per cent in the case of houses, and by as much
as 99 per cent in the case of barns.” The same bureau, in its valuable
publication, “Protection of Life and Property Against Lightning”
(Washington, 1915), supplies the answers to a multitude of questions
that are constantly asked about lightning rods.

Buildings with metal roofs and frames connected with the ground are
generally well protected from lightning (except as to nonmetallic
chimneys) without rods.

During actual thunderstorms, and also at other times when there
are high potential gradients in the atmosphere, luminous electric
discharges of a more or less continuous character are sometimes
observed to occur in the shape of small jets and flames, chiefly from
pointed objects, including lightning rods, the masts and spars of
vessels, the angles of roofs, etc. These are identical in character
with the “brush” discharges, or incomplete sparks, produced by electric
machines. They are accompanied by a hissing or crackling sound. Their
luminosity is comparatively feeble, and for this reason they are much
more often observed by night than by day. They are especially common
during snowstorms.

This phenomenon is known as _St. Elmo’s fire_ or _corposants_ (not to
mention a score of other names, ancient and modern). As seen at sea,
corposants sometimes take the form of one or two starlike objects at
the trucks of the masts or the ends of yard arms, but occasionally the
spars, rigging and other parts of the ship are lighted up with a great
number of stationary or moving flames, producing a weird spectacle. The
finest examples of corposants are, however, observed on high mountains,
and the phenomenon has been carefully studied at certain mountain
observatories, such as those on Ben Nevis and the Sonnblick.

Of its occurrence on Ben Nevis, Angus Rankin writes: “The most frequent
manner in which it makes its appearance is as caps of light on the tips
of the lightning rod, but occasionally it appears as jets of flame
projecting from all objects on the top of the tower and from the cowl
of the kitchen chimney, which rises from the roof at some distance
from the tower. These jets are at times from 4 to 6 inches in length,
and make a peculiar hissing sound. During a very brilliant display,
the observer’s hair, hat, pencil, etc., are aglow with the ‘fire,’
but, except for a slight tingling sensation in the head and hands, he
suffers no inconvenience from it. On such occasions, if a stick be
raised above the head, jets of electric light will be seen at its upper
end. The only drawback to observing it with advantage is the unpleasant
character of the weather in which it appears, namely blinding showers
of snow and hail, and squally winds, causing a good deal of snowdrift.”
Rankin records that it was sometimes _heard_ in the daytime, when its
light was invisible. On the Sonnblick a display of St. Elmo’s fire has
been observed to last as long as eight hours.

No luminous electrical phenomenon is more beautiful or, at first sight,
more mysterious than the _aurora_, popularly known, in the northern
hemisphere, as the “northern lights.” This phenomenon is due to the
passage of electrical discharges through the rarefied gases of the
upper atmosphere, and it now appears to be settled beyond controversy
that the discharges are caused by corpuscles or radiations of some kind
emitted from the sun.

Most accounts of the aurora describe the typical appearances that it
assumes as seen from a single place on the surface of the earth, but
say little, if anything, about the form of the phenomenon as a whole
or about its position in space. We shall follow a different plan here,
and ask the reader, first of all, to imagine himself viewing the aurora
from a point some thousands of miles away from our planet.

The solar emission above mentioned, when sufficiently intense, produces
in the upper atmosphere a glow like that seen in a vacuum tube when
an electrical discharge passes through it. From our vantage point in
outer space we shall notice that this glow is not spread over the
whole globe, but forms two rings, which encircle the polar regions of
both hemispheres, though neither the geographic nor the magnetic poles
lie at their centers. The rings do not extend down into the lower
atmosphere, but hang about 60 miles above the earth’s surface.

The reason for this segregation of the aurora in high latitudes is
that the earth is a great magnet, and magnets have the power of
deflecting an electrical discharge in their vicinity. An appearance
much resembling the two auroral zones of the earth was produced, on
a small scale, by the late Professor Kr. Birkeland of Christiania,
who magnetized a metal globe and allowed an electrical discharge to
play upon it in a vacuum. The surface of the globe was coated with
a phosphorescent substance, which glowed under the discharge in two
rings, corresponding roughly to those of the aurora. In both cases the
discharge follows what are called the magnetic “lines of force.” Our
earth, like other magnets, is enveloped and penetrated by such lines.
At any point on the earth the direction of the neighboring lines of
force is shown by the dipping needle, which assumes a position parallel
to them. At a magnetic pole the needle points straight up and down,
and everywhere in high latitudes it has a position not very much
inclined to the vertical, while in low latitudes it is more or less
horizontal.

If, now, for the sake of simplicity we confine our attention to the
northern hemisphere, and imagine ourselves maintaining our watch for
months and years together, we shall discover that much of the time
there is no ring to be seen; at other times there may be a small or
partial ring; and occasionally there is a very broad, conspicuous ring,
spreading so far south that it overlies the northern part of the United
States and most of Europe. Evidently the emission from the sun that
causes the auroral discharge varies greatly in strength, and this is in
accordance with what we know about solar activities in general.

Next let us take a closer look at the ring, whether from outer space or
from the earth’s surface. We shall find that it is made up, at least
in part, of a multitude of luminous beams directed out into space and
undergoing rapid changes in position and form. These beams, which
really mark out the streams of the discharge in the upper air, follow
the lines of force. In high latitudes they are nearly vertical with
respect to the underlying surface of the earth. Even in the United
States (when the aurora extends so far south) they are much more nearly
vertical than horizontal. A dipping needle will show, at any place,
just how they should stand.

[Illustration: PHOTOGRAPHIC ANALYSIS OF LIGHTNING. This double
photograph was made by Dr. B. Walter of Hamburg, the leading expert in
the field of lightning photography. The picture at the left was taken
with a stationary camera. The photograph at the right, taken at the
same time with a revolving camera, shows that one of the main flashes
(the one to the right) was a single discharge, and the other a multiple
discharge.]

From any distant point on the earth’s surface either north or south of
it, the visible portion of the auroral ring presents the appearance
of an arch across the horizon. Arctic explorers, far within the
Arctic Circle, see this arch to the south of them. In our latitudes
it spans the northern horizon. Separate beams or streamers may be
distinguishable or not, according to the brightness of the discharge
or its distance from the point of observation. Combinations of beams
constitute so-called “draperies.”

[Illustration: PHOTOGRAPH OF LIGHTNING, SHOWING “BLACK” FLASHES.
(_graph by F. Ellerman._)]

[Illustration: ATMOSPHERIC ELECTRICITY INSTRUMENTS ON BOARD
THE “CARNEGIE.” Left: Penetrating radiation apparatus. Right:
active-content apparatus. Below: Arrangement for supplying potentials
to electroscopes and ionization chambers, (_Carnegie Institution._)]

Occasionally, at times of great solar activity, part of the ring
actually overlies our Northern States, and the aurora then becomes a
magnificent spectacle in this part of the world. The whole sky may be
filled with the shifting streamers, along which travel rapid pulsations
of light, so that the phenomenon then suggests strongly what it really
is--a vast electrical discharge passing down through the atmosphere
from outer space. When the observer is thus surrounded by the beams,
they seem, on account of perspective, to converge toward a point south
of the zenith, where they form a beautiful _corona_ or crown. The
position of this crown depends upon the slant of the beams, which, as
already explained, follow the lines of force.

A brilliant aurora is always accompanied by disturbances of the
magnetic needle, which moves about erratically, so that compasses can
no longer be depended upon. At the same time there are strong “earth
currents,” which interfere with the operation of telegraph lines.

Observations with the spectroscope seem to show that the light of the
aurora is chiefly due to glowing nitrogen, though the most prominent
line in the auroral spectrum has sometimes been referred to an
unknown atmospheric gas. The various colors seen in bright auroras,
including reds, greens, and yellows, are believed by some authorities
to depend upon the varying speed of the electrical discharge.
Experiments with vacuum tubes show that nitrogen, especially, gives
great changes of color with changes in the velocity of the discharge.
Another interesting revelation of the spectroscope is that there is
apparently, a faint auroral illumination in the sky at all times and in
all parts of the world, the so-called “permanent aurora.”

Photography has been used with great success in studying the aurora,
especially by the Norwegian physicists Störmer, Vegard, and Krogness.
Simultaneous photographs of a single detail are taken from two points
several miles apart against a background of stars. The apparent
position of the auroral detail among the stars will differ in the two
pictures, and a comparison of them makes it possible to determine the
actual position of the aurora in space. A slow-moving cinematograph has
also been used to obtain series of pictures. The measurements of these
observers show that the base of the aurora is, generally between 60 and
70 miles above the earth with a strong tendency to assume a definite
location at an altitude of about 61 or 67 miles. Its upper limits are
not well defined, but it has been photographed up to an altitude of
more than 300 miles. Earlier observers reported seeing the aurora at
altitudes of only a few miles, and even down to the earth’s surface,
but recent authorities are inclined to discredit these observations.

One more phenomenon of atmospheric electricity requires brief mention,
viz., the electric waves that produce the erratic disturbances known to
wireless telegraph operators as “strays” or “static.” As heard in the
receiver of a wireless outfit the noise of strays has been described as
“like hailstones beating against a sheet of tin,” or “short hisses from
a steam pipe,” or “periodic discharges of coal down a chute.” Another
characteristic sound is a sharp “click.” The study of strays has been
carried out on a world-wide scale by a committee of the British
Association for the Advancement of Science, but their nature is not yet
fully understood. Some strays are undoubtedly due to near or distant
discharges of lightning, and special forms of wireless apparatus, known
as “thunderstorm recorders” or “ceraunographs,” have been used to give
notice of the approach of thunderstorms. On the other hand, strays
seem frequently to have no connection with thunderstorms, and their
principal origin is now sought in electrical disturbances in the upper
atmosphere, perhaps similar to those which cause the aurora, and, as in
the case of the aurora, having their ultimate source in the sun.



CHAPTER X

ATMOSPHERIC OPTICS


When we look up into the sky on a cloudless day we behold a continuous
canopy, the prevailing color of which is blue. This canopy is a veil
that hides the starry hosts beyond, and its presence seems, at first
sight, incompatible with the fact that the air is a transparent medium.
We see the stars by night through the same intervening atmosphere.
Why are they cut off from our sight by day? The answer to this
question can, perhaps, best be made plain by a simple experiment.
Place a lighted candle behind a sheet hung across a room not otherwise
illuminated. The flame of the candle will be distinctly visible through
the sheet. Next, let the room be brightly lighted, say with electric
light or daylight. The candle can now no longer be seen through the
sheet, owing to the bright illumination of the latter as compared with
the feeble light of the candle.

In the atmosphere the counterpart of our sheet is a layer, several
miles in depth, of minute particles, which by day are lighted up by
the sun. Some of the particles are tiny dust motes, others are fine
droplets of water or bits of ice, and the rest are the molecules of the
atmospheric gases themselves. It is the light that comes to us from
these particles that makes our eyes insensitive to the fainter light of
the stars, and makes the sky itself a visible luminous vault.

Next, why is the clear sky generally blue, rather than some other
color? To answer this question, we must recall the fact that sunlight
is made up of ether waves of many different sizes. In combination,
these waves produce upon our eyes the sensation of white light. When
they are separated, as by passing through a prism, the smallest
waves--or, in more technical language, the vibrations of shortest wave
length--register the sensation of violet, and the largest or longest
waves that of red. The whole sequence of colors runs in the order
violet, indigo, blue, green, yellow, orange and red (easily fixed
in the memory by means of the word VIBGYOR, formed from the initial
letters of these words).

Now the passage of sunlight through the atmosphere is obstructed to
a certain extent, not only by suspended dust particles, but also by
the molecules of the air. Let us consider, first, the effect of air
molecules and of the finest dust particles, not much above molecular
size. These tiny objects have different effects on light waves of
different lengths. The longest waves are little disturbed by them, just
as ordinary waves in water are little affected by a floating cork, for
instance. The shortest waves are so small in proportion to the size
of the obstacles that they are diffused or scattered by them, as a
tiny ripple in water might be broken up by a floating cork. It is this
diffuse light, of short wave length, that gives the sky its color. A
large part of the violet and indigo light is lost by further scattering
before it reaches the earth, leaving a preponderance of blue in the sky
as we see it. When the air contains a considerable amount of suspended
particles larger than those above considered--whether in the form of
solid dust or crystals of ice or tiny droplets of water--light of all
wave lengths is reflected by them, and the sky looks white or grayish.

On account of the action of atmospheric particles in filtering out the
shorter light waves, as just described, sunlight becomes relatively
rich in red and orange in passing through the air. When the sun is
high, the path of the sunbeams to the earth is short, and the color
of their light is but little affected. Near the time of sunrise and
sunset, however, sunlight comes to us through a much greater extent
of air, and the filtering process is much more effective. Hence the
sunshine is both enfeebled and reddened when the sun is near the
horizon. The diffuse light of the sky around the sun is filtered in the
same manner, and therefore is commonly red when the sun is low.

A gray sunset sky after a clear day is due to the presence of water
drops in the air, and indicates conditions favorable for rain,
since, unless the air were saturated to a considerable altitude, the
comparatively warm sunshine of the afternoon would favor evaporation
rather than condensation of moisture. A gray sunrise sky has, as
a general rule, just the opposite meaning. It often indicates the
presence in the air of water drops formed on dust particles during
the night, after the manner of dew, because the upper air has been
_dry_ enough to permit rapid radiation from the dust. These drops will
be speedily evaporated by the rising sun, and the general dryness of
the atmosphere will not favor further condensation. Several familiar
weather proverbs are thus justified, e. g.:

  Evening red and morning gray
  Help the traveler on his way;
  Evening gray and morning red
  Bring down rain upon his head.

There are many other interesting optical phenomena connected with
sunrise and sunset, including, first of all, the morning and evening
twilight. When the sun, or any other heavenly body, is only a little
below a clear horizon, it is still visible, on account of the
bending of its rays by the atmosphere. This lifting effect, known
as _astronomical refraction_, amounts to about half a degree (at
the horizon), which is about equivalent to the apparent diameter of
the sun or moon. As the sun sinks farther below the horizon, in the
evening, the only daylight that comes to us is that reflected from
the upper levels of the atmosphere, which are still illuminated. This
is called twilight, and it lasts until the sun is about 18 degrees
below the horizon, when total darkness sets in. The period as a whole
is sometimes called _astronomical twilight_, in distinction from the
briefer period known as _civil twilight_, during which there is light
enough for outdoor occupations; the latter lasts from sunset until the
sun is about 6 degrees below the horizon. Morning twilight is more
commonly called “dawn.”

An interesting succession of light and color effects is observed before
and after sunset and, in inverse position and order, about sunrise.
Considering sunset only: After the sun has sunk out of sight, a broad
band of golden light, called the _bright segment_, is seen along the
western horizon. Above this, in the western sky, appears a more or less
circular expanse of rosy glow, known as the _purple light_. In the
eastern heavens, after sunset, there rises steadily from the horizon
the so-called _dark segment_, which is the blue or ashy shadow of
the earth on the sky. This is bordered above by the pink or purplish
_antitwilight arch_. As time goes on, the purple light in the west,
after increasing in brightness for a while, finally sinks behind the
bright segment; while in the east the rising dark segment encroaches
upon and finally obliterates the antitwilight arch. Sometimes, in clear
weather, there is a fainter repetition of these lights and colors
(_second purple light_, etc.).

Among the Alps and other snow-capped mountains, these sunset and
sunrise phenomena assume a particularly beautiful form, known as the
_Alpenglow_. In fine weather, just before sunset, the peaks to the
eastward begin to show a reddish or golden hue. This fades gradually,
but in a few minutes, when the sun is a little below the horizon of
the observer, but the peaks themselves are still bathed in direct
sunlight, an intense red glow, beginning down the slopes, moves upward
to the summits. This is identical with the antitwilight arch described
above. Presently this glow is succeeded by an ashy tint, as the peaks
are immersed in the rising shadow of the earth (the dark segment).
Their rocks and snows assume a livid appearance, aptly described by
the inhabitants of Chamonix, whence the phenomena in question are
well seen on the summit of Mont Blanc, as the _teinte cadavéreuse_.
In ordinary weather darkness succeeds without any further notable
phenomena, but occasionally there occurs a remarkable renewal of rosy
light upon the peaks, known as the _recoloration_ or _afterglow_. At
Chamonix this is termed the “resurrection of Mont Blanc.” The afterglow
has been variously explained, but it is probably due, mainly at least,
to the reflection of the purple light in the western sky. Sometimes
it lasts until an hour after sunset, and it passes away from below
upward. On very rare occasions there is a second afterglow, presumably
the reflection of the second purple light mentioned above. Similar
phenomena are often seen in reverse order at sunrise.

A pretty phenomenon observed chiefly in the late afternoon and early
morning consists of beams of light radiating from the sun, known
technically as _crepuscular rays_. The beams are made visible by the
presence of abundant dust or water droplets in the atmosphere, and the
intervening dark spaces are the shadows of clouds. When the sun is
above the horizon and the beams are directed downward, the phenomenon
is popularly described as “the sun drawing water” and is regarded as a
sign of rain. Sailors call these beams the “backstays of the sun,” and
they have several other names based upon the legendry associated with
them in different parts of the world. After sunset or before sunrise
a fanlike sheaf of the beams often extends upward from the western or
eastern horizon, respectively. The Homeric expression “rosy-fingered
dawn” probably refers to this phenomenon. In all cases the apparent
divergence of these beams is an effect of perspective, as they are
really parallel. A rarer phenomenon is that of _anticrepuscular rays_,
which appear to converge to a point opposite the sun. In this case the
beams and shadows are projected entirely across the sky, but their
paths can very seldom be traced in the upper part of the heavens
because in this direction the observer’s line of sight passes through a
comparatively shallow extent of dusty atmosphere.

An analogous phenomenon is seen in the shadows which near-by isolated
mountain peaks frequently cast upon the sky opposite the sun at sunrise
and sunset. Travelers have described such shadows cast by Adam’s Peak
in Ceylon, Pike’s Peak in the Rocky Mountains, and Fujiyama in Japan.
The phenomenon is said to be especially striking in the polar regions,
where the air is often heavily charged with particles of ice.

One more optical phenomenon of sunrise and sunset that requires mention
here seems to be comparatively little known to the nonscientific
public, notwithstanding the fact that it has supplied the subject and
title of a diverting novel by Jules Verne. The conditions required
for its appearance are a clear and steady atmosphere and a sharply
defined horizon, such as that of the ocean. At the instant the sun is
appearing or disappearing, and when only a very small segment of its
disk is visible above the horizon, this portion appears to be colored
a bright emerald green, sometimes blending into blue. This transient
phenomenon is known as the _green flash_. It is best explained as due
to the different degrees of refraction undergone by rays of different
wave lengths coming to us from the sun. The effect of refraction in
elevating the solar image as a whole when near the horizon has already
been mentioned. This effect is a little greater for the green and blue
rays than for the orange and red. It is still more pronounced for the
violet and indigo rays, but these are mostly sifted out of the solar
beams in their long passage through the atmosphere when the sun is low.
Hence at the upper edge of the solar image there is a narrow green or
blue fringe, which is not, however, perceptible except when a screen
is interposed between the eye and the bright image of the sun. A sharp
horizon furnishes such a screen. Through a telescope it is possible,
in suitable weather, to see the green flash--and also a corresponding
“red flash” at the lower edge of the sun--by placing an opaque
diaphragm in the focal plane of the object glass. Another explanation
of the green flash--which could, however, account only for its
appearance at sunset and not at sunrise--is that it is a physiological
effect; the eye, fatigued by the reds and yellows that predominate in
the light of the setting sun, sees an “after image” of complementary
hue the instant after the real image has disappeared.

We now turn to a group of phenomena, also due to atmospheric
refraction, which includes some of the most bizarre of optical
illusions. The simplest of these phenomena consists of a slight
apparent elevation of all objects in the surrounding landscape through
_terrestrial refraction_, which is identical in principle with
astronomical refraction and depends upon the difference in density, and
hence of refractive power, of the air at different levels above the
earth.

Normally the air decreases in density at a nearly regular rate with
increasing altitude. Sometimes, however, this change in density is
greatly modified by local effects of temperature. Over a cold surface
of land or water the adjacent air may be abnormally dense, resulting
in an unusually rapid decrease of density with altitude. Over a hot
surface, as in the case of a desert under strong sunshine, the adjacent
air may become so much rarefied that, for a certain distance upward,
there is actually an increase of density with ascent, instead of the
reverse. The rays of light coming to us from distant objects are bent
in different directions and to various degrees by virtue of these
abnormalities in the density of the atmosphere. The apparent positions
of such objects depend upon the angle at which the light rays, coming
from them, strike the eye of the observer. Sometimes the objects appear
to be lifted far above their true positions (a phenomenon known as
_looming_) and sometimes depressed far below them; and occasionally
local irregularities in air density produce curiously distorted images
of these objects.

Most of these strange effects are known collectively as _mirage_. There
are many varieties. There is the “desert mirage,” first made famous
through the experience of Napoleon’s soldiers in Egypt. There are
mirages that suspend the images of remote objects in the sky; sometimes
inverted, sometimes right side up. There is the lateral mirage,
occasionally seen when one looks along the face of a heated wall or
cliff. Lastly, there are the complex displacements and distortions
of objects known as the _Fata Morgana_--a name originally applied to
a phenomenon of this kind visible, on rare occasions, at the Straits
of Messina, but now used generically for similar appearances in other
parts of the world. Some of the finest examples of Fata Morgana are
witnessed in the polar regions.

In the desert mirage an image of the lower part of the sky is brought
down to earth and simulates the appearance of water, while the images
of terrestrial objects, also depressed and inverted by the mirage,
look like the reflections of the same objects upon the liquid surface.
Humphreys says: “This type of mirage is very common on the west coast
of Great Salt Lake. Indeed, on approaching this lake from the west one
can often see the railway over which he has just passed apparently
disappearing beneath a shimmering surface. It is also common over
smooth-paved streets, provided one’s eyes are just above the street
level.” The confusing and obscuring effects of the desert mirage
were illustrated during the fighting between the British and Turks in
Mesopotamia, in April, 1917, when, according to the report of General
Maude, a battle had to be suspended on account of one of these optical
disturbances.

[Illustration: FATA MORGANA ON THE COAST OF GREENLAND

(_From drawing by Scoresby_)]

The strong vertical contrasts in air temperature that occur in the
polar regions produce many remarkable examples of mirage. The pictures
and descriptions of those observed a century ago along the coast of
Greenland by Captain William Scoresby, Jr., have become classical. A
recent episode connected with mirage was the expedition sent north in
1913 to explore “Crocker Land,” which Peary believed he had sighted
from an elevated point in Grant Land in 1906, and which for a time
figured on all maps of the Arctic. The later explorers found no land at
the place indicated, but they observed the same mirage that Peary had
mistaken for distant hills and mountains.

Currents of air of different densities produce, through their varying
effects on atmospheric refraction, the twinkling or scintillation of
the stars, as well as of distant terrestrial lights. Twinkling is much
more violent near the horizon than near the zenith, and more pronounced
on some nights than others. The shimmering of the air over heated
surfaces faces and the “boiling” of celestial objects as seen in the
telescope are analogous phenomena.

[Illustration: INFERIOR MIRAGE

(_From American Museum Journal. Drawing by Chester A. Reeds._)

Out-of-doors when a layer of warm rarefied air arises from contact
with heated ground or warm water, occupying a position below the
colder, more dense normal air, two images of a distant object may be
seen--one inverted beneath the other. This is “_inferior mirage_” and
is explanatory of the appearance of trees and their reflections, which
haunts the desert traveler with the hope of water.]

[Illustration: SUPERIOR MIRAGE

(_From American Museum Journal. Drawing by Chester A. Reeds._)

When a zone of warm rarefied air is sandwiched between normal air above
and colder air below, a “_superior mirage_” of distant objects may be
seen. Three images are produced, one above the other, the middle one
inverted.]

In the refraction phenomena that we have thus far considered the air
is the medium in which the light rays are bent and distorted. In the
production of the _rainbow_, light undergoes refraction, dispersion
(separation of the spectral colors) and reflection by passing through
drops of water in the atmosphere; especially falling raindrops.

The rainbow, perhaps because it is such a common sight, is seldom
observed with careful attention. Hence few people realize that there
are many varieties of this beautiful meteor, and various erroneous
ideas about it are prevalent. The rainbow is always seen in the part
of the sky opposite the sun--or the moon, in the case of the lunar
rainbow--and is high in the heavens when the luminary is low, and low
when the luminary is high. Generally less than a semicircle of the
bow is visible, and never more, except from an eminence. (Aeronauts
occasionally see a complete circle.) The outer border of the bow is
red and the inner blue or violet. Contrary to popular belief and to
statements sometimes found in reference books, it is almost never
possible to distinguish all seven of the spectral colors in a rainbow;
four or five is the usual limit.

The ordinary or _primary rainbow_ has a radius of about 42 degrees at
its outer edge. Very commonly a _secondary_ rainbow is seen, concentric
with the primary bow, and having a radius of about 50 degrees. The
secondary is fainter than the primary, and its colors are in opposite
order--red inside and violet outside. Additional bands of color,
chiefly red and green, may often be detected adjacent to the inner
edge of the primary bow and, less frequently, along the outer edge of
the secondary bow. These are known as _supernumerary bows_. The space
between the primary and secondary bows is somewhat darker than the rest
of the sky.

The common rainbows differ much among themselves in the number and
purity of their colors, the width of the bows, etc., these differences
depending especially on the size of the raindrops. The minute drops of
a fog sometimes give rise to a bow that is almost devoid of color--the
“white rainbow,” or “fog bow.” The rainbows produced by the moon
commonly show little color, on account of the relative faintness of
the light, but the brighter lunar rainbows are often very distinctly
colored.

[Illustration: INTERSECTING RAINBOWS

(_After a sketch by T. Hodge. Courtesy of Scientific American._)]

_Reflected rainbows_ are sometimes seen upon a sheet of water; and
again the image of the sun, as reflected by such a surface, may give
rise to both primary and secondary rainbows in the sky, which appear
to intersect those produced by the sun directly. A horizontal layer of
water drops below the level of the observer’s eye occasionally produces
the so-called _horizontal rainbow_. This may be formed over a bedewed
field or other surface (the “dew bow”); or the drops may be those of
a low-lying sheet of fog, or of water deposited on a floating film of
oil, or, finally, actual raindrops, seen from an elevation, such as the
summit of a mountain. Horizontal rainbows formed by rain have been seen
from the Eiffel Tower.

The common saying,

  A rainbow in the morning
  Is the shepherd’s warning;
  A rainbow at night
  Is the shepherd’s delight,

is, on the whole, well justified for the following reasons: We see the
rainbow where rain is falling, while the sun is shining in the opposite
part of the sky. Our rainstorms usually come from the west and pass
away to the east. A morning rainbow can only be seen in the west, and
indicates that rain is approaching us. An evening rainbow (ignoring
lunar bows) is seen only in the east, and shows that the rain area is
receding from us, giving place to clear skies.

[Illustration: THE CIRCUMZENITHAL ARC

(_From a drawing by L. Besson in La Nature._)

Parts of the halos of 22° and 46°, upper tangent arc of the 22° halo,
and two parhelia are also shown. The circumzenithal arc is always
brightly colored.]

Ice crystals in the atmosphere, such as those composing the higher
clouds, produce a great variety of optical phenomena, known as _halos_.
Some phenomena of this class are common, others exceedingly rare.
Moreover, there are several theoretically possible forms of halo
of which observations have never yet been reported, so that halo
observing can be recommended to the amateur meteorologist as offering
opportunities for making interesting discoveries.

Halos take the form of narrow rings of definite angular size around
the sun or moon (not to be confused with the coronas, of variable
dimensions, described below), rings passing through the luminary, arcs
in various other positions, and roundish spots of colored or white
light. They may be seen separately or in combination. In rare cases,
a dozen or more different forms of halo are visible at the same time,
producing a most spectacular display. One of the most remarkable
displays of this kind in the history of science was seen, in different
degrees of development, over the eastern United States on November 1
and 2, 1913; an event which greatly stimulated interest in the study
of halos in this country. Complex halos are quite common in the polar
regions; where they are seen not only in the sky, but also in the air,
charged with ice particles, close to the earth.

Whenever a thin veil of cirrus or cirro-stratus clouds overspreads the
sky there is a likelihood that halos will be visible. Those formed
near the sun, however, frequently pass unnoticed, on account of the
dazzling brightness of that luminary. Smoked or tinted glasses greatly
facilitate their observation.

[Illustration: DIAGRAMS OF THE PRINCIPAL FORMS OF HALO

(_After Besson, Monthly Weather Review, July, 1914._)

1. (Upper). Perspective view of the sky, showing the sun (_S_);
ordinary halo of 22° (_a_); great halo of 46° (_b_); upper tangent arc
of the halo of 22° (_c_); lower tangent arc of the halo of 22° (_d_);
ordinary parhelia of 22° (_e_, _e’_); Lowitz arcs (_f, f′_); parhelia
of 46° (_g, g′_); circumzenithal arc (_h_); infralateral tangent arcs
of the halo of 46° (_i_); the parhelic circle (_m_); a paranthelion of
90° (_q_); light pillar, (_u, u′_); the observer (_O_). 2. (Lower).
Perspective view of the sky, showing the observer (_O_); the parhelic
circle (_m_): ordinary paranthelia of 120° (_p_); the paranthelion
of 90° (_q’_); the oblique arcs of the anthelion (_r, r′_); and the
anthelion (_n_).]

The commonest halo is a circle of 22 degrees radius (the _22-degree
halo_) about the sun or moon. When formed by the sun it generally
shows a distinct reddish inner border and traces of other spectral
colors. The lunar 22-degree halo usually appears colorless. This halo
is visible, in whole or in part, to the attentive observer about once
in three days, on an average. Less common, but by no means rare, are
the _parhelia_ or “sun dogs” of 22 degrees (called _paraselenæ_ or
“moon dogs” when formed by the moon), the beautiful _circumzenithal
arc_, and a few other members of the halo family. Most forms of halo
are so uncommon that their appearance is an event of some scientific
importance.

The accompanying diagrams, by Dr. Louis Besson of the Observatoire de
Montsouris, show the positions, with respect to the sun (or moon), of
the majority of known halo phenomena. The upper diagram shows the halos
that occur on the same side of the sky as the sun (or moon), and the
lower those that appear on the opposite side. Most of these halos, when
bright, show the spectral colors. The circumzenithal arc, _h_ (commonly
described, by the uninitiated, as a “rainbow”), and the parhelia of 22
degrees, _e, e′_, are especially brilliant in their coloration. The
_parhelic circle, m_, which sometimes extends entirely around the sky,
is white, and so are a few of the rarer forms of halo.

The _upper_ and _lower tangent arcs of the halo of 22 degrees_, _c_ and
_d_, undergo striking alterations, with changes in the altitude of the
sun. When the luminary is more than about 40 degrees above the horizon,
these two arcs become joined at their tips to form the _circumscribed
halo_, and at still greater solar altitudes this halo contracts from an
elliptical to a circular form, thus blending into the 22-degree halo
as shown on the next page, where the solar altitudes corresponding to
the different forms of the halo are indicated. The positions of the
parhelia of 22 degrees, _e, e′_, also depend upon solar altitude. When
the sun is on the horizon these “sun dogs” are 22 degrees from the
luminary, and therefore lie in the 22-degree halo; at greater solar
altitudes they lie outside this halo.

The reader who wishes to acquaint himself further with the
different forms of halo and the methods of observing them will find
a comprehensive article on the subject (devoid of mathematical
discussions) in the “Monthly Weather Review” (Washington, D. C.) for
July, 1914.

[Illustration: SUCCESSIVE STAGES OF THE UPPER AND LOWER TANGENT ARCS OF
THE 22° HALO

When the sun is high they unite to form the “circumscribed halo.”
(Altitude of sun shown in the center of each figure.)]

The ice crystals that produce halos consist of hexagonal plates or
columns, occasionally including complications of structure, such as
pyramidal bases, combinations of plates and columns, etc. These have
the well-known effect of prisms in refracting and dispersing light
that passes through them. It is evident that there are many possible
paths for the light rays through the sides and bases of such crystals,
resulting in different deflections and corresponding differences in the
forms and positions of the halos produced. The attitudes assumed by the
crystals as they slowly sink through the air, and the oscillations
they undergo, are further points to be considered in working out the
theory of each form of halo by the application of the laws of optics.
Nearly all the known forms have been fully explained. A few species of
halo--notably the parhelic circle (called _paraselenic circle_ when
formed by the moon)--are due to simple reflection from the faces of the
ice crystals, and not to refraction.

The last group of optical phenomena that we shall consider consists of
those due to the process called _diffraction_, which occurs when light
is bent around objects in its path, instead of passing through them, as
in refraction. The process involves separation of the prismatic colors.
The diffraction phenomena of the atmosphere are produced by the water
drops of clouds and fog, or sometimes by fine dust.

Everybody is familiar with the nocturnal spectacle which Tennyson
describes as

  ... the tender amber round
  Which the moon about her spreadeth,
  Moving thro’ a fleecy night.

This diffuse reddish or rainbow-tinted circle is called a _corona_.
It occurs about the sun as well as the moon (though not easy to see
on account of the glaring brightness of the luminary), and also about
street lamps and other terrestrial lights when viewed through a misty
atmosphere. Unlike the halos, it has no definite angular size. It is
usually only a few degrees in radius. Small coronas are produced by
large water drops and large coronas by small drops, while the largest
of all coronas, known as _Bishop’s ring_, is due to exceedingly
fine dust in the atmosphere, and has been seen after great volcanic
eruptions.

In its commonest form the corona consists of a brownish-red ring,
which, together with the bluish-white inner field between the ring
and the luminary, forms the so-called _aureole_. If other colors are
distinguishable, they follow the brownish red of the aureole (in the
direction away from the luminary) in the order from violet to red; the
reverse of the order seen in halos. Sometimes the sequence of colors is
repeated three or four times.

Patches and fringes of iridescence are sometimes seen in the clouds at
a greater distance from the luminary than that of the ordinary corona.
Probably they are fragments of coronas of unusual size produced by
exceedingly fine cloud particles.

Similar in appearance to the corona is the _glory_; a series of
concentric colored rings seen around the shadow of the observer, or
of his head only, cast upon a cloud or fog bank. Such a shadow, with
or without the glory, constitutes the _specter of the Brocken_, often
seen from mountain tops and from aircraft. The colored circles are
sometimes called _Ulloa’s rings_, from the name of a Spanish _savant_
who observed the phenomenon among the mountains of South America in the
eighteenth century and has left us a vivid description of it.

The Brocken specter, though it owes its name to legends associated
with the famous German mountain where witches were once believed to
assemble on Walpurgis Night, is actually less frequently witnessed
there than in many other parts of the world. Whenever the sun is low
on one side of a mountain and a wall of mist arises from a near-by
valley on the other, the mountaineer is likely to see his shadow upon
the mist. If the latter consists of fine droplets of approximately
uniform size, the colored rings will probably appear, and occasionally
there is also a white fogbow outside of the glory. As all shadows cast
by the sun taper rapidly (on account of the angular breadth of the
solar disk), a well-defined Brocken specter can never be more than a
few yards away from the observer. Its distance is, however, commonly
overestimated--some observers have supposed it to be miles away!--and
hence the erroneous idea prevails that the specters are of enormous
size.

Rarely from a favorable point of vantage on a mountain, and very
frequently from aircraft, the specter, instead of being seen on a
vertical wall of mist when the sun is low, appears on a horizontal
sheet of cloud below the observer when the sun is high. The aeronaut
may thus observe the complete outline of his balloon or aeroplane,
encircled with the rainbow tints of the glory. During the World War the
appearance of the luminous rings was likened to the emblem painted on
the wings of the Allied aeroplanes and was regarded by superstitious
aviators as an omen favorable to their cause.

The glory is due to the light that is reflected back to the observer
after penetrating the cloud or fog a little way and is diffracted by
the superficial layer of drops in emerging.

The Brocken specter and the glory have occasionally been photographed.



CHAPTER XI

ATMOSPHERIC ACOUSTICS


Though air is but one of an unlimited number of elastic substances that
transmit sound, it is the one through which sounds ordinarily reach
our ears. Hence the acoustic properties of the atmosphere are of great
interest to mankind.

Science deals with several kinds of “waves,” and those of the
atmosphere that produce the sensation of sound are quite different
from the waves of the sea. In quiet unconfined air sound travels in
concentric spherical waves, consisting of successive condensations
and rarefactions of the medium. Sound is not transmitted through a
vacuum. A familiar laboratory experiment is to install an electric bell
inside the receiver of an air pump and notice the dying away of the
sound as the air is exhausted. The colossal eruptions that astronomers
witness on the surface of the sun would probably be audible on earth if
interplanetary space were filled with air. In the rarefied air of high
mountains the intensity of sounds is much reduced. Thus we are told
that on the top of Mont Blanc the report of a pistol sounds no louder
than that of a firecracker at sea level.

The speed with which sound travels through air depends upon the
temperature. At 32° F. (the freezing point) it is 1,087 feet per
second, and at 68° F. it is 1,126 feet per second. The increase of
speed with increase of air temperature is very close to 2 feet per
degree Centigrade, or a little more than 1 foot per degree Fahrenheit.
The sounds of violent explosions travel considerably faster than
ordinary sounds near the place of explosion, but slow down to the
normal speed at greater distances. This is true of heavy claps of
thunder. The effect is not important enough, however, to invalidate
the well-known rule that, if you count seconds between the flash and
the detonation and divide the result by 5, you get approximately the
distance of the source of sound in miles.

Since the speed of sound varies with the temperature of the air,
differences in the latter cause deviations of the paths of sound waves
similar to the deviations which rays of light undergo on account of
differences in the density of the air. Sound is also reflected by
obstacles, in the same manner as light. Moreover, whereas light travels
too swiftly to be affected by the wind, this is not true of sound. The
latter travels faster with the wind than against it, and sound waves
are more or less broken up by the gusts and irregularities that are a
feature of most winds near the earth’s surface. For all these reasons
the acoustic qualities of the air are subject to marked variations, as
everybody has observed.

Unusual audibility of distant sounds is a popular prognostic of
rain. The fact underlying this belief is that when the air is full
of moisture it is likely to be of uniform temperature, and therefore
favorable for transmitting sound.

It is impossible to assign any limit to the distance at which loud
sounds may occasionally be heard. No fact of nature has yet, so far
as we know, matched Emerson’s metaphor of the “shot heard round the
world,” but it is literally true that the sounds of eruption of
Krakatoa, in August, 1883, were heard, like the roar of distant heavy
guns, in the island of Rodriguez, in the Indian Ocean, 3,000 miles from
the volcano. Moreover, the atmospheric waves set up by this outburst
actually made the circuit of the globe, not only once, but at least
three times, and the successive journeys were registered by barometers,
if not detected by human ears. During the World War gun firing in
Flanders was very commonly heard at places in England 140 to 150 miles
distant. Several observers also reported that pheasants appeared, from
their disturbed behavior, to hear cannonading over the North Sea that
was beyond the range of the human ear.

Writers have often commented on the fact that thunder cannot be heard
so far as the sounds of artillery. It has been affirmed that 10 miles
or thereabouts is its maximum range of audibility. As a matter of fact,
however, thunder has occasionally been heard at much greater distances,
up to 20 or 30 miles; but it remains true that the distance is always
much less than that at which loud terrestrial sounds are audible. The
reasons why this should be so are not far to seek. In the first place,
the intensity of a sound depends upon the density of the air in which
it is generated, and not upon that of the air in which it is heard. The
air, as we know, diminishes in density upward. Balloonists thousands
of feet above the earth hear with remarkable clearness sounds from
the ground below, but people on the ground cannot hear similar sounds
from the balloon. As thunder is mainly produced at the level of the
clouds, it is subject to this peculiarity. Again, cannonading is heard
at great distances only when the air is comparatively calm, and perhaps
only when it is arranged in well-defined horizontal layers of such a
character as to keep the sound from spreading far aloft. Very different
conditions prevail during a thunderstorm; in fact the conditions are
then just such as would scatter and dissipate the sound waves. Lastly,
the noise of a cannon or the like comes from a single place and the
energy of the disturbance is concentrated to produce a single system of
sound waves; while the disturbance due to lightning is spread over the
long path of the discharge.

The audibility of sounds at abnormally great distances is not usually
a matter of practical importance, but the converse phenomenon--the
failure of sounds to carry to normal distances--has been responsible
for a great number of marine disasters on such fog-ridden coasts as
those of the British Isles, eastern Canada and California. Hence some
of the ablest physicists of both the Old World and the New have tried
to ascertain the conditions under which this phenomenon occurs.

The scientific study of fog signals, dating especially from Tyndall’s
well-known investigations at the South Foreland, in England, in 1873,
and those of General Duane and Professor Joseph Henry in America,
begun somewhat earlier but continued contemporaneously with Tyndall’s,
has probably raised more questions than it has answered. The caprices
of these signals take the shape of variations in the range of
audibility--a signal may at one time carry 10 miles and at another
only 2--and the formation of “zones of silence,” comparatively near
the signal, within which the sound is not heard though audible at
much greater distances. The silent zones are sometimes more or less
permanent and are then generally due to peculiarities of topography;
but in many cases they are transient and opinions differ as to their
cause or causes. Since it is only when fog prevails that fog signals
are sounded (except for experimental purposes), and that vessels meet
with accidents on account of the failure to hear such signals, the idea
has become rooted in the public mind that these acoustic eccentricities
are entirely due to fog. When, however, experiments are made in clear
weather, similar phenomena are observed.

In foggy weather audibility is often better than the average, because
fog prevails chiefly when the air is still and of uniform temperature,
and such conditions favor the transmission of sound. Tyndall strongly
denied that either fog or falling rain, snow, and hail have, as has
been commonly believed, a muffling effect on sound, and he attributed
the peculiar behavior of fog signals to the presence in the atmosphere
of invisible “acoustic clouds,” consisting of patches of air containing
irregularities of temperature and humidity. To the same cause he
ascribed the occurrence of mysterious “aerial echoes,” not due to
any visible object. Several recent investigators have disputed these
conclusions. Thus it is asserted that when the fog signal is in fog
and the observer in a clear atmosphere, or _vice versa_, or when the
signal and the observer are in different fog banks, the fog reflects
the sound very strongly. Apart from the possible effects of fog itself,
the very extensive investigations made by Prof. L. V. King, of McGill
University, at Father Point, Quebec, led him to conclude that the
effects are chiefly due to eddies in the atmosphere. Prof. King used in
his observations the latest devices for obtaining exact measurements
of sound and such up-to-date meteorological apparatus as pilot balloons
for measuring the wind at various levels. He discovered, among other
things, that existing types of fog-signal machinery are very wasteful
of energy, and he has pointed out how their “acoustic efficiency” may
be much improved. Before we dismiss this subject it should be noted
that submarine bells and the radio compass have made mariners much less
dependent than they formerly were upon the types of signals that are
affected by meteorological conditions.

“Zones of silence” on a much more extensive scale than those that
disturb the operation of fog signals have been frequently observed, in
recent years, in connection with great explosions, cannonading, and
volcanic eruptions. The first case of this kind to attract scientific
notice was that of a dynamite explosion at Förde, Westphalia, on
December 14, 1903, the acoustic phenomena of which were investigated
by Dr. G. von dem Borne; and among the many cases that have since
been studied was that of the bombardment of Antwerp in October, 1914.
Without describing these various cases separately, we may state that
when reports were collected from the surrounding country to determine
the places at which the sounds were audible and these reports were
entered on a map, it was found that there was a large and usually
very irregular area of audibility surrounding the source of sound,
beyond which lay a broad, more or less circular zone of inaudibility,
and finally, beginning about 100 miles from the source, there was a
second large region of audibility, extending perhaps 150 miles from
the source. In some cases a single sound at the source gave multiple
reports (double, triple, or quadruple), chiefly in the outer zone of
audibility.

In his attempt to explain these curious silent zones, Von dem Borne
pointed out that the atmosphere at very high levels is supposed to
consist mainly of hydrogen, in which sound travels nearly four times
as fast as in the common gases of the lower air, and that sound
waves ascending to such heights along a slanting course would be
bent strongly toward the earth. Another student of this phenomenon,
Dr. A. Wegener, who is the champion of the idea that the atmosphere
contains an unknown gas lighter than hydrogen (called “geocoronium”
or “zodiacon”), sees in the prevalence of this gas at high levels
the cause of a similar quasi-reflection of sound waves. Probably the
majority of investigators, however, believe that the effect is due
chiefly or entirely to the refraction of sound by wind.

Of acoustic phenomena that belong especially to the domain of
meteorology, probably thunder is the one that excites most general
interest. The sudden expansion of the air along the path of a lightning
discharge, due partly but probably not entirely to the heat generated,
appears to be an adequate explanation of the explosive sound of
thunder, though somewhat different explanations have been suggested. If
the discharge is near at hand, we generally hear a single loud crash.
More distant lightning is usually attended by rumbling. The common and
obvious explanation of rumbling is that it is due to the arrival of the
sound progressively from different points along the path of discharge,
which may be a mile or more in length. A crooked path would account
for reenforcements and diminutions of the sound. Another cause of
irregularities in the sound is probably “interference” (combinations
of waves that tend either to strengthen or to neutralize each other),
especially in the case of multiple lightning discharges, such as we
have described elsewhere. Lastly, thunder is further complicated by
echoes from the ground and probably also from the air (not exclusively
from clouds), though much uncertainty prevails concerning these aerial
echoes. The sounds of thunder have been the subject of some interesting
investigations on the part of an Austrian meteorologist, Dr. Wilhelm
Schmidt, who has devised apparatus for making an automatic registration
of the sound waves that constitute a thunderclap. He finds that there
is a great preponderance of waves of very long period, including many
of too low a pitch to be audible, though perceptible through the
rattling of windowpanes, etc. In fact, the greater part of the energy
involved is represented by these long, inaudible waves, so that one
really _hears_ only a small part of a clap of thunder.

The statement has often been made, on the authority of Humboldt, that
thunder is never heard at sea, at any point far from land. This matter
was investigated by the magnetic survey yacht _Carnegie_ during a long
cruise in the Pacific in 1915. Of twenty-two displays of lightning, six
were accompanied by thunder.

The late war gave prominence to certain acoustic phenomena which,
though hardly mysterious, were novel to the world at large. One of
those was the double report (triple in the case of an exploding shell)
heard near the line of fire of large guns. This effect is due to the
fact that modern projectiles travel much faster than sound. The moving
projectile sets up its own waves in the air, like those at the bow
of a steamer, which may reach the ear of the observer and produce the
sensation of a sharp sound before he hears the sound coming from the
mouth of the gun. Another phenomenon frequently observed when heavy
firing was in progress was the appearance in the sky of rapidly moving
parallel arcs of light and shade. These were generally seen against
clouds, but sometimes they swept across blue sky. They probably
occurred only in calm weather. These arcs were the result of the
successive condensations and rarefactions of the air constituting waves
of sound--visible sound waves. Their visibility was due to contrasts in
the refraction of light passing through air of different densities; the
same sort of refraction contrasts that cause the tremulous appearance
of the air over a hot stove, for example. The same “flashing arcs” of
light have been described by Prof. F. A. Perret as attending explosive
volcanic outbursts at the craters of Vesuvius and Ætna.

The humming of telegraph wires has been the subject of a certain
amount of discussion in meteorological circles, but without altogether
satisfactory results. This sound is not, of course, caused or affected
by the electric currents passing along the wire, and it is almost
certainly due solely to the wind, though the suggestion has been
made that it might be caused by the microseisms, or small and rapid
earthquake tremors, that are so commonly registered by seismographs
while imperceptible to the human senses. The humming is best heard
when one’s ear is placed against a telegraph pole. Several persons
have made systematic observations of these sounds from day to day,
and it has often been alleged that they vary with the temperature,
the movements of storms, etc., and even constitute a safe basis for
weather predictions. They are sometimes heard when the air appears to
be perfectly calm, but in such cases there might be some movement of
the air at the level of the wires, though there was none at the lower
level of the observer. From what is known about “æolian tones” (such
as those of the æolian harp), it would seem that the humming requires
a wind more or less at right angles to the wire, and that the pitch of
the sound depends upon the force of the wind and the diameter (but not
the length or tension) of the wire. For a given wire, the stronger the
wind the higher the pitch of its sound.

Of all the sounds that haunt the air, probably the most mysterious are
those which are best called by the generic name “brontides” (coined,
in the Italian form _brontidi_, by Prof. Tito Alippi from two Greek
words meaning “like thunder”), though they rejoice in scores of other
names in various parts of the world. Brontides take the form of
muffled detonations, resembling the sound of distant cannon or peals
of thunder, and are heard chiefly in warm, clear weather. The first
systematic investigations of these phenomena were made in India. The
fact that they were frequently reported from the neighborhood of
Barisal, a town in the Ganges delta, led to their being called “Barisal
guns,” under which name they were first made known to European science
in 1890. A few years later they were discussed in an extensive memoir
by E. van den Broeck, who had collected numerous reports of their
occurrence in Belgium, especially on the seacoast, where they are
known as “mistpoeffers” (i. e., “fog belchings” or “fog hiccups”).
The majority of descriptions, however, have come from Italy, where
the sounds appear to be extremely common, though peculiar to certain
localities, and where they bear a great variety of names. In Australia
the noises are called “desert sounds,” in Haiti, “gouffre,” etc.
They have been reported from parts of the United States, including
California and, above all, from the vicinity of Moodus, Conn., which
owes its original Indian name, Morehemoodus (“place of noises”), to the
brontides which appear to have formerly been much more common there
than they are to-day. There is a reference in one of Lord Bacon’s
works to “an extraordinary noise in the sky when there is no thunder”;
apparently a description of brontides.

The source of these sounds is undoubtedly subterranean in a great
many cases, though perhaps not in all. Prof. W. H. Hobbs, who has
made a painstaking study of the seismic geology of Italy, concludes
that the brontides of that country are due to the slow settling of
the blocks of the earth’s crust; a process which, in its more abrupt
and violent phases, causes definite earthquakes. Alippi believes that
in order that the sounds may be heard they must be reenforced by a
peculiar configuration of the ground, above or below the surface, and
he attaches special importance to the effects of caverns, which he
suggests act as resonance boxes in the production of audible brontides.
Occasionally an apparent brontide may be due to the explosion of an
unseen meteor. Lastly, a certain proportion of these thunderlike
sounds, if not merely distant thunder, may be such noises as
cannonading, blasting, or the like, made audible at unusual distances
by the refraction of sound waves.



CHAPTER XII

CLIMATE AND CLIMATES


Some day the meteorologists of the world will join forces to produce
a great encyclopædia of climate. No work of science is more sorely
needed, but the magnitude that it would, ideally, assume is simply
staggering.

Few people realize the multiplicity and complexity of climates. It
is a common occurrence for a prospective traveler or a business man
to write to a meteorological establishment requesting, for example,
a description of the climate of South America. Of course, no such
thing exists. A continent does not have a climate, but a multitude
of climates. Even to set forth, in general terms, the more important
types of climate that prevail between Cape Horn and Panama is no small
undertaking. Moreover, general descriptions often fail to supply the
needs of those who make inquiries about climate.

Suppose, instead of the wholesale order above mentioned, the
meteorologist receives the relatively modest request to describe
the climate of Buenos Aires or Rio de Janeiro. Is it easy to comply
with such a request? That depends. If the information is sought by
a tourist who wishes chiefly to know whether he will need light or
heavy clothing at a specified season, or whether his excursions are
likely to be hampered by frequent rains, we can enlighten him in a
few brief paragraphs. If the inquiry comes from a manufacturer who
aspires to invade the South American markets, we must know, before
replying, what kind of goods he purposes to export, and just how they
are affected by climatic conditions. Are they liable to injury by high
or low temperatures, dryness or humidity? Does the demand for them
depend, as in the case of rubber coats, upon the prevalence of rain,
or, as in the case of electric fans, upon the occurrence of hot weather
during at least a part of the year? For each branch of the export trade
certain elements of climate are important, and the more detailed and
explicit the information that can be obtained about them the better.
Suppose, again, climatic data are desired by a horticulturist who has
to solve the problem of introducing a South American plant into the
United States. In order to find the best environment for it in this
country, he should know something about the climate of its original
home. The data he requires are, however, different from those sought by
the tourist or the manufacturer. Is the plant’s habitat a region where
frosts occur? How long is the growing season? Is the rainfall rather
evenly distributed over the year, or are there definite dry and rainy
seasons? Such are some of the questions he will ask. For the purposes
of medical climatology a different set of data will be sought. The
astronomer, selecting a site for a new observatory, will ask about
freedom from clouds, and also about the pureness and steadiness of the
air that insure good “seeing.” The aviator will want information about
winds and fog. And so on.

Thus it appears that climate means very different things to different
people.

Climate has been variously defined as the sum total of weather, average
weather, typical weather, etc., but the conception is still somewhat
indefinite. We know that, while the weather of any place is subject
to incessant changes, its climate persists; but we need not assume
that it persists indefinitely. The geological record proves, on the
contrary, that vast changes of climate have occurred in the course of
long ages. In Antarctica and in Spitzbergen are found deposits of coal,
constituting the débris of ancient forests such as could not exist in
the climates now prevailing in those regions. There are plenty of other
proofs that great climatic changes have taken place from one geological
period to another; but what of changes in shorter intervals of time?

An immense amount of zeal and energy has been devoted to the study of
supposed changes of climate. Evidence of such changes is sought, on the
one hand, in a painstaking examination of weather records (a process
often involving the tabulation of hundreds of thousands of figures),
and, on the other, in the collection of geographical and historical
data bearing on the question. There have been numerous reports of the
gradual drying up of African and Asiatic lakes, of the discovery of
ancient ruins indicating that prosperous agricultural communities once
flourished in regions that are now deserts, and of various other tokens
that marked vicissitudes of climate have occurred within historic
times. A recent ingenious method of studying climatic variations is
to measure the successive annual rings seen in cross sections of old
trees. Thick rings are supposed to have been formed during periods of
abundant rainfall, and thin rings when the rainfall was deficient. This
method has been applied to the giant _Sequoias_ of California, some of
which are more than 3,000 years old.

The net result of a wide range of investigations appears to be that,
on the whole, climate has everywhere been remarkably constant since
the dawn of human history. There is much evidence that, in certain
regions, there have been alternate increases and decreases--recurrent
oscillations--of temperature, rainfall, etc., but there is little
evidence of progressive changes in one direction.

In contrast to the uncertainty that still prevails in the scientific
world on the subject of climatic changes is the confidence with which
the average layman may be heard to assert that such changes have taken
place within his own recollection. The popular idea that climate has
changed perceptibly within a single human lifetime is a world-wide
delusion, and one that has, apparently, always flourished. In the
United States we hear of the “old-fashioned winter,” with its unlimited
sleighing, and also of a marked increase or falling off in the rainfall
in certain districts. It is an interesting fact that a century and more
ago Americans were indulging in the same sort of retrospections.

In the year 1770, when Benjamin Franklin was president of the American
Philosophical Society of Philadelphia, a paper was read before that
society entitled: “An Attempt to account for the Change of Climate
which has been Observed in the Middle Colonies of North America.” It is
published in the first volume of the society’s Transactions. Barring
the long _s_’s and the use of the word “colonies,” the greater part of
it might have been addressed to the owners of automobiles and Liberty
bonds. We are told of a “very observable change of climate,” remarked
by everybody who has resided long in Pennsylvania and the neighboring
colonies. “Our winters,” says the author, “are not so intensely cold,
nor are our summers so disagreeably warm as they have been.” These
changes he ascribes to the clearing and cultivation of the country.

Another firm believer in old-fashioned winters and old-fashioned
summers was Thomas Jefferson. In his “Notes on Virginia,” written in
1781, he says:

“A change in our climate is taking place very sensibly. Both heats and
colds are become much more moderate, within the memory even of the
middle-aged. Snows are less frequent and less deep. They do not often
lie, below the mountains, more than one, two or three days, and very
rarely a week. They are remembered to have been formerly frequent,
deep, and of long continuance. The elderly inform me, the earth used to
be covered with snow about three months in every winter.”

Samuel Williams, who published a “History of Vermont” in 1794, uses
almost identical language in reference to the climate of that State.
“Snows,” he says, “are neither so frequent, deep, or of so long
continuance as they were formerly; and they are yet declining very fast
in their number, quantity, and duration.” That these changes, he adds,
“are much connected with and greatly accelerated by the cultivation of
the country cannot be doubted.”

What are the facts? When the statements above quoted were written few
regular records of the weather had been maintained for any length of
time in this country. The earliest instrumental record was begun at
Charleston, S. C., in 1730. Much information was, however, available
concerning the dates of harvest, of the formation and breaking up
of ice in rivers and harbors, and other events dependent upon the
weather, which, if anybody had taken the trouble to collect and analyze
it, would have dispelled the universal belief that marked changes of
climate had recently taken place. Nowadays it is much easier to refute
the common assertion that the climate has changed within the memory
of living men. The meteorological history of our country for more
than three-quarters of a century has been recorded from day to day by
a host of careful observers in every State of the Union. The records
show that, while the weather of one year has often differed strikingly
from that of the next, there has been no real change in climate.
“Old-fashioned” winters, for example, were neither more nor less common
half a century ago than they are to-day.

Our memories of past weather mislead us, chiefly because we remember
the exceptional weather and forget that which commonly prevailed.
Other circumstances may contribute to the illusion. Thus many people
who now live in cities, where modern appliances make them more or less
independent of the weather, passed their childhood under the more
primitive conditions of the country.

If climates were not fairly constant for long periods of years, it
would be a waste of time to compile the climatic statistics that, as
we have seen, are wanted by so many different kinds of people for so
many different purposes. Such statistics are based upon past events,
but their practical value depends upon the fact that, within certain
limits, they are a safe guide to the future.

The climatic data for any place are a sort of digest of the
meteorological observations that have been made there, special
emphasis being given to those features of the meteorological record
that bear important relations to the life and activities of mankind.
Temperature and rainfall are the leading elements of climate; others
are wind, humidity, evaporation, cloudiness, etc. We have not space
to enumerate here all the kinds of data found in elaborate climatic
tables; but in order to illustrate how the records of a meteorological
station are utilized in compiling climatic statistics and to show what
complications may arise in this process, we shall consider the question
of temperature alone.

The instruments used in measuring temperature have been described
in another chapter. From these instruments are obtained the current
temperature of the air, the wet bulb temperature (used to compute
the humidity), and the maximum and minimum temperatures of the day.
Readings are made at fixed hours, known as “term hours.” At regular
stations of the United States Weather Bureau the term hours for the
observation of all the meteorological elements are 8 a. m. and 8
p. m., Eastern Time, and an observation of temperature, humidity, and
clouds is made at noon. In most other countries tri-daily readings
have been the rule, though in Europe four or more observations a
day are now taken at many stations in order to supply the frequent
weather bulletins required by aeronauts. Important stations are
generally equipped with thermographs, which make a continuous record of
temperature.

Theoretically, the mean temperature of any day is the average of 24
hourly observations, from midnight to midnight. In practice, the mean
is generally computed from the observations at the term hours, or from
the maximum and minimum. Having obtained the mean daily temperature
for each day of a month, the average of these values gives us the
mean monthly temperature. The average of the mean temperatures for the
twelve months of the year is the mean annual temperature.

These data for each day and month, and for the year--sometimes also for
other intervals, such as five-day periods, or “pentads,”--are computed
year after year, and eventually the values for all the years of the
record are averaged to form what are called “normals.” We thus obtain,
for example, for a given station, the normal temperature for January
21, the normal temperature for the month of March, the normal annual
temperature, etc.

All this is a mere beginning toward the complete discussion of a
body of temperature observations for the purposes of climatology. We
have still to obtain from the readings of the maximum and minimum
thermometers the normal maximum and minimum temperatures and range
of temperature for each day, each month, and the year; also the
“absolute” maxima, minima, and ranges (i. e., the extreme values that
have occurred during the entire record) for corresponding intervals of
time. These data furnish answers to such questions as: What was the
lowest temperature ever recorded on January 21? What is the lowest on
an _average_ January 21? What is the average range of temperature in
March? What was the highest temperature ever recorded, on any day, at
the station?

Having thus disposed of the extremes and ranges, we may compute
what is called the “variability” of temperature, i. e., the average
difference between the means of two successive days in a given month,
and the corresponding average for the entire year. These data are of
considerable importance in medical climatology. We may also compute
the frequency of occurrence of various values among the temperature
data above enumerated. The most frequent value is often quite different
from the average value. Many climatologists compute the number of
days, in an average year, on which the temperature rises to 77 degrees
(Fahr.) or above (“summer-days”), and the number of days on which the
temperature does not rise above the freezing point (“winter-days”).
Especially valuable in agricultural regions are data of the average
and extreme dates of the last frost in spring and of the first frost
in autumn. These define the length of the “growing season.” Statistics
of the temperature of the ground at the surface and at various depths
below the surface are also of agricultural interest.

From the foregoing outline it will be seen that a bewildering variety
of climatic statistics may be computed merely from observations of
temperature, and the same is true of the other elements. Moreover, the
list set forth above is by no means exhaustive even for temperature.
In fact, there is almost no limit to the number of ways in which
the raw material of climatic data--i. e., the original records of
observation--may be grouped, averaged, or otherwise treated in order to
bring out certain features of the climate that may conceivably serve
some useful purpose. The reader will now be able to understand why a
treatise on the climate of a single locality often fills a substantial
volume.

The numerical data contained in such a work are generally supplemented
by text descriptions and by various graphic devices, such as curves
showing the normal fluctuation or “march” of a weather element during
a day, year, or other interval. Works which deal with the climates
of larger areas, such as whole countries, are usually accompanied by
climatic charts. These charts furnish a quick and easy way of getting
a general idea of the climate of a region. Among the more important
climatic charts are the following:

1. _Temperature (isothermal) charts._ These include charts showing
the distribution of normal temperatures for months, seasons, and the
year; normal range of temperature for similar periods; highest and
lowest temperatures ever recorded at the different stations; etc. Lines
known as _isotherms_ are so drawn as to pass through places having
identical values of the element in question (mean temperature, highest
temperature, etc.).

2. _Rainfall (isohyetal) charts._ These show the distribution of
rainfall (including snowfall, expressed in equivalent depth of water);
especially for each month and for the year. Other charts may show the
average number of rainy days; average snowfall (actual depth, not water
equivalent); seasonal distribution of rainfall; etc.

3. _Wind charts._ These are drawn in various forms, to show the
prevailing wind directions, the frequency of winds from different
directions, the average force of the winds, etc. Charts of the winds at
different levels above the earth’s surface will eventually be drawn for
the use of aeronauts, but such charts are still in a tentative stage.

[Illustration: A MODERN ISOTHERMAL CHART OF THE GLOBE. (_Hann, 1901_)
The isotherms show the mean annual temperature in centigrade degrees.]

As in the case of tabulated climatic data, the number of charts
that might be drawn to bring out different features of climate is
practically unlimited. Sunshine, cloudiness, humidity, barometric
pressure, and the frequency of various special phenomena, such as
thunderstorms, hail, tornadoes, droughts, etc., are all charted in
some of the more extensive works on climate. Large atlases have been
published to portray the climates of certain countries. The study of
the actual distribution of climates over the earth, as distinguished
from that of climate in general, is sometimes called _climatography_.

Climates are variously classified, usually on the basis of one or
more of the climatic elements, but sometimes with reference to their
effects. The most familiar classifications refer to temperature. We
speak of tropical, temperate, and polar climates; but in using these
terms it should not be forgotten that other things besides latitude
control the distribution of temperature. Location with respect to the
ocean or other large bodies of water is almost equally important. A
land surface grows warm by day and in summer, and grows cold by night
and in winter, much more rapidly than a water surface, and the adjacent
air varies in temperature accordingly. Hence we have a classification
of climates as _marine_ and _continental_. The former, under the
influence of oceanic winds, have a moderate range of temperature, while
the latter are subject to extremes of heat and cold. With increase
of altitude temperature is diminished, but rainfall is generally
increased. The distribution of rainfall is also determined to a great
extent by the paths of cyclonic storms. Such are a few of the many
things that control the complex distribution of climates.

People who never travel far from their own homes usually cherish quite
erroneous ideas regarding the climates of distant lands. It is hard for
most Americans to realize, for example, that the Isthmus of Panama,
in the heart of the tropics, never experiences temperatures nearly so
high as those which occur every summer in the United States. A citizen
of South Dakota, where the mercury, in the shade, frequently rises
above 100° Fahr., and has been known to reach 115°, will be inclined
to revise his definition of the term “tropical” when he learns that
at Colon, the Atlantic terminus of the Canal, a temperature as high
as 90° is decidedly exceptional, and that the maximum reading during
a period of six years was only 92°. In thirteen years Canal Zone
vital statistics showed only two deaths from sunstroke and twenty-one
non-fatal cases of heat prostration among a population of 120,000. It
will also surprise most Americans to learn that the highest natural
air temperatures that have been recorded anywhere on earth were not
observed near the equator, but in a California desert. At a place on
the edge of Death Valley, rejoicing in the ironical name of Greenland
Ranch, a temperature of 134° Fahr. was registered in July, 1913. The
thermometer which furnished this remarkable reading was a tested
instrument, installed in a standard screen over an alfalfa sod, and
not exposed to the reflected heat of the desert. At the same place the
temperature reached 100° or more on 548 days in four years. Outside
of the United States the highest temperature ever recorded at a
meteorological station was 127° Fahr. at Wargla (Ouargla), Algeria.

The lowest temperatures encountered by polar explorers are considerably
higher than those experienced each winter by the inhabitants of
northern Siberia. The “record,” so far as instrumental observations go,
is held by the town of Verkhoyansk, at which the temperature fell to
90° below zero (Fahrenheit) in February, 1892. Strange to say, this
“winter cold pole” of the earth has warm summers. At Verkhoyansk the
temperature sometimes rises to 80° above zero, or higher. At Yakutsk,
Siberia, the thermometer has been known to fall to 84° below zero in
winter and to rise 102° above zero in summer; a range of temperature
exceeding the interval between the freezing point and the boiling point
of water!

Another climatic paradox is that experienced by mountaineers who, in
scaling peaks mantled in eternal snow, often suffer with the heat,
on account of the intensity of solar radiation in the pure, dry air
of high altitudes. At the health resort of Davos, in the high Alps
(altitude 5,250 feet), invalids sit out-of-doors without wraps in
midwinter, and, indeed, are sometimes driven into the shade to escape
the too ardent rays of the sun. At the same time the temperature of the
air itself may be far below freezing, and the ground covered with snow.

Certain parts of the world are often loosely described as “rainless,”
but, as we have stated elsewhere, there is actually no spot on earth
at which rain (or snow, in the polar regions) has never been known to
fall. In the driest part of the Sahara--the Libyan Desert, between
Dakhel and Kufra--the explorer Rohlfs experienced a drenching rainstorm
of three days’ duration in 1874. Neither is the Sahara, in spite of its
proverbial heat, exempt from touches of real winter. Snow is a common
occurrence in many parts of this desert, even at moderate altitudes.
On the higher Saharan peaks snow lies on the ground all winter, and
is sometimes found, in sheltered spots, in summer. Occasional falls
of snow occur in all parts of Algeria, and several falls have been
recorded in Lower Egypt.

When all is said and done, the whole fabric of what now constitutes
the science of climatology leaves much to be desired. Climate is
of practical interest, first of all, on account of its effects
on human life and health, and secondly because of its influence
upon the crops that are the mainstay of man’s material prosperity.
Under both these heads climatic data, as now commonly presented,
ignore certain atmospheric activities of the utmost importance. For
biological purposes no description of a climate can be regarded as even
approximately complete that does not furnish, for the region under
discussion, a detailed account of the different kinds of radiation
received from the sun, their intensities and fluctuations; and there
are few places in the world at which even a beginning has been made
in the collection of such data. Again, the phenomena of atmospheric
electricity, including radioactivity, are probably of real climatic
significance, but we are still in the stage of speculation with regard
to this subject. Possibly there are still other elements of climate,
now wholly neglected, that will figure prominently in the climatology
of the future.



CHAPTER XIII

ORGANIZED METEOROLOGY


No other branch of science is so dependent upon the constant systematic
cooperation of a multitude of workers as meteorology. There are, to
be sure, some kinds of atmospheric phenomena that can be studied
advantageously by the individual meteorologist, with no further aid
from his scientific confrères than the same sort of interchange of
ideas that prevails in all departments of knowledge; but the widespread
processes that constitute weather and climate require for their
observation--whether the purpose in view be weather forecasting, or
the collection of climatic statistics, or the assembling of data from
which to deduce the laws of atmospheric movements--a veritable army of
colaborers, equipped with standardized instruments and keeping their
records according to a uniform plan.

Probably few people, in looking at the charts portraying the climates
of the world that are found in reference books, realize how many
observers have contributed to the preparation of such charts or the
number of separate instrumental observations upon which they are
based. In the United States alone there are something like 6,000
meteorological stations, at which upward of two and a quarter million
observations are made every year--and a climatic chart is, of course,
the fruit of _many_ years of observations. At the beginning of the
present century it was estimated that there were 31,000 meteorological
stations in operation throughout the world. The present number is
doubtless much greater. At some of these stations observations have
been made regularly, once, twice or three times a day, for 100 or 150
years. In round numbers one may say that, during the last few decades,
meteorological observations have been made, the world over, at the
rate of ten million a year, and the total number, since the keeping of
regular weather records began, runs far up in the hundred millions.

Organized meteorological observations were not unknown to antiquity--we
have mentioned elsewhere the early rainfall measurements in India and
Palestine--but the present era of such undertakings dates back only
to the middle of the seventeenth century. In the year 1654 the Grand
Duke Ferdinand II of Tuscany, through his chaplain and secretary,
Luigi Antinori, secured the cooperation of several observers in Italy
and the adjacent countries, to whom were distributed instruments and
forms for maintaining daily records of the principal meteorological
elements. Antinori and most of the observers belonged to the Jesuits,
an order which has displayed extraordinary zeal in the furtherance of
meteorology down to the present day. The observations thus inaugurated
appear to have been kept up until about 1667, but unfortunately few
of the records have been preserved. Several undertakings of similar
character were launched during the next hundred years in France,
England, and Germany. The most notable of such enterprises, however,
antedating the foundation of the present official weather services,
was the international system of observations maintained by the
Meteorological Society of the Palatinate, founded at Mannheim in
1780 under the auspices of the Elector Karl Theodor. The chief credit
for the epoch-making work of this society is due to its secretary,
J. J. Hemmer. The society distributed standard instruments to its
observers, who were widely scattered over the world; viz., fourteen in
Germany, two in Austria-Hungary, two in Switzerland, four in Italy,
three in France, four in Belgium and Holland, three in Russia, four in
Scandinavia, one in Greenland, and two in North America (at Bradford
and Cambridge, Mass.). The very detailed observations of this network
of stations down to the year 1792 were published in twelve large
volumes.

Although the activities of the Mannheim society came to an end in
the troublous days of the French Revolution, the records that it had
collected served as the groundwork for fruitful studies during the next
generation. There are two distinct uses that can be made of statistics
of this sort. First, they can be digested in such a way as to bring out
the characteristic features of the climate at each of the localities
included in the collection, and likewise to illustrate the distribution
of climates over the globe. Second, the data for individual days from
the various stations can be charted separately, so as to illustrate the
_instantaneous_ distribution of barometric pressure, wind and weather,
and, by a comparison of the charts for successive days, to provide a
sort of moving picture of the atmospheric machinery in operation.

Charts based on approximately simultaneous observations showing
the state of the atmosphere at a particular moment of time over an
extensive area of the earth are called _synchronous charts_, or
sometimes _synoptic charts_, though the latter term is also applicable
to charts showing average values for a particular month, year, etc.
Synchronous charts, as used nowadays for the purpose of making
forecasts, are prepared from data collected by telegraph; but the
same kind of charts can be prepared in a more leisurely manner from
the statistics gathered at any previous time, and such charts were
frequently made for the purpose of study before the days of telegraphy.
The pioneer in such undertakings was the German physicist, H. W.
Brandes, who, about 1820, utilized the observations collected by the
Meteorological Society of the Palatinate, together with some others, in
compiling a series of daily synchronous charts of Europe for the year
1783.

Very similar studies were carried out in America, a few years later, by
J. P. Espy, W. C. Redfield and Elias Loomis. Early in the nineteenth
century a copious fund of meteorological observations had already
accumulated in this country. The first undertaking in the nature of a
meteorological organization, foreshadowing the present Weather Bureau,
was due to Josiah Meigs, Commissioner of the General Land Office, who
in 1817 established a system of tri-daily observations at the various
land offices. At an almost equally early period the Surgeon General of
the Army inaugurated regular weather observations at the military posts
throughout the country. Local systems of observations were established
by the authorities of New York State in 1825 and Pennsylvania in
1837, and systems of broader scope by the Patent Office in 1841
and the Smithsonian Institution in 1847. Experiments in collecting
weather reports by telegraph for the purpose of forecasting storms
were undertaken by the Smithsonian Institution as early as 1849. At
about the same period Lieut. M. F. Maury, of the navy, was gathering
meteorological reports from mariners and laying the foundations of
marine meteorology. Finally, in 1870, Congress was induced to establish
a full-fledged telegraphic weather service, similar to those that were
already in successful operation in Europe. One of the great promoters
of this enterprise was Dr. I. A. Lapham of Wisconsin; it had been
repeatedly advocated by Maury; and a convincing object lesson in its
behalf was furnished by the local service of reports and forecasts
conducted by Prof. Cleveland Abbe, at the Cincinnati Observatory, with
the aid of the Western Union Telegraph Company, in 1869 and 1870.
During the first twenty years of its existence, from 1870 to 1890, the
Federal weather service was under the Signal Corps of the army. Since
1890 it has been a branch of the Department of Agriculture, as the
United States Weather Bureau, though the name “Signal Service” stuck to
it, in popular speech, long after it ceased to belong to the army.

In this country weather forecasts are--or once were--said to emanate
from “Old Probabilities,” or “Old Probs.” Our first “Old Probs” appears
to have been Professor Abbe, who has explained the origin of this name
in an account of his pioneer forecasting experiments at Cincinnati. He
says of the initial Cincinnati Weather Bulletin, issued September 1,
1869:

“It contained only a few observations telegraphed from distant
observers and announced ‘probabilities’ for the next day. This
bulletin, in my own hand-writing, was posted prominently in the hall of
the Chamber of Commerce, but unfortunately I had misspelled ‘Tuesday,’
and I soon found below my Probabilities the following humorous line
by Mr. Davis, the well-known packer: ‘A bad spell of weather for “Old
Probs.”’ This established my future very popular name of ‘Old Probs.’”
The name has, however, been more particularly associated with Gen.
Albert J. Myer, who, as Chief Signal Officer, was the first head of the
Federal meteorological service.

Desultory experiments in the collection of current weather reports and
their use in constructing weather maps were first carried out in Europe
at about the same time as the early undertakings of this character in
America. Such reports were gathered and published by James Glaisher,
with the cooperation of the British railways, in 1849. The existing
national weather services of the Old World owe their origin to an
episode of the Crimean War. In November, 1854, a violent storm wrought
havoc among the French and British warships in the Black Sea and sank
many vessels containing invaluable stores intended for the Allied
armies in the Crimea. The French astronomer Le Verrier, director of
the Observatory of Paris, collected information showing the progress
of this storm across Europe, and the results of this inquiry were so
significant that he submitted to the Emperor Napoleon III the plan of
organizing an international system of telegraphic reports, by means
of which timely warning could be obtained of similar atmospheric
disturbances. The French Government, with the aid of other European
countries, established such a system in 1855. Within the next two
decades most of these countries organized their own services, and at
the same time maintained an international exchange of observations by
telegraph. Before the close of the nineteenth century nearly all the
civilized countries of the world, including many colonial possessions,
such as Canada, Australia, Algeria and the Philippines, had established
meteorological services, entailing more or less extensive arrangements
for collecting daily reports by telegraph and issuing storm warnings
and weather forecasts. The chief exceptions were several of the
Latin-American republics and the Ottoman Empire, in which such
organizations are still lacking.

Meteorology is essentially an _international_ science. The atmosphere
knows no political boundaries, and the more it is studied the more
strongly meteorologists are impressed with the fact that intimate
relations exist between the atmospheric events of widely separated
regions of the world. Thus, the great anticyclone that is built up
every year over the cold interior of Siberia exercises an influence
upon the weather of the United States; the behavior of the Indian
monsoons has been found to have some connection with barometric
conditions in South America; and fluctuations in the force of the
trade winds are apparently of world-wide significance--whence these
winds have been described as the “pulse” of the general atmospheric
circulation. The French meteorologist L. Teisserenc de Bort called
attention many years ago to the existence of what he called “centers
of action”; viz., large permanent or semi-permanent areas of high and
low barometric pressure, the variations of which correspond strikingly
with the vicissitudes of wind and weather in countries thousands of
miles distant. Last but not least, persistent attempts have been made
to interpret all the weather happenings on our globe in terms of a
fluctuating supply of radiant energy received from the sun.

Fortunately meteorology has possessed an international organization
for a great many years. The International Meteorological Organization
was founded at a conference held at Leipzig in 1872, and was perfected
at a formal congress of meteorologists convoked at Vienna in the
following year. The International Meteorological Committee, which is
the permanent working body of the organization, was established at
the Vienna Congress. Finally, the organization was reconstituted at a
conference held at Paris, by invitation of the French Government, in
1919.

The International Committee consists of not more than twenty members,
all of whom are directors of official meteorological services. It
is supposed to meet at least once in three years. At less frequent
intervals are held “conferences,” to which are invited representatives
of all the meteorological services and the principal independent
meteorological observatories of the world. Attached to the organization
are several international “commissions,” which supervise and coordinate
the work of meteorologists in various special fields. At the close of
the year 1921 there were commissions on the following subjects:

Agricultural Meteorology, Weather Telegraphy, Marine Meteorology,
Solar Radiation, Application of Meteorology to Aerial Navigation,
Réseau Mondial, and Polar Meteorology, Investigation of the Upper Air,
Terrestrial Magnetism and Atmospheric Electricity, Study of Clouds.

Each commission includes in its membership at least one member of the
International Committee, besides a number of experts, from different
countries, in the particular subject with which the commission is
concerned.

The resolutions adopted at the various international meetings of
meteorologists have been collected in the “International Meteorological
Codex,” the chief object of which is to secure uniformity in methods of
observation, forms of publication, etc.

One of the most notable international undertakings in the history of
meteorology was the plan of simultaneous observations, at Greenwich
noon, both at land stations and on board ships, adopted by the Vienna
Congress at the suggestion of General Myer, and carried out under
the auspices and mainly at the expense of the United States Signal
Service. The results of these daily observations, from 1875 to 1887,
were published in detail, with charts, by the Signal Service. The many
bulky volumes of this series, illustrating the meteorology of the globe
(or mainly the northern hemisphere) day by day for a period of more
than a decade, are the modern analogue of the “Ephemerides” issued a
century earlier by the Meteorological Society of the Palatinate--which
cover very nearly the same length of time. In recent years the efforts
of meteorologists have been bent toward establishing a so-called
“réseau mondial,” or world-wide network of stations, which will not
only provide telegraphic reports for the use of forecasters, but will
also send their detailed records to an international commission to be
compiled and published. The telegraphic feature of this project now
bids fair to be realized in a manner that was not contemplated when the
plan was originally proposed; viz., by the broadcasting of weather
reports from high-powered radio stations all over the world.

An effective “world weather bureau,” with permanent headquarters
and staff, is at present the most urgent desideratum of practical
meteorology. Such a bureau would not only tie together the national
weather services of the world and greatly facilitate their operations,
but would also digest the great mass of existing climatic statistics
and provide for extending the climatological survey of the globe to
regions where meteorological stations are scarce or lacking.

A typical national meteorological service comprises a central station
or institute, usually, but not always, situated at the national
capital, and a network or “réseau” of subordinate stations, which are
sometimes classified, according to the extent of their observations,
as stations of the first, second, and third order. They may also
be classified, from another point of view, as telegraphic and
nontelegraphic stations. The former provide telegraphic reports of
their observations, which serve as the foundation for forecasts, while
the latter are maintained chiefly for the purposes of climatology. In
some countries--notably in the United States--there are additional
classes of stations engaged in particular lines of work; these include
storm-warning stations, river stations (which report river stages and
rainfall in the river basins), stations for agricultural meteorology,
etc. Several of the great maritime nations collect reports from vessels
on the high seas, including a small percentage of wireless reports. A
number of the national meteorological services carry on work in other
branches of geophysics, such as seismology and terrestrial magnetism.

The United States Weather Bureau is an exception to the rule that,
apart from the central offices and a few special stations and large
observatories, meteorological stations are not generally manned by
professional meteorologists, nor are the observers paid specifically
for their meteorological work, though in a great many cases they
are public functionaries who are expected to take meteorological
observations in addition to their other duties. In this country there
are about 200 stations at which the observers, of whom there are from
one to a dozen or more at each station, devote all their time to the
work of the stations and are salaried employees of the Weather Bureau,
and there are several hundred minor stations manned by part-time paid
employees. But even in the United States the great majority of the
meteorological stations are operated by unpaid observers. There are
about 4,500 of these so-called “cooperative stations,” which provide
the bulk of the climatic statistics of the country.

Some of the leading meteorological services of the world, and the
places at which their central offices are located, are as follows:

United States Weather Bureau (Washington), Meteorological Service of
Canada (Toronto), Meteorological Office (London), Office National
Météorologique (Paris), Reale Ufficio Centrale di Meteorologia e
Geodinamica (Rome), Zentralanstalt für Meteorologie und Geodynamik
(Vienna), Indian Meteorological Department (Simla), Central
Meteorological Observatory (Tokyo), Commonwealth Bureau of Meteorology
(Melbourne), Oficina Meteorológica Argentina (Buenos Aires).

In Germany there are several mutually independent meteorological
establishments, of which the Prussian Meteorological Institute,
with headquarters in Berlin, is the most important with respect to
climatology and research, while the Deutsche Seewarte, at Hamburg,
is the chief center for telegraphic weather reports and issues the
principal weather map. Russia, before her debacle, had one of the
most splendidly organized meteorological services in the world, with
headquarters at the Central Physical Observatory in Petrograd, and a
separate service for agricultural meteorology, which was the model
institution of its kind. The Philippine Islands have a Weather Bureau
which is entirely distinct from that of the United States. This Bureau,
with headquarters at the Manila Observatory, was founded by the
Jesuits, who also maintain a quasi-official meteorological service in
China, with headquarters at the Zikawei Observatory, near Shanghai.

Many meteorological societies have done much for the progress of the
science, and in some cases have shared the duties of the official
meteorological services, especially in maintaining stations for
climatology. These include the Royal Meteorological Society and the
former Scottish Meteorological Society, in Great Britain, the French,
Italian, German, Austrian, and Japanese meteorological societies, and
the American Meteorological Society, which was founded in December,
1919.



CHAPTER XIV

WEATHER MAPS AND FORECASTS


“Forecast”--with the stress on the first syllable when it is a noun,
but often on the second when it is a verb--is a word that meteorology
has made peculiarly its own. This fact is not the result of accident,
but of design.

The founder of scientific weather prediction in Great Britain was
Admiral Robert FitzRoy--the same talented officer who explored
the coasts of South America in the _Beagle_ and had Darwin for a
fellow-voyager--and his first predictions were issued in 1861 from
the Meteorological Department of the Board of Trade, which was under
his charge. The boldness of this pioneer undertaking is not easily
realized by the present generation, which is accustomed to see the
official weather forecast at the head of every daily newspaper.
Weather prognostication had previously been the undisputed province of
charlatans and quacks. For a civilized government to embark upon such
an enterprise must have seemed, to the educated public, very much like
charging the Astronomer Royal with the duty of casting horoscopes.

[Illustration: A GLASS WEATHER MAP OF THE UNITED STATES WEATHER BUREAU

(_Courtesy of U. S. Weather Bureau._)]

There is much virtue in a name. A few years ago the United States
Bureau of Fisheries persuaded the American public to eat dogfish
by changing its name to “grayfish.” Similarly, FitzRoy induced the
British public to take his weather predictions seriously by calling
them “forecasts.” The name has stuck; and nowadays, throughout the
English-speaking world, the expression “weather forecast”--except
as applied comprehensively to predictions of the “long-range”
variety--means something decidedly less chimerical than the average
weather prophecy.

[Illustration: THE SUN DRAWING WATER

(_Photograph by P. K. Budlong._)]

It is still necessary, however, to emphasize the distinction. There
are probably many people among us, well above the illiterate level,
who have no clear idea as to what constitutes a scientific weather
forecast. The distinguishing feature of such a prediction, apart from
the fact that it is made by a trained meteorologist, is that it is, in
all cases, based upon a weather map.

The forecasting machine is a big one, with its human gear spread over
a wide territory. Eventually it will be spread over the entire globe,
and then we shall have better forecasts. A little manual entitled “The
Weather Map,” published by the British Meteorological Office, says:

“The making of a single forecast in any one of the meteorological
offices of Europe, America, Australia, or the Far East requires the
organized cooperation of some hundreds of persons; about a hundred
observers who note the necessary observations simultaneously at as
many separate places and hand in their reports to the telegraphists
who transmit them to one center, where the meteorological expert
charts them on a map and draws therefrom the conclusions on which the
forecasts are based. The preparation of the map is an essential part of
the process. No meteorologist in the modern sense attempts to forecast
the weather without reference to a map prepared either by himself or
by some one with whom he is in direct communication, from observations
transmitted by telegraph for the purpose. No amount of weather wisdom
or weather lore or experience is a substitute for the map. The more
expert and accomplished the meteorologist, the more certain he is that
all he can do without the materials for constructing a map, though
he may have a barometer and other instruments at hand, is to make a
guess at what the map is like and think out from that what the weather
changes are likely to be. It is a common experience of professional
meteorologists away from their base to find themselves appealed to for
an opinion about the weather, judging from the signs of the sky alone,
because they are learned in such things. That is exactly what they are
not. Accustomed to refer everything to a map, without one they feel
themselves to be rather worse off than those who are unaccustomed to
its use. A modern meteorologist thinks in maps; his language and modes
of expression are formed thereby.”

While the weather map is prepared, first of all, for the use of the
forecaster, who makes his predictions from the map before it has passed
beyond the manuscript stage, it has other important uses, which justify
its publication and widespread distribution. The weather map is a
weather newspaper. Like other newspapers, it is founded on a system of
telegraphic dispatches and is designed to keep us in touch with what
is going on in the world. Weather news is of general interest because
weather plays a part in most of the doings of humanity. Sometimes the
news we read on the face of the map merely satisfies our curiosity; at
other times it renders us more substantial service.

By way of illustrating the manifold purposes served by weather maps,
let us set down two cases that are, perhaps, at opposite ends of the
scale of utility. Our first case is that of the traveler who scans the
map to see whether the atmospheric conditions at his distant home are
propitious, that day, for some outdoor pleasure event on the family
program. This we may describe as a sentimental use of the map. The
second case is that of an aviator embarking on a flight some time in
the fore part of the day, soon after the morning map has made its
appearance. Here is a case in which the map is of vital utility, purely
as a record of current conditions. The aviator is not concerned with
the forecast of the morrow’s weather, unless he is making an unusually
long flight, but he is immensely concerned with the winds and weather
prevailing along his route at the time he flies, and these will not, as
a rule, differ radically from the conditions shown on the map of the
same morning.

Since weather affects business in a variety of ways, people who have
business interests away from their places of residence frequently have
occasion to consult the weather map. The influence of the weather on
crops explains why the map is watched with keen interest by dealers in
agricultural products. Owners of vessels navigating the ocean or the
Great Lakes take a practical interest in the present as well as the
future location of storms. And so on. It is not necessary to prolong
this list of those who use the weather map, because the popular demand
for it speaks for itself. It is worth while to record the fact that the
demand far exceeds the supply. In this country the Weather Bureau has
been constantly harassed with urgent requests for the publication of
maps at places where, in consequence of limited appropriations, it has
not been possible to issue them.

The weather maps published in various parts of the world exhibit much
diversity in detail, though they have, of course, many features in
common. As a rule a weather map covers a wider area than that of the
country in which it is published. The aim has always been to make
these publications international, as far as practicable. The longest
continuous file of printed daily weather maps in existence, viz.,
that established in France by Le Verrier in 1863 and still published,
has been called from its beginning the “Bulletin International.” It
embraces nearly the whole of Europe, a little of Africa, Iceland,
and the Azores. The other European maps now cover the same area or a
considerable part of it. Before the war the Russian meteorological
service was issuing a map that included, in addition to Europe, a wide
zone of Asia extending all the way to the Pacific Ocean. The United
States map, as published in its most extensive form at Washington,
comprises the whole of this country and southern Canada, besides
presenting tabulated statistics for more distant parts of the world.
Manuscript maps prepared daily at Washington have a still broader
outlook; they are drawn on a base map that covers the northern
hemisphere, and the printing of a map of this sort, including a chain
of stations extending around the globe, was undertaken in 1914,
but was interrupted by the war. The map published by the Argentine
Meteorological Office, at Buenos Aires, covers more than half of South
America. Most national meteorological services issue weather maps, but
there are a few that do not. No such maps are published in South
Africa or any of the South American countries except Argentina.

[Illustration: LAND AREAS EMBRACED IN DAILY WEATHER MAPS AS PUBLISHED
IN 1921]

Thus there is still much room for the horizontal extension of the
weather map, and there is even more room for its vertical extension.
Daily weather maps for aeronauts (chiefly wind maps) are now more or
less on the programme of all the leading meteorological services,
and in a few cases their publication has already begun. Probably the
first maps of this character, showing the winds at various levels over
a whole country, were those that began to appear in Italy in 1913.
The British Meteorological Office now publishes such maps, showing
winds and clouds at different levels over the British Isles at three
hours of the day. In the United States maps of the “wind aloft” are
prepared daily, at Washington, from the reports of kite and balloon
stations, but they are not yet published. The Weather Bureau has,
moreover, invented an ingenious method of depicting the winds at
several levels on a single map; in other words, constructing a map in
three dimensions. This consists of attaching arrows to little metal
posts erected on an ordinary weather map at points corresponding to
the location of the upper-air stations. Each post bears a series of
arrows--one arrow for each level charted--and the arrows are set in
positions showing the direction of the wind at each level. Numbers on
the arrowheads indicate the force of the wind. When the map is finished
it is photographed from two different angles so as to make a pair of
pictures suitable for viewing through a stereoscope. These stereoscopic
pictures were formerly made every day and a file of them is available
for reference and study.

[Illustration: STEREOSCOPIC MAP OF THE “WINDS ALOFT”]

There are a few conspicuous points of difference between the weather
maps issued in foreign countries and those issued in this country.
Thus a majority of the foreign weather services publish two or more
charts on the same sheet; either for the sake of showing different
meteorological elements separately or, in most cases, to represent
the conditions prevailing not only at the hour of the current morning
observation, but also at certain hours of the previous day. One of the
three editions of the British map includes four charts, corresponding
to observations at four different hours. By means of such series of
charts one can observe the recent changes of weather as well as the
current conditions. Weather maps published in the United States show
primarily the conditions at 8 a. m., Eastern Standard Time, of the
morning of issue; though certain features of past weather are also
indicated, including changes of temperatures, movements of storm
centers, etc. Evening maps are drawn at Washington and at many other
places, but are not published.

In this country the publication of weather maps has been carried out on
a much more liberal scale than elsewhere. Instead of issuing maps at
only one or a few places, as is the custom in other parts of the world,
it has been the policy of the American service to publish them at
populous centers all over the country. In some cases they are printed
or manifolded at the local Weather Bureau station, and distributed
by mail and messenger; in other cases they are published in the
newspapers. The daily circulation of the maps has thus, at times, run
up into the millions. This comprehensive duplication of the chart is
made possible by special arrangements with the telegraph companies. The
reports of observations are, to a large extent, sent over circuits,
along which the telegraph offices, besides forwarding the local report,
copy the reports from other stations as they pass over the wires.
Certain stations, forming connecting links between the circuits, effect
the transfer of collected reports from one circuit to another; so
that, in a very short time, upward of 150 stations receive the reports
from a large number of other stations. The maps issued at stations or
published in newspapers are generally rather crude, though they answer
their purpose; but the large lithographed map issued every day at the
Central Office, in Washington, is much the most artistic production of
its kind published anywhere in the world.

[Illustration: ENLARGED SECTION OF THE STEREOSCOPIC WIND MAP]

Large weather maps, drawn with colored chalk on a ground-glass base,
may be seen at certain produce exchanges and railway stations, on
the “boardwalk” at Atlantic City, and in the Capitol at Washington.
Motion-picture weather maps, made from series of maps showing
conditions at successive intervals of time, have been prepared
experimentally in this country and abroad.

The reports used in the construction of weather maps are telegraphed
from the stations in cipher, in order to save expense. In Europe groups
of figures are used for this purpose, but the United States Weather
Bureau makes use of a word code, which offers the advantage over a
figure code that, as a rule, mistakes in the telegrams can easily be
detected by anybody familiar with the code. The American weather code
is something of a literary curiosity. In each of the many thousand
words it contains there are certain significant letters, and these must
fall in certain sequences in order to convey the information required.
The English language has been ransacked--and somewhat stretched--to
secure the necessary words. Observers consult the code book in
enciphering their reports, but translating is easily done without the
book by those who have mastered the relatively simple principles on
which the code is constructed.

[Illustration: SIMPLIFIED WEATHER MAP FOR JAN. 25, 1905, 8 A. M.,
EASTERN STANDARD TIME]

The language of lines, shadings and symbols used in weather maps can
be learned in a few minutes, and it is, as a rule, fully explained on
the face of the map. This is true of foreign maps as well as American.
A full-fledged weather map is hardly susceptible of reproduction
in a book of ordinary dimensions. The simplified map that we show
here, taken from a Weather Bureau bulletin, will, however, serve to
illustrate some of the features of such publications. The reader should
first fix in his mind the explanations printed at the lower left-hand
corner of the map and then study the map in the light of what has been
said in Chapter VIII about the circulation and movements of highs and
lows. On this map we have an exceptionally well-developed high over
the middle of the country and a pronounced low on the Atlantic coast.
The former, with clear skies and very cold weather, constitutes a cold
wave. The latter is attended by a widespread snow storm and, as may be
inferred from the crowded isobars, by stormy winds.

Now bear in mind the fact that charts identical with this one, except
that they contained much more detailed information, were issued in
all the more important cities and towns of the United States on the
morning of January 25, 1905, about a couple hours after the taking
of the morning observations, at 8 a. m., Eastern time. The same
morning, weather forecasts, cold-wave warnings and storm warnings,
deduced from this map, were issued to some hundreds of thousands of
addresses by telegraph, telephone, mail, and messenger. The map itself
conveys information comparable in interest to the news of public
events published on the first page of the newspapers the same day.
The forecasts and especially the warnings are, in such a case, worth
millions of dollars to the people of the United States. The following
account of the cold wave appears in the Annual Report of the Chief of
the Weather Bureau for 1905:

“A severe cold wave appeared over the Dakotas, Minnesota, Nebraska, and
Iowa on January 24, 1905, and on the 25th covered the central and upper
Mississippi valleys and extended over the northern portions of the
east Gulf States, the line of zero temperature reaching into northern
Tennessee. On the 26th the cold wave covered Florida, and temperatures
below freezing were reported as far south as Tampa and Jupiter. At the
latter place, the minimum temperature, 24 degrees, equaled the lowest
ever recorded since the establishment of the Weather Bureau station at
that point. Considerable damage was done to orange trees where groves
could not be fired or protected. Ample warnings had been given of the
expected low temperatures.”

The subject of making forecasts from a weather map is one concerning
which some big books have been written, and it cannot be dealt with
very satisfactorily in the brief space at our disposal. There are two
cardinal rules--viz., (1) the weather has a characteristic distribution
in relation to the distribution of barometric pressure, and (2)
pressure systems, with their attendant winds and weather, move, in
a general way, from west to east--but these rules require various
qualifications and are subject to various exceptions. Thus highs and
lows generally take rather circuitous routes in getting, eventually, to
the eastward, and sometimes they break up or fade out. The high shown
on the annexed map actually moved much more south than east during the
following twenty-four hours, while the low moved up the coast; i. e.,
more north than east. These were, however, the movements expected by
the experienced forecaster.

Well-developed highs are nearly always regions of clear weather and, in
winter, of cold weather. Lows are attended by clouds and precipitation;
rising temperature usually precedes them and falling temperature
follows them. The professional forecaster recognizes several types of
pressure distribution other than ordinary highs and lows, and they,
also, have their characteristic winds and weather. The seven typical
forms of isobars, as classified many years ago by R. Abercromby, are:
Cyclone; anticyclone; secondary; V-shaped depression, or trough;
wedge of high pressure; col, or saddle, between two anticyclones; and
straight isobars. These may be combined in a variety of ways on the
weather map. The forecaster learns to classify these combinations
consciously or subconsciously, and grows familiar with their habits and
mannerisms. Forecasters spend a good deal of time in studying the files
of weather maps for past years; but the results of such studies are not
easy to reduce to definite statements. Scientific forecasting is, in
its present stage, almost wholly empirical. The dependence of weather
changes upon the phenomena of atmospheric circulation is generally
easy to make out, but the vagaries of winds and pressure are still in
the main mysterious; notwithstanding such interesting developments as
(1) the much-discussed rules of M. Gabriel Guilbert for predicting the
movements of barometric depressions from abnormalities in the force and
direction of the winds; (2) the systematic charting of “isallobars,”
or lines of equal pressure change, associated especially with the name
of Dr. Nils Ekholm; and finally (3) the hypothesis of a sharp line
of demarcation between masses of equatorial and polar air along the
so-called “polar front,” forming the basis of a system of forecasting
that originated at the Geophysical Institute of Bergen, Norway, and has
had a marked influence upon the methods of forecasters in other parts
of the world.

As to the practical results of this empirical art, one point of the
utmost importance is commonly overlooked by the public when it
complains about the mistakes of the forecaster. He is required to make
forecasts of weather day after day, regardless of the kind of map
that is laid before him. Sometimes the map is so featureless (or, as
the forecasters say, “flat”) that there is little in it on which to
build a forecast. At other times there is an abundance of features,
but they are in process of rapid and disconcerting change. In either
case the ordinary day-to-day weather forecast is likely to go astray.
The brighter side of this picture is that the atmospheric phenomena
that count heavily in terms of dollars and imperiled human lives are
not found on “flat” maps, and, when they appear on the map, generally
behave in a simple, straightforward way. In other words, such events as
great storms and cold waves are far easier to forecast than everyday
weather, and it is the successful prediction of these events that
furnishes the principal _raison d’être_ of an expensive telegraphic
forecasting service.

Since weather predictions serve a variety of purposes, many different
kinds of forecasts and warnings have been developed by meteorological
services. In this country there are, first of all, district forecasts
and local forecasts; the former, covering whole States and groups of
States, being issued at a few main forecasting centers, while the
latter, applying to a single town and its vicinity, are issued at a
large number of the ordinary Weather Bureau stations. There is a long
list of forecasts and warnings intended for special classes of the
community; indeed the specialization of forecasts is carried so far
that an individual citizen or a single business firm can generally
obtain, by asking for it, a forecast of any specified predictable
feature of the weather for a particular place. The established types
of special prediction issued regularly, or when conditions warrant,
include wind and weather forecasts and storm warnings for mariners;
shippers’ forecasts, relating to temperatures injurious to perishable
goods; aviation forecasts; “fire-weather” warnings, issued when the
weather is conducive to fires in the western forests; avalanche
warnings; and several different kinds of advices for the benefit
of agriculture and horticulture. The United States Weather Bureau,
although it is not the only branch of the Government that carries
on work in hydrology, is the one charged with the duty of issuing
river-stage predictions and flood warnings. An elaborate organization
is maintained for this purpose, and the results are extremely
successful.

The period of time covered by an official weather forecast is
generally from one to two days. In this country the morning forecast
is ordinarily for 36 hours from 8 a. m., and the evening forecast for
48 hours from 8 p. m., but occasionally the period is extended for an
additional day. For some years the Weather Bureau has issued every
Saturday a forecast in quite general terms relating to the whole of
the following week. These long-range forecasts are made for extensive
areas of the country, such as the North and Middle Atlantic States, the
Ohio Valley and Tennessee, and the Great Lakes region. “From the weekly
forecast,” says an official publication, “a farmer may know whether
it is safe to cut his hay at the beginning of the week or whether
it would be better to wait till the last of the week; and a produce
dealer may know whether it is safe, at a particular time in the early
spring, to start a carload of strawberries to a northern market.” The
British Meteorological Office follows a more cautious plan. Its regular
forecasts are for twenty-four hours, but occasionally, when conditions
are fairly settled, announcements are made of what is termed the
“further outlook.” The same office sends notices to the agricultural
districts when a spell of fine weather, favorable for haying or the
like, appears to be on the programme.

There are a few official meteorological establishments that have
embarked on much more ambitious undertakings in long-range forecasting.
The classic example is furnished by the Indian Meteorological
Department, which has issued seasonal forecasts of rainfall ever since
1882. These were originally based upon reports of the snowfall in
the Himalaya, abnormalities of which, as noted in the spring, appear
to be related to the intensity of the subsequent monsoon rainfall.
Eventually the Indian meteorologists began to seek in more remote
regions for clues to the character of the Indian seasons, and they
believe they have found them; the barometric pressure in South America
and at Mauritius, the rainfall at Zanzibar and Seychelles, the Nile
flood, and summer rains in Australia all seems to bear some relation to
meteorological conditions in India.

The study of world-wide interrelations of weather, although it has
not generally, as in the case just mentioned, furnished the basis
of official forecasts, has engaged the attention of a great many
able meteorologists. We have spoken on another page of the “centers
of action” that seem to be such important indexes to changes in the
circulation of the atmosphere, with concomitant fluctuations in
weather. Telegraphic reports from some of these centers, including the
Iceland and Aleutian lows and the Siberian and Azores highs, have
helped to guide the Weather Bureau in making its weekly forecasts. Many
attempts have been made to predict the weather months in advance from
variations in the temperatures of the water in different parts of the
ocean or from the distribution of sea ice in high latitudes. Another
line of attack upon the problem of long-range forecasting is through
observations of solar activities, as indicated by fluctuations in solar
radiation, the prevalence of sun spots, etc. Lastly, an immense amount
of energy has been expended in efforts to detect definite cycles or
periodicities in the weather itself, without regard to their causes.
The thirty-five year period of rainfall and temperature variations,
announced in 1890 by Prof. E. Brückner, has found a place in all the
current textbooks on meteorology, and several other alleged weather
periods have been the subject of serious discussion.

From all of which it appears that the professional meteorologist is
not at all inclined to discountenance attempts at long-range weather
prediction, provided they are made both honestly and intelligently.
Unfortunately the vast majority of people who, in all ages, have
indulged in this sort of vaticination--and their name is legion--have
been either dishonest or ignorant, or both. The world is still well
supplied with them, and they are, undeniably, a thorn in the flesh
of the scientific forecaster, who sometimes sees his predictions
confounded with theirs by the public, and who commonly incurs the
charge of jealousy and narrow-mindedness because he declines to
acknowledge brotherhood with the cranks and impostors who hang about
the outskirts of his profession.

Quack weather predictions are nearly always made for long periods in
advance, and their popularity depends upon the fact that they give
the public something--however fallacious it may be--that science
does not attempt to give. The making of such predictions appears to
be a particularly easy way of acquiring both fame and fortune. In
this country there has hardly ever been a time when some exponent
of this industry did not enjoy a nation-wide reputation. It is a
satisfaction to record, however, that foreign countries produce the
same sort of celebrities. Dr. Gustav Hellmann, writing in Germany,
has recently published an extremely interesting account of the famous
“weather prophets” of the 19th and 20th centuries. Their geographical
distribution is given as follows: Belgium, 2; Germany, 36; England,
25; France, 14; Italy, 2; Austria-Hungary, 8; Russia, 1; Sweden, 1;
Switzerland, 5; Spain, 2; North America, 9. The list for the United
States is, to be sure, conspicuously incomplete, but we need not grieve
over the fact that the fame of the American prophets omitted from the
list has not spread to the Old World.

The almanac is, as it has always been, the chief stronghold of
long-range weather predicting. Nobody knows to what extent the almanac
prognostications are taken seriously by the public, or are meant
to be by the publishers. It is to be feared that the percentage of
the population that “swears by them” is not inconsiderable. Almanac
publishers would undoubtedly perform a public service, and perhaps
save themselves some pangs of conscience, if they would append to
their weather predictions the statement that, like the portrait of the
gentleman who displays his anatomy to the signs of the zodiac at the
front of the book, they are published merely for the sake of keeping
up an old custom, and if they would conclude every almanac with the
following candid avowal, which we find in Gabriel Frende’s “Almanack
and Prognostication” for 1589:

  Thou hast my guess at daily weather
    Here present in thy view.
  My credit shall not lie thereon
    That every word is true:
  Yet some to please I thought it best
  To shew my mynde among the reste.



CHAPTER XV

AGRICULTURAL METEOROLOGY


Two farmers are grumbling about the weather. The scene is Ohio, the
time July, and the prevalent crop corn (i. e., maize).

Farmers have grumbled about the weather from time immemorial. The
point of interest in this particular case is that the two grumblers do
not agree about what is wrong. Farmer A thinks the corn needs rain.
Farmer B declares that at this stage rain would do more harm than
good. Plenty of warm sunshine is, he thinks, the right prescription to
insure a “bumper” crop. Of course Providence will do as it pleases,
and whatever weather comes, since it cannot be cured, must be endured;
but it is a matter of practical as well as academic interest to get
some inkling betimes as to how your crop is going to turn out, and
the weather is likely to be the decisive factor. Moreover, it is a
very significant fact that our two farmers are not of the same mind
about which atmospheric blessing is in default. It is painful to
reflect that an enormous amount of grumbling about the weather on the
part of the rustic community must, at one time or another, have been
misapplied. It is a plausible assumption that farmers have sometimes
worried themselves to death over meteorological events that were either
harmless or actually beneficial to their crops.

How can we arrive at the facts? Admitting, as everybody does, that the
weather has a preeminent influence upon plant life, is this influence
susceptible of analysis? Is there anything definite about it? Are not
the effects of various atmospheric conditions so entangled with one
another, and with the effects of soil and methods of cultivation--to
say nothing of insects and plant diseases--as to baffle all attempts to
gauge them separately?

There is a new branch of applied science that teaches farmers how to
grumble right about the weather. It is called Agricultural Meteorology.
As a coherent branch of knowledge, this subject is so new that the
first formal textbook about it in the English language was published
in the year 1920. It happens that the author of this book, Professor
J. Warren Smith, of the United States Weather Bureau, began his
investigations in the new field by making a careful study of the
relation of weather to the yield of corn in Ohio. Let us see what light
his studies shed upon the question at issue between our friends A and B.

Day after day, and year after year, the principal atmospheric
conditions are observed and measured at a great number of points
scattered over the State of Ohio, as they are elsewhere throughout the
Union, and the records thus obtained are carefully compiled, summed up,
averaged and otherwise discussed by officials of the Weather Bureau.
Thus a great fund of detailed statistical information about the weather
is available for comparison with the statistics gathered by other
agencies concerning the yield of crops and their condition at different
stages of growth.

Professor Smith’s analysis of the Ohio records revealed a fact of so
much practical importance that this discovery alone suffices to place
agricultural meteorology among the most fruitful branches of knowledge
cultivated by mankind. He discovered that the success of the Ohio corn
crop depends chiefly upon the amount of rain that falls during the
month of July. The normal rainfall of that month for the State is 4
inches, while the average yield of corn during the past sixty years has
been 34.5 bushels per acre. Comparing the values for individual years,
it is found that the yield is strikingly sensitive to variations from
the normal July rainfall, and especially so when the rainfall is a
little more or less than 3 inches. Near this critical rainfall point, a
variation of _one-fourth inch_ of rain in July means a variation in the
value of the corn crop of Ohio of nearly $3,000,000, and a variation of
one-half inch makes an average variation in the value of the crop of
more than $7,500,000. When the rainfall for July averages over 5 inches
the probable yield of corn will be more than 27,000,000 bushels greater
than it will be if the rainfall averages less than 3 inches. In other
words, this difference of 2 inches in the rainfall for the month of
July adds $13,650,000 to the income derived by Ohio farmers from corn
alone.

Variations of the temperature in July, in Ohio, have been compared with
variations in the yield of corn, with the result that the temperature
of the month appears to have little effect upon the crop. Thus we find
Farmer A to have been right and Farmer B wrong; but both were merely
expressing personal opinions based upon an insignificant sum-total of
experience. Science rests upon a surer foundation.

Although the case of the Ohio corn crop is probably simpler than
most of those that agricultural meteorology has to deal with, for
the reason that a single meteorological element is, in this case, of
decisive importance, it illustrates a rule of quite general application
that has recently come to light; viz., that in the growth of any
particular crop there is usually a rather brief “critical period,”
when it is most sensitive to the influence of weather. For corn, in a
considerable area of the northern United States, this period is July,
or more specifically, in Ohio, the interval from July 11 to August 10.
The rainfall and temperature of other months have, however, definite
though minor influences, which can be evaluated for the same regions.

With respect to the American “corn belt” in general, it is not certain
how far the rules deduced for Ohio are applicable. Professor Smith
has been inclined to look upon July rainfall as the dominating factor
for the whole of that region; so that, for example, a difference of 1
inch in the rainfall (viz., a total for July of 4.4 inches or more,
as compared with 3.4 inches or less) has been held responsible for
an increase of 500,000,000 bushels of corn in the eight principal
corn-growing States. It has also been stated that in the four States
of Indiana, Illinois, Iowa and Missouri an increase of half an inch
of rain in July meant an increase of $150,000,000 in the value of the
crop. These figures have, however, been challenged, and the subject is
still under discussion.

The study of the critical periods of different crops, and the
determination of the amounts of heat and moisture most favorable
to the success of the crop at such periods, may be regarded as the
leading task of the agricultural meteorologist. The most elaborate
researches of this character have been made in Russia by Professor
P. Brounov, who founded in 1896 a meteorological bureau, attached to
the Ministry of Agriculture, with an extensive network of stations
scattered over the Russian Empire. This bureau was quite distinct from
the ordinary meteorological service, under the direction of the Central
Physical Observatory in St. Petersburg. Just before the war Professor
Brounov had 150 stations in operation; most of them for observing the
effects of weather on the leading cereal crops, though some studied
the corresponding relations of horticulture or the animal industries.
Each agricultural station comprised a small plot of land, on which a
certain sequence of crops was grown year after year under conditions
of cultivation as nearly uniform as possible, the only variable factor
being the weather. Meteorological instruments were installed in the
immediate proximity of the plants under investigation. Prior to 1914
Brounov had determined the critical periods of most of the crops grown
in Russia, and had published a great deal of information on this
subject that could be turned to practical account by Russian farmers.

It will perhaps not be immediately apparent to the reader just how
such information can be utilized. Its practical applications vary,
in fact, according to circumstances. First of all, a knowledge of
critical periods and of the weather requirements of crops at these
periods enables the farmer to select his crops and time his farming
operations on the basis of climatic statistics. Brounov published a
series of charts showing the probability of dry weather, as deduced
from many years of observations, for each ten-day period throughout
the agricultural year for every part of European Russia. With such
charts at our disposal, and knowing how long after planting each crop
arrives at its critical period with respect to moisture, we can readily
estimate the probable success of a given crop planted at a given
time and place; at least, so far as this is determined by rainfall.
If temperature or other meteorological conditions are of special
importance at the critical periods, we shall need additional climatic
charts. Of course, the weather in any particular year may differ widely
from the climatic averages; but in the long run crops will prosper in
proportion as their critical periods coincide with the occurrence of
favorable weather as shown by the climatic record. It will be seen
that this is quite a different idea from the traditional one that a
certain crop needs a “moist climate,” another a “hot climate,” etc. The
agriculturist now asks the man of science to tell him _when_, between
planting and harvesting, heat or moisture is of vital importance to the
crop, and _how much_ of each will produce the biggest yield.

In regions where irrigation is practiced it is obviously advantageous
to the farmer to know at what stage of its growth a crop becomes
sensitive to the amount of moisture received. During the greater part
of its life the plant may be quite indifferent to moisture, and at
such times irrigation would be wasteful. The farmer needs to know not
only when the critical period has arrived, but also what the water
requirements are at that period. Too much water may be as bad as too
little.

Even when agricultural practice ignores the rules laid down by the
agricultural meteorologist, a knowledge of these rules may be applied
with great advantage to the prediction of crop yields. It is hardly
necessary to tell any farmer or business man that accurate crop
forecasts are an economic desideratum of the utmost importance. The
United States Government maintains an army of more than 200,000
volunteer crop reporters, supervised by a staff of experts, for
the purpose of determining month by month the condition of every
agricultural crop and its prospective yield. With regard to the monthly
announcements of the Bureau of Crop Estimates, Professor H. L. Moore,
of Columbia University, says:

“The commodity markets are in a state of nervous expectancy as the time
approaches for the official forecasts, because great values are at
stake. It has been estimated that in the case of the cotton crop alone
an error in the forecasts which should lead to a depression of one cent
a pound in the price of cotton-lint would--assuming a crop equal to
that of 1914--entail a loss of eighty million dollars to the farmers.
The vast values at stake and the dangers when no official estimate
is available of the manipulation of the markets in the interest of
speculators are held to justify the large recurrent annual cost of the
employment of the numerous correspondents, clerks, and experts.”

Professor Moore is one of those who have pointed out that the forecasts
based upon the actual condition of the growing crops can be vastly
improved by a mathematical analysis of the weather reports from the
various regions in which the crops are grown. In fact, he goes so far
as to assert that much better forecasts can be made from the weather
reports alone than from reports on the condition of the crops. Whether
or not this view is unduly optimistic, it goes without saying that the
precise data which agricultural meteorology is now acquiring cannot
fail to enhance greatly the accuracy of crop forecasts.

Of course, the weather has always been watched with keen interest by
everybody concerned with the purchase or sale of agricultural products
and has been one of the chief factors determining the rise and fall of
prices. At produce exchanges throughout the United States daily weather
bulletins are received from the agricultural districts, and at many of
them a large weather map is drawn every morning by an employee of the
Weather Bureau detailed for this purpose. The Bureau has made various
other arrangements for supplying the information that is so eagerly
desired concerning the weather as it affects crops, as well as the
animal industries. During the “growing season” in the cotton, corn,
wheat, sugar, rice, broom-corn and cattle-producing areas, designated
centers receive telegraphic reports of rainfall and the daily extremes
of temperature from substations in the regions concerned, and these are
distributed in bulletin form. Each local center, besides publishing
detailed reports from its own area, issues condensed reports from all
the others. The Bureau also issues every week during the agricultural
season a “National Weather and Crop Bulletin,” with text and charts
setting forth the current conditions of moisture, temperature, etc.,
and the state of the crops in all parts of the country.

The use which dealers and farmers make of these weather reports is,
however, very far from having been reduced to science. Some of these
persons, it is true, are frequently able, by a purely instinctive
process of deduction, to make successful forecasts of crop yields from
a close study of the weather, and others have worked out crude rules
of their own for the same purpose; but the agricultural meteorologist
approaches the problem in a different way. Immense progress has
been made in the past decade in applying the mathematical theory of
_correlation_ to this problem. This branch of mathematics, originally
developed chiefly for statistical studies in biology by Galton,
Pearson, and others, is now extensively used by meteorologists not only
for studying the effects of weather on crops, but also for finding
out what correspondences or relationships exist between variations of
weather in different parts of the world, as well as between weather and
sun spots, weather and vital statistics, etc.

Sometimes, when the farmers do not disagree on the subject of
favorable and unfavorable weather for the crops, they hold opinions
in common that agricultural meteorology is unable to substantiate. An
illustration is found in the idea that a good covering of snow during
the winter is favorable to the yield of winter wheat. Apparently this
is one of the host of popular ideas that are based merely on the
delusive foundation of “everybody says so.” Smith has investigated the
statistics of wheat for Ohio and C. J. Root those for Illinois. In both
cases their results negative the prevailing opinion. Professor Smith
finds “some evidence to indicate that wheat has a better prospect if
it is not covered by snow during the month of January,” while Mr. Root
states that, in general, “the winters of light snowfall are followed by
good wheat yields and the winters of heavy snowfall by light yields.”

The study of the relations between weather and crops is really a branch
of a science of broader scope, known as _phenology_. This science is
devoted to the investigation of all periodic phenomena of plant and
animal life that are controlled by the weather. There are, in some
parts of the world, large corps of phenological observers, who maintain
records year after year of the leafing, flowering, and fruiting of both
wild and cultivated plants, the migrations and first songs of birds,
and various other events of a biological character that recur with the
seasons. In the course of time it becomes possible to compute from such
records the normal dates of these events; and then, in any particular
year, a comparison between the actual dates and the normal shows
whether the season is early or late, and by how many days. Phenological
observations on plants also make it possible to draw charts showing the
normal march of the seasons over a country, expressed in terms of plant
life, and such charts are often more valuable to the agriculturist or
horticulturist as a guide in selecting varieties for cultivation and
in timing his operations, than any charts that can be compiled from
ordinary climatic data. Some admirable charts of this kind have been
drawn for parts of Europe.

There are many practical applications of phenology to agriculture,
and there would be more if phenological observations had been made
more extensively throughout the world. Good phenological charts of
different regions would, for example, greatly facilitate the work of
foreign plant introduction carried on by the United States Bureau of
Plant Industry. In the United States phenological observations were
made systematically between 1850 and 1863, but only desultory work has
been done in this line subsequently. The most comprehensive individual
record is that maintained by Thomas Mikesell from 1873 to 1912, at
Wauseon, Ohio, and published in full by the Weather Bureau in 1915.

The old rule of American farmers, inherited from the Indians, that the
time to plant corn is when the leaf of the white oak is “the size of a
mouse’s ear,” illustrates the use that can be made of so-called “index
plants” of the native flora as guides for farming operations. Professor
A. D. Hopkins writes on this subject:

“If such guide plants do not occur on the farm, they can be found among
the ornamental trees and shrubs and hardy flowering plants of other
localities or countries and transplanted. The periodical event of the
falling of the flower catkins of the Carolina poplar has been found to
be one of the best guides to the general early or late character of one
season as compared with the average, while the opening of the leaf buds
and unfolding of the leaves serve as reliable guides to the progress of
spring. The various magnolias in their succession of flowering events
serve as excellent guides to the rate of progress of spring and the
time to do various kinds of work. The ornamental Spiræas, Deutzias,
Diervillas, climbing roses, and Clematis among the ornamentals, and
the dogwood, service tree, redbud, and oaks among the native trees of
the middle and eastern regions of the United States are more or less
constant in their response to prevailing local influences which are
indicative of the time to plant certain field and garden crops. The
opening of the leaf and flower buds and the flowering of the common
fruit trees and shrubs of almost every farm serve as more or less
reliable guides to the time to spray for certain insect and plant
diseases.”

Dr. Hopkins has worked out an interesting rule known as the
“bioclimatic law,” according to which the periodical events of plant
and animal life advance over the United States at the rate of 1
degree of latitude, 5 degrees of longitude, and 400 feet of altitude
every four days--northward, eastward, and upward in spring, and
southward, westward, and downward in autumn. Thus, when the date of
any phenological occurrence is known for one locality, it may be
approximately determined for any other. This law has enabled the
Department of Agriculture to publish rules of general application
concerning the best time to plant winter wheat in order to escape
the ravages of the Hessian fly, thus saving many millions of dollars
to American farmers. The same law is susceptible of various other
profitable uses.

[Illustration: ORCHARD HEATERS IN OPERATION. The economical use of this
method of frost protection depends upon accurate forecasts of the right
time to “fire” the orchard. (_Courtesy of Hamilton Orchard Heater Co._)]

The United States Weather Bureau has been a branch of the Department
of Agriculture since 1890, and a very large share of its routine
work is devoted to the agricultural interests of the country. The
climatological statistics that it has assembled are indispensable in
many departments of agricultural research, besides furnishing varied
information of practical value to farmers. The Bureau has developed a
number of special types of forecasts for the rural industries; such as
predictions, three or four days in advance, of favorable weather for
cutting alfalfa; forecasts of weather unfavorable for sheep-shearing;
notices to fruit growers of dry-weather periods in which fruit trees
should be sprayed; and warnings of the occasional summer showers that
would do so much damage to the great raisin-drying industry of
California but for the vigilance of the forecasters and the efficient
arrangements made by the industry itself for disseminating and acting
upon the warnings. Of course, the ordinary daily weather forecasts,
storm warnings, and cold-wave warnings are valuable in many ways to
agriculturists, and the Bureau has made great efforts to give such
information prompt and general distribution in the rural districts.
The forecasts are generally displayed in post offices, and in many
cases the rural telephone exchanges are pressed into service to
distribute weather information regularly to all their subscribers.
Some exchanges sound a signal every morning when the forecast is ready
for distribution. Lastly, the wireless telegraph and the wireless
telephone, which, in the immediate future, will form part of the
equipment of every up-to-date farm, afford ideal channels for the
dissemination of weather news and are already extensively used for this
purpose.

[Illustration: A SNOW SURVEYOR AT WORK. Note the cylindrical snow
sampler, with its serrated cutting edge, and spring balance for
weighing the sample of snow (_Photographed by J. C. Aller._)]

[Illustration: SNOW ROLLERS, OR WIND-BLOWN SNOWBALLS ON A LAWN AT
POTSDAM, N. Y. (_Photographed by T. J. Moon._)]

There remain to be mentioned the various steps the Weather Bureau
has taken to protect the rural industries from the night frosts of
spring and autumn, in the shape of special forecasting arrangements,
the publication of frost charts, and a wide range of scientific
investigations. The Bureau’s undertakings in this line are merely a
part, though a leading one, of a great campaign of frost protection
that is being carried on by scientific and official agencies in this
country on a larger scale than anywhere else in the world.

Frosts, classified according to their severity as “light,” “heavy,”
and “killing,” are most likely to occur in spring and autumn, when
an extensive area of high barometric pressure brings its usual
accompaniment of clear skies and calm nights. They are predicted on
a general scale from the weather map, and locally from indications
of temperature and humidity and a knowledge of important topographic
influences, such as those due to hills and valleys and neighboring
bodies of water.

In agricultural usage the term “frost” is applied to the occurrence of
a temperature low enough to kill or injure tender vegetation, such as
growing vegetables or the buds, blossoms, and fruit of fruit trees. The
occurrence of a frost, in this sense, is not necessarily identical with
the deposit of ice crystals known as “hoarfrost.” Different species
and varieties of plants are, of course, susceptible in very different
degrees to the effects of low temperature; i. e., they differ greatly
in “hardiness.” In the case of fruits and vegetables the danger point
generally lies a little below the freezing point of water (32 degrees
F.).

The occurrence of frost is favored by the rapid cooling, by radiation,
of the earth and its plant covering, which goes on at night under a
clear sky and in still air. Under these conditions a layer of stagnant,
cold air forms close to the ground, with warmer air lying above it. The
difference in temperature at different levels is often so pronounced
that fruit on the lower branches of a tree is killed while that growing
on the higher branches remains uninjured. Similarly, frost will occur
in the bottom of an inclosed valley but not on the surrounding slopes.
In the case of a valley the layer of cold air that forms at the bottom
is commonly deepened by additional cold air draining down from the
hills.

Many large orchards have their “warm spots” and their “cold islands” or
“north poles,” well known to the orchardist; due in some cases to the
nature of the soil rather than to topography. Certain mountain regions
in North Carolina are famous for their “thermal belts” or “verdant
zones”; i. e., areas part way up the slopes that escape the frosts
occurring both above and below them. These frostless belts, which have
been the subject of numerous investigations for three-quarters of a
century, seem to mark the upper level of the pool of cold air that
collects in the valley by drainage from the mountainsides. A detailed
temperature survey of the thermal belt region of North Carolina was
made during the years 1912-1916 by the United States Weather Bureau
and the North Carolina State Board of Agriculture. In some places the
minimum temperature at night was found to be 15 or 20 degrees higher in
the thermal belt than at the bottom of the valley, a few hundred feet
below.

Clouds, by checking radiation from the earth, and wind, by mixing
the colder and warmer layers of air together, both prevent frosts
that would otherwise occur. Artificial methods of protection include
covering plants with screens of wood, paper, or cloth, building smudge
fires to provide a blanket of smoke (a method of doubtful value),
and, above all, heating by means of wood fires or various types of
“orchard heater,” burning either oil or coal. An elaborate technique
of orchard heating has been developed, having in view especially the
most economical use of fuel and labor consistent with the object to be
attained. In many cases orchards are provided with alarm thermometers,
which ring a bell when the temperature approaches the danger point in
the orchard.

The local prediction of frost from the readings of meteorological
instruments is a problem that has not been fully solved. The idea
formerly prevailed that the temperature of the dew point, as determined
from readings of the dry-bulb and wet-bulb thermometers in the early
evening, was a safe guide to the fall of temperature to be expected
during the night, but this belief has not stood the test of accurate
observations. At the present writing certain formulas involving data
of both temperature and humidity are being used experimentally by
Weather Bureau specialists for predicting the lowest temperature of
the night when the general conditions indicate that frost is possible.
A comprehensive discussion of this subject has been published by
the Bureau as Supplement No. 16 of the “Monthly Weather Review.”
(Washington, 1920.)



CHAPTER XVI

COMMERCIAL METEOROLOGY


It is a significant fact that the American Meteorological Society,
which was organized in 1919, has a Committee on Commercial Meteorology.
The appointment of this committee was one of the earliest tokens of
the fact that the applications of meteorology to business, always
recognized to be important and far-reaching, had at last been
segregated as a distinct field of inquiry. The time is near at hand
when this field will have its corps of specialists and its textbooks.
Courses in commercial meteorology will be given in business colleges,
and meteorologists will be attached to the staffs of large business
enterprises. The chamber of commerce of a wide-awake western city
already maintains a Department of Meteorology, with a former Weather
Bureau official at its head, and “consulting meteorologists,” now
practicing their novel profession in other parts of the country, find
their principal clientele among business concerns.

Weather not only influences most kinds of business, but is the
foundation of many of them. Plenty of illustrations of the latter fact
will be found in every large department store. Umbrellas, rubbers,
and mackintoshes are made and sold because of rain; their best market
is in countries with rainy climates, and their sale from day to day
fluctuates with the state of the sky. Electric and palm-leaf fans
are a drug on the market or the reverse, according to the readings
of the thermometer. Sleds and ice skates are sold where and when the
weather is cold. This list may be prolonged _ad lib_. If we leave the
department store and walk along any business thoroughfare, we shall
discover other striking examples of commercial undertakings that owe
their existence chiefly or entirely to the weather. Abolish cold
weather and you abolish the dealer in furnaces and heating stoves,
besides reducing the rank of the coal-dealer considerably below the
“baronial” level. Eliminate hot weather from the meteorological program
and the ice dealer will likewise tumble from his high estate.

All this is so obvious that it seems hardly worth while setting down;
and yet the paradox must somehow be explained that business men
have not, in general, paid much attention to meteorology, and that
they have made only fragmentary use of the official meteorological
establishments that were created, in part, for their benefit. Probably
this paradoxical situation is merely a case of mental inertia. During
the long ages of traffic before there were any weather maps, scientific
weather forecasts, or climatic statistics, the weather was necessarily
an unknown quantity in the mathematics of buying and selling. That it
is not so to-day is a fact to which the business mind has been very
slow in adjusting itself.

We have mentioned some of the obvious relations of weather to commerce,
but there are others that are not so obvious. Many of these are
indirect. Thus the effects of the weather on agriculture are nearly
always reflected in the commercial world. It is not the farmer alone
who suffers from a prolonged drought, for example. It has been
asserted that every severe financial panic in our history has been
closely associated with a protracted period of deficient rainfall, and
that there has been no period of protracted drought without a severe
financial panic except one that occurred during the Civil War. Mr.
H. H. Clayton, who published this assertion in 1901, has cited the
case of the wheat crop as illustrating the magnitude of the effect
that rainfall exercises on economic conditions in general through its
effects on agriculture.

“If,” he says, “the amount of wheat raised in the United States were
reduced one-half or even one-third by a year of deficient rainfall, it
is easy to imagine an enormous strain on the business of the country,
and with a succession of such years the effect might mean disaster.
Such a deficiency in the wheat supply, with wheat at 80 cents a bushel,
would mean for a single year a direct loss in wealth of more than
$100,000,000; it would mean that nearly all the wheat which is usually
shipped abroad would be needed at home; it would mean that thousands of
railroad cars and ships which ordinarily transport this grain would lie
idle, that thousands of men who usually handle this grain in transport
would be out of employment, that farmers in large numbers would be
unable to meet their obligations, and consequently that banks and
business of all kinds would suffer.” Recent prices of wheat give added
force to these statements.

In contrast to such broad relations of the weather to business, it may
be interesting to point out certain relations which are of so special
a character that, although familiar to hosts of business men, they
have generally escaped the attention of writers on economics. On this
subject Mr. John Allen Murphy says:

“Retail sales are influenced tremendously by the weather. This is one
factor that makes it impossible for a retailer to equalize the peaks
and valleys in his sales chart. Favorable weather will bring him a
rush of business. A bad day will keep patrons from his store. There is
nothing he can do to prevent it. Many merchants have tried the plan of
offering ‘stormy day specials,’ but at best such a scheme is only a
makeshift that seldom works. The weather also affects the buying moods
of people. A dark, dreary day in summer seems to influence humans to
take on the same cast as the atmosphere. They are grouchy and hard
to please. On the other hand, a cold day in winter has the opposite
effect. The warmth and cheer of the store is such a pleasant contrast
to the out-of-doors that shoppers like to linger over the wares and
indulge themselves more readily in the luxury of buying.

“A stormy day or a series of them always helps the mail-order business.
In such weather people are inclined to stay at home. In passing the
time, they are likely to thumb the pages of such catalogues as they
have and thus see in them articles that they want. On the farm,
especially in the bleak days of winter, it is often the custom to order
garden seeds, incubators, tools, and many other things that will be
needed as soon as spring opens up. On a bad day traveling salesmen find
it easier to get the ear of a merchant. Not being busy with customers,
he is prone to be more lenient toward the ‘boys with the grips.’”

Another writer on this subject, Mr. F. C. Kelly, says:

“In a large city, the business of a department store is seriously hurt
by rain in the forenoon, but rain in the early afternoon is usually
a big help. Most customers of a big-city department store are women,
and nearly all of them live some distance from the store--at the edge
of the city or in the suburbs. If it rains along about eight or nine
o’clock in the morning, the woman who had planned to go shopping
that day is quite likely to change her mind, even though she did not
intend to go until afternoon. The rain not only suggests discomfort in
getting about, but diminishes her desire or immediate need for certain
articles, and drives the shopping idea out of her head. On the other
hand, if it is bright and clear in the morning, but clouds up about
noon for a heavy downpour which lasts most of the afternoon, it is
the best thing that could happen for the department store, because
shoppers get in and cannot comfortably get out. They shop all over the
store, buy luncheon there, and shop some more. While the rain is thus
helping the department stores, it may hurt the smaller shops, because
many customers who would otherwise look around are obliged to do their
buying all under one roof.”

No aspect of business more faithfully reflects the weather, or, in
a somewhat less degree, the weather forecasts, than advertising. So
important is it, in many lines of business, to make advertising fit
the weather that one might expect merchants to be, as a class, as
weather-wise as sailors and farmers. A page of advertising in a great
metropolitan newspaper is a costly investment. If, for example, it
invites the public to pay a Sunday visit of inspection to some haven
for homeseekers in the suburbs and Sunday turns out to be the kind
of day that converts building-lots into bogs, the advertiser will
perhaps be led to inquire whether there is not some means of avoiding
another such fiasco; and he may thus make the surprising discovery that
meteorology is not entirely a theoretical science. The conjunction of a
conspicuously advertised sale of rubbers and a soppy week-day morning
may be either a lucky accident or the result of studying the weather
map. In the former case, supposing the business to be conducted in
the northeastern United States, where dry weather is about twice as
common as wet, the odds would be two to one against the occurrence even
of light showers on the day the advertisement appeared, and three or
four to one against the occurrence of such weather as would make the
advertisement decidedly _à propos_.

These remarks about newspaper advertising are, to a great extent,
applicable also to the dressing of windows and the display of goods
inside the shop. In both cases a moderate amount of foresight in the
matter of weather will result in placing before the customer the right
goods at the right time. One of the minor ways in which the merchant
can turn the science of meteorology to advantage consists of using
meteorological instruments and the official weather forecasts and
bulletins for the purpose of attracting attention to his windows or
to his stock-in-trade. The drug-store thermometer is the illustration
of this process that comes first to one’s mind. There is no reason
why, with the progress of civilization, this celebrated instrument
should not be made a trustworthy index of temperature as well as an
effective advertisement. In continental Europe weather instruments are
displayed along with miscellaneous advertising matter in many of the
street “weather columns” (_Wettersäulen_), which furnished the idea of
the meteorological kiosks installed by the Weather Bureau in American
cities.

One of the most important branches of commercial meteorology relates
to the effects of weather upon transportation. This is a many-sided
subject. In the first place, the railway and steamship companies,
and other concerns engaged in the transport of goods and passengers
have their manifold weather problems, among which one may mention,
at random, that of dealing with the snow blockades of railways,
precautions against the skidding of taxicabs in wet weather, the
avoidance of iceberg-infested routes at sea, and the selection of
climatically favorable sites for aerodromes. The shipper has a somewhat
different, but overlapping set of weather problems.

“In the building of railroads,” says Mr. E. L. Wells, “many phases of
climate are to be considered, including the probability of floods,
deep snows, high winds, sand storms, etc. It is not long since a
considerable length of railroad line in one of the Western States
was found to be practically worthless because of having been built
too near the bed of a stream and therefore being too much subject to
damage from floods, so it was replaced by a line built higher up. The
writer remembers two railroads entering the same town in one of the
northern plains States, one of which is seldom blockaded, while the
other is sometimes closed by snow for months at a time. In the former
case the cuts are parallel to the wind, while in the latter the wind
blows directly across the cuts. In operating railroads a knowledge of
the climate is essential. This is particularly true in the shipment
of perishable products, which may require icing or ventilation as a
protection against high temperature, or insulation against cold.
Not only is a knowledge of climatic conditions essential in taking
precaution against loss in transportation, but weather records are
playing an increasingly large part in the settlement of claims for
products and property damaged in transit. The claim agents of the
leading transportation companies and the traffic managers of the
commission houses and producers’ associations keep complete files of
climatological data, and a large percentage of claims for damaged
goods, whether they be for a trainload of chilled bananas or for a
traveling man’s samples ruined by rain, are now settled out of court on
the basis of the weather records. Claims for car demurrage are often
settled on the basis of the weather reports.”

Detailed information concerning the effects of temperature on all sorts
of food products, both in transportation and in storage, was collected
by the Weather Bureau some years ago and published as the Bureau’s
Bulletin No. 13.

While the domestic shipper can easily obtain from the Weather Bureau
detailed climatic statistics for all parts of the United States, as
well as the current weather reports for this country, and can profit
greatly by regulating his operations in accordance therewith, it
is not quite so easy for the shipper to foreign markets to obtain
the corresponding data of foreign countries. With the expansion
of our foreign trade, the demand for such data has grown to large
proportions. The Weather Bureau, which has an unrivaled meteorological
and climatological library in Washington, is naturally the place
where such data are most frequently sought, but the labor entailed in
extracting and digesting the information in response to individual
requests is often too great to be undertaken by a Government office,
where the time of the employees is absorbed in routine duties. There
is, therefore, a promising field here for the private commercial
meteorologist. Unofficial work in this line is already carried on to
some extent. Thus a great steel company in New York has a salaried
“consulting geographer” on its staff, who advises on meteorological
questions. One of the problems he has been called upon to solve was to
determine the proper dates for shipping steel from Atlantic and Pacific
ports of the United States to various places in India, so that it would
never arrive during the monsoon rains. Records of current and very
recent weather in distant countries are, in a great majority of cases,
unobtainable anywhere in the United States. The interchange of detailed
weather reports between the different meteorological services of the
world involves in the first place, with few exceptions, a painfully
slow process of publication, and then distribution by mail; so that,
for example, records of observations at some places in South Africa
or Australia, or even many parts of Europe, do not reach the Weather
Bureau Library, in Washington, until two or three years after the
observations are made. Undoubtedly the time will come--and probably in
the near future--when there will be a world-wide exchange of weather
news by wireless telegraphy.

One kind of business wholly dependent upon the weather, which we have
not yet mentioned, is weather insurance. There are several kinds, but
hail insurance and tornado insurance are those extensively practiced;
the former much more widely and systematically in the Old World than
in the New, while the latter is confined to America. Insurance
against frost is said to have been practiced in Germany, and there
appears to be an excellent field for it in the United States. The
insurance of outdoor events, such as games, shows, and _al fresco_
parties, against rain has been carried on for a good many years by
speculative underwriters at Lloyd’s, in England, and has more recently
been undertaken in this country. Of course the weather element enters
to a considerable degree into other kinds of insurance. Ordinary
marine insurance is, to a large extent, insurance against storms; fire
insurance is partly insurance against lightning; window and plate glass
insurance involves the risk of breakage by wind and hail; and even life
insurance is greatly concerned with the effects of weather and climate.



CHAPTER XVII

MARINE METEOROLOGY


That it behooves a sailor to be weather-wise has always been admitted,
but there was a time, almost within the memory of men now living, when
neither seamen nor landsmen had the remotest conception of the benefits
that a systematic study of the meteorology of the sea was capable of
conferring upon the maritime world. The man who first grasped the
importance of such a study and translated his ideas into facts was the
American naval officer, Lieutenant Matthew Fontaine Maury.

During his brief career at sea Maury became impressed with the
meagerness of the information then available concerning the winds and
currents that aid or hinder the voyages of sailing ships. When, in
consequence of an accident that incapacitated him for shipboard duties,
he was assigned to service in Washington, he began to explore the old
logs of naval vessels, filed in the Navy Department, for notes on
meteorological conditions, and eventually developed a plan of securing
regular observations from both the Navy and the merchant marine. The
result of this undertaking was the publication of the famous Wind and
Current Charts, which revolutionized navigation throughout the world.

The practical value of these charts, of which 200,000 copies were
distributed to the masters of merchant vessels of all nationalities,
was promptly recognized. By taking advantage of the favorable winds
and currents shown on the charts, and avoiding those that were
unfavorable, mariners were able to reduce the average time of a sailing
voyage between the Atlantic and Pacific ports of the United States
by forty days. The money value of the charts to vessels sailing from
the United States to South America and the Far East was estimated at
$2,250,000 per annum. British shipping on all seas is said to have
benefited to the extent of $10,000,000 per annum. Neither was the
utility of the charts limited to the saving of money. The following
episode is cited in Maury’s biography:

“When the _San Francisco_, with hundreds of United States troops on
board, foundered in an Atlantic hurricane, and the rumor reached port
that she was in need of help, everyone looked to Maury as the only man
in the country who could tell where to find the drifting wreck. To him
the Secretary of the Navy sent for information. He at once set to work
and showed how the wind and currents acting upon a helpless wreck would
combine to drift her ‘just here’, pointing to a spot on the chart,
and making a cross mark with the blue pencil he had in his hand. Just
there the relief was sent, and just there the survivors of the wreck
were picked up. This was an incidental result of his study of winds and
currents.”

A further outcome of Maury’s enterprise was the holding, at his
suggestion, and by invitation of the United States Government, of an
International Maritime Conference, which met in Brussels in 1853,
and worked out a world-wide plan for meteorological observations at
sea. The work thus begun has since been carried on by the leading
maritime nations of the world. In the United States the duty of
gathering weather reports on a uniform plan from vessel masters has
been intrusted, at different times, to the Hydrographic Office, the
Signal Service, and the Weather Bureau. It is now performed by the
last-named institution, through its Marine Division, but the Pilot
Charts and books of sailing directions (“Pilots”) in which the compiled
information is published, are issued by the Hydrographic Office of the
Navy.

The modern successors of Maury’s Wind and Current Charts are,
especially, the Pilot Charts for the different oceans issued monthly
in Washington, and the monthly charts of similar character published
by the British and German governments. Apart from these periodical
publications, valuable collections of meteorological charts for oceans
or smaller marine areas have been published by the British, German,
Dutch, Indian, Japanese and other authorities.

The value of such publications has not been lessened by the gradual
substitution of steam for sails on ocean-going vessels. While wind is
no longer all-important, it is a factor in determining the speed, and
hence the earning capacity, of all classes of ocean shipping, and the
same is true of marine currents. Fog and drifting ice are, in general,
more serious obstacles to steamers than to sailing ships. A glance
at one of the Hydrographic Office Pilot Charts will suffice to show
that these publications are indispensable to the mariner. On these
charts we find, first of all, in the center of each five-degree square
of latitude and longitude a “wind rose” showing the frequency of the
winds that have been observed in that region from each of the cardinal
points, and their average force from each direction. On the charts
will also be found the routes recommended for full-power and low-power
steamers and sailing vessels, lines of magnetic variation, tracks of
storms in past years for the month in question, location and force of
currents, average limits and prevalence of fog for the month, recent
information about drifting ice and derelicts, descriptions of storm
signals, and an abundance of other information of vital importance to
the seaman.

[Illustration: OCEAN WEATHER MAP PREPARED FROM VESSEL REPORTS

JAN. 11, 1913, GREENWICH MEAN NOON

_U. S. Weather Bureau_

Solid black lines are isobars. Arrows fly with the wind, the center of
the arrowhead marking the position of the vessel, and the number of
feathers denoting the force of the wind on the Beaufort scale. Shading
of the head shows degree of cloudiness.]

Most of the material used in the preparation of the charts above
described is obtained from a great corps of volunteer marine observers,
who enter their observations at stated hours in forms provided for
the purpose and send these records to the establishment in charge
of the work at the end of each voyage. The forms furnished by the
United States Weather Bureau prescribe only one regular observation
a day, to be taken at Greenwich mean noon. Each observation shows
the position of the ship, the direction of the wind, the force of
the wind on the Beaufort scale, the height of the barometer, the
readings of the dry-bulb and wet-bulb thermometers, the temperature
of the water at the surface, the state of the weather, and the kind,
amount, and movement of clouds. In order to check the accuracy of the
barometric readings, the observer is instructed to read his barometer
at prescribed hours on three successive days when in port and send the
readings to the Weather Bureau. On receipt of these readings the Bureau
compares them with those of the nearest meteorological station, and
then mails the observer a “barometer tag,” showing the results of the
comparison and the error of his instrument. Besides keeping up these
routine observations, the observer keeps a record of fog encountered
at any hour of the day and makes detailed reports on storms. Many
marine observers also report observations at stated hours by wireless
telegraphy.

The enormous fund of information thus collected over the ocean is
applied to several purposes besides the construction of Pilot Charts.
Our Weather Bureau and certain foreign meteorological institutions
prepare daily charts, showing approximately the instantaneous
conditions over great oceanic areas, especially the North Atlantic.
These maps are analogous to the daily weather maps published for land
areas, but the drawing of each map is, necessarily, delayed for
several months after the date to which it refers, in order to allow
time for the receipt of as many reports as possible from ships at sea.
As a rule such charts are prepared in manuscript only, but, though they
cannot be distributed after the manner of ordinary weather maps, they
are valuable for studies in the institution itself on the movements
of storms and other atmospheric processes. They also enable the
meteorological officials to answer inquiries concerning the winds and
weather that have prevailed over a particular part of the ocean on any
specified date. Such inquiries come from vessel owners, underwriters,
and others, and the replies are frequently used as evidence in
admiralty suits.

In the case of one series of such maps--viz., the daily synoptic
charts of the North Atlantic, begun by Niels Hoffmeyer, of Copenhagen,
and now prepared jointly by the Danish Meteorological Institute and
the Deutsche Seewarte, in Hamburg--the charts have actually been
published and sold, though they are so costly that the number of sets
in libraries throughout the world is probably small. These remarkable
charts present daily pictures of the winds and barometric pressure over
the North Atlantic Ocean and the adjacent continents from 1873 to 1876,
and from 1880 down to a recent date.

From what we have already said it will be seen that the marine
observers cooperating with the United States Weather Bureau and kindred
institutions abroad are all contributing toward the great task of
recording the history of the weather over the oceans from day to day
and assembling data that can be digested in the form of marine climatic
statistics and used as the basis for many scientific investigations.
This concerted undertaking does not, however, constitute the whole
scope of marine meteorology. Every intelligent mariner finds it
necessary to acquaint himself with the laws of the winds, indications
of coming storms, means of determining the proximity of icebergs, the
systems of storm signals used in different countries, the method of
constructing weather maps from wireless bulletins, etc. He ought, in
short, to become an accomplished meteorologist.

One of the classic problems of the navigator is that of handling his
ship in a violent cyclonic storm, especially a tropical hurricane. The
reader will recall that a cyclone, besides traveling as a whole at a
rate of several hundred miles a day, consists of a system of winds
rotating around the center. The result of this double motion is that
the winds on one side of the center are not only more violent than
those on the other, but they are also so directed as to drive a vessel
running before the wind, across the storm track ahead of the advancing
center, while those on the other side tend to drive a vessel to the
rear of the storm. The two halves of the storm area are accordingly
known as the “dangerous” and “navigable” semicircles, respectively.
While this simple statement sets forth the fundamental facts involved,
the actual problem is complicated by many features, such as the fact
that the winds do not blow in circles, but more or less spirally, that
the area of the storm cannot be readily determined, that two storms may
occur in close proximity to each other, etc.

The accompanying diagram, published by the United States Hydrographic
Office, represents a cyclonic storm in the northern hemisphere, the
circles being isobars, or lines passing through places at which the
same barometric pressure prevails (indicated in inches), and the arrows
indicating the direction of the winds. The diagram is thus explained:

“For simplicity the area of low barometer is made perfectly circular
and the center is assumed to be ten points to the right of the
direction of the wind at all points within the disturbed area. Let
us assume that the center is advancing about north-northeast, in the
direction of the long arrow, shown in the heavy full line. The ship _a_
has the wind at east-northeast; she is to the left of the storm track,
or technically in the navigable semicircle. The ship _b_ has the wind
at east-southeast and is in the dangerous semicircle.***A vessel hove
to at the position marked _b_, and being passed by the storm center,
will occupy successive positions in regard to the center from _b_ to
_b_4, and will experience shifts of the wind, as shown by the arrows,
from east through south to southwest. On the other hand, if the storm
center be stationary or moving slowly and a vessel be overtaking it
along the line from _b_4 to _b_, the wind will back from southwest to
east, and is likely to convey an entirely wrong impression as to the
location and movement of the center. Hence it is recommended that a
vessel suspecting the approach of proximity of a cyclonic storm should
stop for a while until the path of the center is located by observing
the shifts of the wind and the behavior of the barometer.”

The movement of the winds around the storm center shown in this diagram
is that of cyclones of the northern hemisphere; i. e., contrary to the
direction of the clock hands. In the southern hemisphere they blow in
the opposite direction around the center.

[Illustration: NAVIGATION OF A SHIP IN A CYCLONIC STORM

(_U. S. Hydrographic Office._)]

By observing the rise or fall of the barometer, the shift of the
winds, and the state of the sea and sky, the experienced navigator
is generally able to lay down on a chart the approximate position of
the storm center and steer his vessel so as to avoid danger. Various
devices, known as “storm cards,” “cyclonoscopes,” etc., have been used
to aid in the process of locating a storm from shipboard observations.
In the Far East mariners use for locating typhoons an ingenious
combination of the storm card and the aneroid barometer, called the
“barocyclonometer,” an invention of the Rev. J. Algué, director of the
Philippine Weather Bureau.

The most important development in marine meteorology in recent years
has been the rapidly increasing use of radiotelegraphy, both by marine
observers in transmitting reports of observations to shore and to other
ships, and by meteorological institutions in issuing weather bulletins
and storm warnings to vessels at sea. The first regular undertaking
in this line was carried out under the auspices of the London _Daily
Telegraph_ in the year 1904. This newspaper arranged with some of
the leading transatlantic steamship lines to furnish weather reports
by wireless from their vessels, and these reports were published in
its columns for several months. The following year the United States
Weather Bureau began, in a tentative way, the collection of wireless
weather reports from off-shore vessels, and similar undertakings were
soon afterward launched in other parts of the world, but for some years
such reports were of little practical value, owing to the limited range
of wireless communication.

[Illustration: A SHIPBOARD WEATHER MAP

Vessels off the American coast can make their own weather maps every
night, using data supplied at fixed hours by high-power radio stations
(shown by stars on the map), together with radio reports from other
vessels. Letters near stations are the code letters used in wireless
bulletins to describe the stations. Vessel reports are indicated by
names. Arrows show direction of wind and the force on the Beaufort
scale (shown by the number of feathers). Besides data for constructing
maps, the radio stations issue forecasts and storm reports for each of
the numbered zones shown off the Atlantic coast and over the Gulf.]

At the present time wireless reports from ships on the Atlantic enable
the forecasters on both sides of that ocean to extend the areas of
the weather maps on which their predictions are based, and reports
from ships are also received to a limited extent by forecasters on our
Pacific coast as well as in the Far East, India, and elsewhere. In
this country such reports have been especially valuable in indicating
the movements of West India hurricanes, and thus have helped to solve
the problem of protecting the vast tonnage that has been attracted
to Caribbean waters by the opening of the Panama Canal. The reciprocal
process of transmitting weather intelligence to vessels by wireless
bulletins, broadcasted at certain hours every day by high-powered
radio stations, has made much more progress. Such bulletins include
information concerning the current and prospective weather, winds and
storms over specified ocean areas, as well as reports of observations
made at a number of land stations, from which it is possible for
vessels at sea to construct their own weather maps. They are thus
enabled to take advantage of favorable winds and to avoid unfavorable
winds and storms. Wireless weather reports from other vessels help to
piece out these shipboard maps.

The meteorological services of all civilized countries adjacent to
the sea display signals along their coasts to announce the coming of
storms dangerous to navigation. One of the earliest devices used for
this purpose was the “aeroclinoscope,” a form of semaphore formerly
employed by the meteorological service of Holland. The position of the
arm of the semaphore indicated the region in which the barometer was
low; i. e., the storm center. In the British Isles, in the middle of
the last century, Admiral FitzRoy introduced the use of canvas cones
and “drums” (i. e., cylinders), which, seen from any direction, have
the appearance of solid triangles and squares against the background of
the sky. The British later abandoned the drum and used the cone only,
pointing up or down for northerly or southerly gales, respectively.
The American storm flag--red with a square, black center--was adopted
by the United States Signal Service (the predecessor of the Weather
Bureau) in 1871. This signal was subsequently amplified by the addition
of red and white pennants to show the expected direction of the wind
at the beginning of the approaching storm. Most countries use lanterns
for night storm signals. In the year 1909 a uniform system of signals,
consisting of cones by day and lanterns by night, was recommended for
use in all countries by an international commission which met in London.

In spite of this recommendation some thirty or forty different systems
of daytime storm signals are now in use in different parts of the
world. On the China coast an elaborate system of signals, consisting of
cones, balls, diamonds, and squares displayed on a mast and yardarms,
indicates the existence of a typhoon anywhere in the neighboring seas,
together with its location and movement.



CHAPTER XVIII

AERONAUTICAL METEOROLOGY


During the great war the British Government decided, in its wisdom,
to establish a flying field in Scotland, at which aviators were to be
trained in dropping bombs. The commission having this matter in hand
chose a site on the shores of Loch Doon. In laying out the field a
bog had to be drained; then a railway was constructed, hangars were
erected, and other operations were carried out, entailing altogether an
expenditure of half a million pounds. At a certain late stage in the
proceedings the disconcerting discovery was made that the field could
never be successfully used for the purpose intended, on account of the
gusts and eddies produced by the surrounding hills. The undertaking
was therefore abandoned. The authorities had presumably enlisted the
skill of engineers from the outset of the work--but they had failed to
consult a meteorologist!

A few such object lessons seem to be necessary to demonstrate the
fact--which ought to be obvious--that meteorology is an indispensable
and vital adjunct of aeronautics. This fact is now pretty well
understood. Nearly all the activities of mankind are more or
less influenced by weather, but few, if any, to such an extent
as aeronautical enterprises. Hence a definite branch of applied
science--Aeronautical Meteorology--is rapidly taking shape. Already it
enters into the curriculum of aeronauts; it has profoundly modified the
methods of the ordinary meteorological services of the world; and it
is raising a crop of specialists, some of whom are now employed by the
business firms that manufacture or operate aircraft.

The statement has constantly been made since the war that aeronauts
are becoming “independent” of the weather. This statement has a grain
of truth in it, but no more. It is a fact that, under war conditions,
aviators flew in every sort of weather, and often with impunity. Even
since the war commercial aircraft have negotiated adverse atmospheric
conditions with remarkable success. A spectacular feat of this sort was
achieved on August 28, 1919, when a passenger-carrying aeroplane on the
Paris-London route, piloted by Lieutenant Shaw, flew over this route
through a hurricane blowing in gusts of from 40 to over 100 miles an
hour, accompanied by a torrential rainstorm and such poor visibility
that the pilot was frequently obliged to fly very low in order to pick
up his landmarks and make sure that he was on his course. The flight
was accomplished in 1 hour and 50 minutes--about half an hour less than
schedule time. It is said that “the two passengers in the cabin of the
machine emerged without any appearance at all of strain”--such as they
certainly would have experienced if they had made the crossing by the
Channel steamer on that boisterous day. In fact the land and sea route
was seriously disorganized by the storm, and the Continental trains
were arriving in London hours late.

Lest hasty conclusions should be drawn from this episode it should be
stated at once that the company operating the air route in question,
far from considering itself independent of weather, is not content
with the detailed bulletins furnished to aeronauts by the British
Meteorological Office (which specializes in aeronautical meteorology
more extensively than any other official weather service in the world),
but maintains an elaborate weather service of its own, with an able
meteorologist at the head of it.

An accurate statement of the situation would be that wind and weather
are no longer the grave dangers that they once were to the aeronaut;
but they are still, and will probably always be, factors of the utmost
importance in the successful and profitable operation of aircraft. In
order to make this matter plain it will be necessary for us, first
of all, to devote a few words to some of the fundamental principles
involved in aerial navigation.

The layman who sees nothing mysterious in the ascent of a balloon is,
in general, somewhat puzzled by the phenomenon of a heavier-than-air
machine rising from the ground. Yet, in both cases, the ascent of the
vehicle depends upon the fact that air is not just empty space, but a
material substance, possessing density, weight, and other properties
many of which pertain also to solids. A balloon rises not because
it is light, but because the air about it is heavy. In other words,
gravity pushes the air under the balloon more forcibly than it pulls
the balloon downward. The ability of an aeroplane to leave the ground
depends upon the fact that air offers resistance to bodies moving
through it.

[Illustration:

        A                                B
        |                                   /
        |                                  /
        |                                 /
  ------|------->                 -------/------>
        |                               /
        |                              /
        |                             /

THE EFFECT OF AIR RESISTANCE ON AN AEROPLANE]

Suppose a vertical plane (A)--such, for example, as the wind shield
of an automobile--is moving horizontally through still air. The
resistance of the air impedes its motion, and a part of the motive
power is employed in overcoming this resistance. Now, suppose the
plane (B) is nearly, but not quite, horizontal, and is propelled by a
force tending to make it move in the direction indicated by the arrow.
This is approximately the case of an aeroplane driven by a motor; the
plane representing the wings of the machine. Only a part of the air’s
resistance is now effective in impeding the forward motion of the
plane. The rest of it pushes the plane upward. If you hold your hand
at such an angle and move it through water you will feel an analogous
upward push. Moreover, you will notice that the faster you move your
hand the greater is the push. Not only does this upward pressure of
a fluid upon an inclined plane moving through it vary with the speed
of the latter (to be exact, as the square of the speed), but it also
varies with the angle which the plane presents to the fluid in its
path. If the wing of an aeroplane, for example, cuts the air nearly
edgewise, the upward pressure will be slight. As it departs from an
edgewise position, (with the front edge higher than the rear), the
upward pressure increases, but not indefinitely; beyond a certain
rather small angle it begins to diminish.

In an aeroplane the upward pressure, or “lift,” is increased by giving
the wings a slightly arched shape, or “camber.” The air flows over the
arched wings in such a way as to produce a suction above them which
helps the push from below. The actual amount of lift for a given speed
has been determined by experiments for wings of various shapes and
sizes and set at various angles to the line of motion. If, when the
machine is in the air, the lift is just sufficient to counterbalance
the weight of the aeroplane, the latter flies horizontally. An increase
in lift causes the machine to rise; a decrease in lift permits gravity
to pull it down.

Now suppose the aviator is flying horizontally and wishes to climb.
At the rear of the machine and forming part of its tail is a hinged
horizontal flap called the “elevator,” under the control of the pilot.
By giving this flap an upward tilt he causes the air to exert a
downward pressure on the tail of the machine, and hence the nose of the
machine is carried upward. While the inertia of the aeroplane tends to
carry it along the original path, its wings now present a greater angle
to the air, the lift is increased, and the machine rises. The reverse
of this operation will cause the machine to descend.

[Illustration: THE BED OF POTOMAC RIVER, AT WASHINGTON. From an
altitude of a few hundred or a few thousand feet, submarine features
are clearly revealed to great depths. Objects have thus been
photographed 45 feet under water. The shoals are submerged to a depth
of from 2 to 5 feet. In favorable weather, aerial photographs are
valuable in making hydrographic surveys. (_Photographed from the air by
Dr. W. T. Lee, U. S. Geological Survey._)]

A vertically hinged flap in the tail, acting on exactly the same
principle as the rudder of a ship, enables the pilot to turn
horizontally. Two or more small horizontal flaps, known as “ailerons,”
attached to the wings, are used to preserve the lateral balance of the
machine, and to give it the proper “bank,” or inclination, when making
a turn.

[Illustration: DRILLING WITH COMPRESSED AIR IN A COPPER MINE. The
drill also forces a stream of water into the hole to lay the dangerous
sulphur-bearing dust. (_Courtesy Sullivan Machinery Co._)]

[Illustration: LAUNCHING A WEATHER BUREAU KITE FROM THE “SENECA” DURING
THE INTERNATIONAL ICE PATROL, TO EXPLORE THE AIR OVER THE OCEAN.

(_Photograph, U. S. Weather Bureau._)]

With these few details in mind, we shall be prepared to consider, in a
general way, how the behavior of an aeroplane is affected by the wind
and other atmospheric phenomena.

With respect to wind there is an important difference between aircraft
and marine craft. Mere strength of wind is not dangerous to an
aeroplane, except when starting or landing. An aviator flying above the
clouds, with no landmarks in sight by which to gauge his movements,
is no more conscious of the actual wind at that level, provided it is
steady, than he is of the rotation of the earth on its axis. He feels
the wind produced by the motion of his machine through the air--the
so-called “relative wind”--but no other. The true wind may be a mere
zephyr, or a hurricane blowing 150 miles an hour; the effect is the
same on his machine, so far as he is able to observe. On the other
hand, a strong wind has a very different effect from a light one
upon the course of the aeroplane’s flight with respect to the ground
beneath. If a pilot, with no landmarks to guide him, steers by compass
for a certain point, and if there is a strong cross-wind of which he is
unaware, he will be carried far out of his course; a wind dead ahead
or astern will merely affect the speed of his flight, so that he will
arrive later or sooner at his destination than he expected.

One of the important problems of aeronautics, especially from the
commercial point of view, is to prevent aircraft from being driven
off their course by the wind when flying with no visible landmarks;
i. e., over clouds, fog, the ocean, or an unmapped country. When this
problem is solved, pilots will fly above the clouds much more commonly
than they do now. The winds at high levels are generally both steadier
and stronger than at low. The stronger wind is an advantage or a
disadvantage, according to whether it is blowing in the direction of
flight or the reverse; but as the winds at different levels generally
blow in different directions, a pilot who is independent of landmarks
can choose whatever level affords the winds most favorable for his
intended journey.

Over established air routes quite elaborate measures are now adopted
to keep pilots informed of the direction and speed of the wind at
different levels, so that they can make due allowance for this factor
in shaping their course. In clear weather this information is easily
obtained by sighting the drift of a pilot balloon with a theodolite,
or by observing in a specially designed graduated mirror or pair
of mirrors the drift of the smoke cloud from a shell fired by an
anti-aircraft gun and timed to burst at any desired altitude. In cloudy
weather the smoke trails can often be successfully observed through
small breaks in the clouds. When the sky is completely overcast, a
succession of shells is fired at definite short intervals of time and
the distances apart of the puffs of smoke and the direction of the
line in which they lie are determined from an aeroplane flying above
the spot. Another method, which was devised by the French military
meteorological service during the war, is to send up small balloons
loaded with bombs which burst after a certain time, the position of
each burst being determined by sound-ranging from the ground.

These methods of providing information concerning the winds at flying
levels have, however, their serious limitations, and aeronauts now look
hopefully to the perfection of the existing systems of “directional
wireless,” whereby the pilot will receive whenever desired, or at
regular intervals, a wireless signal from the terminus of his route or
some other known point, the direction from which the signal comes being
indicated by suitable apparatus on the aeroplane. Thus aided, he should
never deviate far from his course, unless he chooses to.

For long journeys, such as the crossing of the Atlantic, the air pilot
will naturally make use of all available information concerning the
great permanent or semipermanent wind systems of the earth, such as the
trade winds of the lower atmosphere, the antitrades above them, and
the fairly constant eastward drift of the atmosphere at high levels
in middle latitudes. The dividend-earning capacity of commercial
aircraft necessarily depends upon taking advantage of favorable
winds, while adverse winds may mean not only a loss of money but the
danger of prolonging a journey until the fuel supply is exhausted--a
serious predicament over the ocean and also over lands remote from
civilization. It is, however, a common error on the part of current
writers to overrate the constancy and reliability of the winds in
various parts of the world, and to lay too much stress on the value
of permanent wind charts. What the aeronaut needs especially to know
is the typical behavior of the winds with respect to the distribution
of barometric pressure, as shown by a weather map, including their
variations with altitude. The time will come when the information
necessary for plotting the winds at various levels will be flashed at
frequent intervals by high-powered radio stations to aerial navigators
in all parts of the world--a system that is already in its initial
stages, especially in Europe. A pilot making a long journey will thus
be able to lay his course so as to utilize the winds that will speed
him on his way. Even violent storms, such as the mariner seeks to
avoid, will be turned to advantage by the airman.

We have now to consider another aspect of wind that is of much more
interest to the airman than to the seaman, and that is the question
of “wind structure.” The layman usually thinks of a wind as a nearly
steady horizontal flow of air. Such winds exist, but they are
exceptional, especially in the lower levels of the atmosphere. A wind
is generally full of gusts and eddies, upcurrents and downcurrents,
and it is these eccentricities that gradually develop in the aviator
a sort of sixth sense--a “feel” for atmospheric fluctuations, that
enables him to adjust his machine instinctively to the forces tending
to disturb its equilibrium. He also learns by experience the conditions
under which irregularities of a pronounced character may be expected.
He becomes well acquainted with the great mound of air that drives his
machine upward when passing over a hill or mountain; with the eddy that
lurks in the lee of such an obstacle; with the downward tendency of the
air over lakes, rivers, swamps and forests.

“The air is so sensitive,” writes the late well-known British flyer,
Gustav Hamel, “that it is affected even by the color of large patches
of vegetation. Whether this be entirely due to the different
heat-radiating power of different colors it is impossible to say, but
invariably an aeroplane on passing from grass land to a field covered
with yellow flowers experiences a certain amount of air disturbances
only less noticeable than the inevitable bump experienced in passing
from green fields to ploughed land, or from ploughed land to meadow.”

When the wind is blowing, the air for at least a few hundred feet above
the ground is nearly always in a state of turmoil. This is partly due
to the friction of the moving fluid against the irregular surface of
the earth, and partly to the ascending and descending currents caused
by differences in temperature. The latter effect is illustrated in
the rapid rise of air over a bare sunlit plain and its fall over an
adjacent forest or body of water. Ascending currents are often made
visible by the formation of detached cumulus clouds, each of which
marks the summit of a rising column of moist air, while in the spaces
between the clouds the air is generally sinking. Measurements with
balloons have shown that vertical currents often attain speeds of 600
feet a minute or more, while the process of hail formation appears
to indicate that in thunderstorm clouds there are violent uprushes
amounting to 2,000 or 2,500 feet a minute, and possibly much more. The
descending air current between clouds is sometimes so strong that an
aeroplane cannot force its way up through it.

[Illustration: THE FLOW OF AIR OVER TWO RIDGES

(_After Dr. Franz Linke._)

Notice the eddy in the valley to the leeward of the first ridge]

[Illustration: A SHELTERED LANDING PLACE MAY BE DANGEROUS

(_After Dr. Franz Linke._)

A landing place surrounded by trees is dangerous in windy weather on
account of the air waves formed between the moving air above and the
calm air below.]

[Illustration: AIR WAVES NEAR THE EARTH’S SURFACE

The waves are made visible by smoke]

The turbulence of the lower air--a phenomenon that adds so much to
the difficulties of starting and landing--extends to various heights,
depending especially upon the strength of the wind. A rough rule,
evolved by the Zeppelin pilots before the war, was to expect turbulent
conditions up to an altitude equal to from ten to twenty times the
force of the wind in meters per second. Thus, for a wind of 10 meters
per second, the turbulent layer would be from 100 to 200 meters thick.
A good picture of the atmospheric ups and downs encountered by the
airman when flying low is furnished by the behavior of the smoke from
a factory chimney with a moderate wind blowing, forming smoke waves.

[Illustration: THE WIND’S AUTOGRAPH ON A GUSTY DAY, RECORDED WITH A
PRESSURE-TUBE ANEMOMETER

The vertical lines are hour lines and the horizontal lines show the
force of the wind in miles an hour and also in pounds a square foot.]

These disturbances give rise to the very marked fluctuations in the
force of the wind known as gusts. There are certain forms of anemometer
especially designed to record the gustiness of the wind. A record of
the wind’s force is traced by a pen on a moving strip of paper, and the
“anemogram” thus obtained shows a continuous series of irregularities,
the extent of which increases with the strength of the wind. The puffs
and lulls often alternate at intervals of a few seconds or less, and
the actual force of the wind at a given instant may be many times
greater than its average force for, say, five minutes. An ordinary
anemometer does not indicate these rapid fluctuations, but merely shows
the time required for a mile of wind to flow past the instrument. Thus
when such an instrument tells us that the wind is blowing at the rate
of 40 miles an hour, it may actually be varying between 20 and 60
miles an hour, or between even wider limits.

Since the matter became of practical importance on account of the
needs of aviation, many interesting studies have been made of the
effects of different kinds of topography upon the overlying air
currents. A striking example of the eccentric winds that sometimes
prevail in mountain valleys has been described by Mr. B. M. Varney,
of the University of California, in the “Monthly Weather Review.”
From the summit of a steep cliff about 1,100 feet above the floor of
Yosemite Valley the writer launched broad sheets of tissue paper, and,
with the aid of powerful binoculars, followed their flight as they
were carried in huge spirals, thousands of feet in diameter, finally
disappearing beyond the mountains on the opposite side of the valley.
The accompanying sketch shows the path of one of these papers. From its
starting point at _A_ until it passed behind the summit of Liberty Cap
(_B_), more than a mile distant, the paper was watched for 7 minutes.
The top of Liberty Cap is some 1,600 feet above the point at which the
flight began. This sketch visualizes one of the ticklish problems that
will some day confront the pilot of a sight-seeing or mail-carrying
aeroplane in the Yosemite National Park.

[Illustration: AIR CURRENTS IN YOSEMITE VALLEY

(_Sketched by B. M. Varney._)

The flight of a sheet of paper across the valley.]

Although, on an average, the air is much steadier at high levels than
near the ground, very unsteady currents are sometimes found at all
altitudes attainable by aircraft. Thunderclouds, thousands of feet
above the earth, are always the seat of violent turmoil, but such
clouds can, as a rule, be avoided by the airman. When a stratum of air
glides over another differing sharply from it in density--and distinct
strata of this sort are not uncommon in the atmosphere--friction
between the strata sets up waves like those produced in water by wind
blowing over it. If the two streams are moving in the same direction,
but at different speeds, the waves are long and regular; when they are
more or less crossed, the waves are short and choppy. The moisture
at the crests of these waves may be cooled to such an extent as to
condense into visible clouds, arranged in long continuous rolls or rows
of detached patches; forms frequently assumed by cirro-cumulus and
alto-cumulus. More often, however, the waves of air remain invisible,
because the conditions of moisture and temperature are not right for
the production of cloud.

Recalling, now, what has been said above about the way in which the
lift of an aeroplane varies with the angle at which the wings meet
the air and also with the speed of the machine relative to the air,
it will be easy to understand some of the difficulties experienced in
maintaining one’s equilibrium when flying in a turbulent atmosphere.
Waves, eddies, vertical currents and other features of wind structure
cause abrupt changes in the attitude and the speed of the machine with
respect to the air stream. The sudden increases and decreases of lift
thus produced have much the same effect upon the machine as if it
were running over a solid obstacle on the one hand or plunging into
a vacuous space in the atmosphere on the other, and hence are aptly
described by aviators as “bumps” and “holes in the air,” respectively.
The latter term, which seems to have become firmly rooted in all
languages (French, _trou d’air_; German, _Luftloch_; etc.), has had
the unfortunate effect of keeping alive in the public mind the idea
that the aviator occasionally runs into a vacuum or semivacuum, such
as could not exist in the atmosphere. (The nearest approach to such a
thing is the rarefaction in the core of a tornado or waterspout, due to
the enormous centrifugal force of the vortex; something that no aviator
has yet encountered.)

To make matters worse, different parts of the sustaining surface of
the machine may receive different impulses. One wing, for example, may
graze a violent uprush of air not encountered by the other, giving
the aeroplane a tilt to one side, or the tail of the machine may be
driven in one direction and the nose in the other. Again, the whole
machine may suddenly enter an air current of quite different speed and
direction from the one in which it has been flying. To take an extreme
case, it may run into a stream of air flowing just as fast, and in
the same direction, as the machine itself, with the result that the
_relative_ wind becomes zero, and the machine, deprived of all lift,
drops like a stone until it acquires a velocity with respect to its new
environment.

When such conditions prevail, the pilot is kept busy with his
“controls”; now moving his elevator to adjust his fore-and-aft balance,
and now his ailerons to set him on an even keel laterally, and
occasionally turning his rudder to offset the effects of horizontal
gusts. The elevator and the ailerons are worked with a single lever,
colloquially called the “joy-stick,” and the rudder with a bar which
the pilot operates with his feet. Ordinary adjustments of this kind
are performed automatically by the trained aviator, but violent
disturbances call for the exercise of skill and judgment. Generally
speaking, no amount of atmospheric turbulence causes any serious
trouble to the trained pilot, except when he is flying close to the
ground, as in starting and landing.

Before we leave the subject of wind it will be well to emphasize once
more the fact, which the average layman has difficulty in grasping,
that the only movements of the air that affect the safety and comfort
of flight are the movements relative to the machine, and not those
relative to the ground. To the aviator, when he is once clear of the
ground, a steady wind of any speed is merely a mass of calm air. Hence
an aviator will sometimes have perfectly smooth flying when the wind,
as measured on the earth, is blowing 40 or 50 miles an hour; and again
he will describe the air as rough and bumpy when flags are hanging
limp from their staffs and dwellers on _terra firma_ declare that not
a breath of air is stirring. In the early days of flying aviators
themselves were afraid of a strong wind. Thus Wilbur Wright, during
his pioneer exhibition flights in France, would never go up unless the
smoke from his cigarette rose in a straight line, and until about the
end of 1909 no aviator attempted to fly in a wind of 20 miles an hour.

At the present time the only atmospheric condition that seriously
hampers flying is fog or low cloud. An aviator flying in a fog or cloud
is not only liable to wander far from his course, on account of the
unknown leeway of his machine, but he is often in great doubt as to his
proximity to the ground. One of the curious effects of such a situation
is that the airman loses his sense of the vertical. On land our sense
of up and down is determined by the force of gravity, pulling us toward
the earth. When riding in a terrestrial vehicle, we are conscious of
other pushes and pulls; such, for example, as the jolt that pitches
us forward when a train stops suddenly, or the outward thrust that we
feel when swinging around a curve. Again, in descending in a lift we
seem to lose weight, as if gravity had suddenly grown weaker. On earth
all such impressions are corrected by the sight of objects around us;
but the aviator enveloped in mist has no such guides, and he often
becomes quite confused about the direction of the ground. A turn, which
involves banking, increases his confusion. Eventually he may be flying
almost upside down without being aware of the fact. Professor Melville
Jones, who has been through such experiences, says of the pilot’s
confusion:

“His first indication that something is wrong is, as a rule, either
an increase or a decrease of speed that is not counteracted by the
accustomed movements of the controls. A period of wild suspense and
utter bewilderment now follows, during which the pilot makes violent
efforts to recover control, but without success. The next thing that he
realizes, if he realizes anything at all, is that he is either on his
back or spinning, and the next thing he knows is that he is out of the
clouds with the earth standing up at a ridiculous angle and spinning
round like a drunken dinner plate. Happy is he that has plenty of air
room under these circumstances.”

Spirit-levels and similar instruments are affected by the same
disturbances that mislead the pilot in his estimation of the vertical;
but fortunately there are certain other devices, due to the exigencies
of the war, during which cloud flying was a part of the tactics of the
military aviator, which have virtually solved this problem, though
their use has not yet become general.

The outstanding difficulty of a fog is the problem of landing. In the
case of a forced landing, at a distance from a regular landing-ground,
the pilot must simply trust to luck. He may descend in the water or the
treetops, or on rough ground that will wreck his machine, but he has no
choice. The only solution of this difficulty is the installation of a
reserve engine, or some other expedient that will obviate the necessity
of forced landings. The task of finding a landing ground in a fog and
descending to it in safety will, in the near future, be comparatively
simple. Most fogs, though by no means all, are so shallow that it is
possible to tether a kite-balloon so that it will float above the fog
and indicate the position of the aerial harbor. Several such balloons,
flying tandem, would afford sufficient lift to support a series of
electric lanterns along the cable, for use at night. Searchlights and
“star shells” have been employed for the same purpose. Directional
wireless and the wireless telephone seem likely, however, to be the
chief dependence of the future aeronaut seeking port in a fog. These
devices will also be the means of averting collisions in a fog or cloud
along crowded airways, and especially in the congestion that will
prevail in the vicinity of important air ports. Last but not least,
the artificial dispersion of fog by means of electrical discharges,
although still in the experimental stage, holds out possibilities
of being the ultimate solution of the fog problem, not only for the
aeronaut, but also for the mariner, the railway manager, and everybody
else who is incommoded by a misty atmosphere.

Even when he is not flying in clouds or fog the aviator by no means
always enjoys a clear view of distant objects. A slight haze impairs
visibility, while a heavy rainstorm or snowstorm may obstruct the
aeronaut’s view as badly as a fog.

Of the meteorological elements that affect aeronautics, other than
those we have mentioned, the most important is the density of the
atmosphere--generally expressed in terms of barometric pressure. The
air diminishes in density upward, and the rarefied atmosphere of high
levels has several effects on aircraft. Its decreased buoyancy imposes
a limit upon the ascent of balloons; its decreased resistance makes
it necessary for an aeroplane to fly at greater speed in order to get
the same lift; it diminishes the power of gasoline engines, on account
of the reduced supply of air; and it has various unpleasant and even
dangerous effects on the aeronaut, similar to “mountain sickness.” The
level that a given aeroplane cannot exceed owing to the combined effect
of reduced lift and reduced engine power is known as its “ceiling.”
Different types of aeroplane have very different ceilings.

At great altitudes the air is always very cold, summer and winter.
The low temperature may interfere with the efficient working of the
engine, and it is, of course, a source of discomfort to the pilot. The
formation of ice and heavy deposits of snow lead to inconveniences
in both aeroplanes and airships. The pelting of hail is sometimes a
painful experience for aeronauts. Lastly, lightning has hitherto left
aviators unscathed, but has caused numerous disasters among balloonists.

The recent rapid development of aeronautics has laid a heavy burden of
additional labor upon the meteorological services of the world, and
is producing something like a revolution in their methods. The history
of these changes is interesting. From the beginning of the twentieth
century until a few years before the World War meteorologists were
engaged in a great campaign of upper-air research, utilizing kites,
captive balloons, pilot balloons, and sounding balloons to measure
the winds, temperature, humidity and pressure at various levels in
the atmosphere. In other words, aeronautical methods were employed in
the service of meteorology, but the investigators hardly entertained
the idea of reversing the relation and making meteorology the
handmaiden of aeronautics. The point of view prevailing in those days
is well indicated by the fact that the organization that had charge
of the upper-air explorations throughout the world was known as the
“International Commission for Scientific Aeronautics,” a name that it
bore until the year 1919.

The plan for providing regular weather reports for the benefit
of aeronauts began with some small-scale enterprises in Germany
about 1909. In the summer of that year Dr. Franz Linke organized a
storm-warning service in connection with the International Aeronautical
Exposition at Frankfort, and at the beginning of the year 1911 an
aeronautical weather bureau for the whole of Germany was established,
with headquarters at the Observatory of Lindenberg. Shortly before the
war a similar undertaking was launched in Italy, under Dr. Matteucci,
whose service was the first one in the world to publish daily charts,
based on telegraphic reports, of the winds at various levels over an
entire country.

During the war the regular meteorological services of the belligerent
countries and the meteorological units attached to the armies and
navies maintained an almost continuous service of weather information
for the great fleets of fighting aircraft. Bulletins, distributed
chiefly by wireless telegraphy, supplied particulars of the current
and prospective winds at the flying levels, the prevalence of fog,
the degree of visibility, etc. New telegraphic weather codes, far
more elaborate than those in use before the war, were devised for
transmitting such information, and the whole business of observing and
reporting weather became immensely more arduous than it had been in the
days when the only interests served by practical meteorology were those
of the land and the water.

Since the close of hostilities great efforts have been made to maintain
these new operations of the meteorological establishments at something
like the level attained during the war. The task is, however, beset
with difficulties, on account of the great expense involved. It is
being accomplished with different degrees of success in different
countries.



CHAPTER XIX

MILITARY METEOROLOGY


One of the most astonishing paradoxes connected with the misapplication
of human brains and energy glorified with the name of the “art of war”
is this--that, while weather has always played an important part, and
often a decisive one, in military operations, no attempt was ever made
until a few years ago to include meteorology in the purview of military
science or to utilize the services of meteorologists at the battle
front.

The most casual survey of the history of warfare reveals the fact that
atmospheric conditions rank high among the “controls” of fighting. From
a military point of view, weather and climate bear a certain analogy to
topography. They are a part of the physical environment with which a
commander has to reckon. Weather, however, differs from topography in
the fact that it is subject to rapid changes, and is therefore doubly
worthy of attention on the part of an army, which must not only take
account of the weather as observed and in progress, but must also, as
far as possible, anticipate that which is to follow.

Everybody will recall the ruin that overtook the French army in Russia
in 1812 on account of untoward weather conditions, but it is less
well known that Napoleon, with his usual sagacity, obtained from his
scientific advisers a report on the Russian climate before he planned
his campaign; that the winter set in much earlier than usual in that
fatal year; and, most interesting of all, that it was actually a brief
period of thawing weather, rather than the intense cold that preceded
and followed it, which, by turning the roads into bogs and breaking up
the ice in the Beresina, brought about the culminating disaster.

Another fateful spell of weather ushered in the battle of Waterloo.
It is described in a well-known passage of “Les Misérables,” which
contains enough truth mingled with hyperbole to be worth quoting:

“S’il n’avait pas plu dans la nuit du 17 au 18 janvier, 1815, l’avenir
de l’Europe était changé. Quelques gouttes de plus ou de moins out
fait pencher Napoléon. Pour que Waterloo fût la fin d’Austerlitz, la
Providence n’a eu besoin que d’un peu de pluie, et un nuage traversant
le ciel à contre-sens de la saison a suffi pour l’écroulement d’un
monde.”

The rains and floods that led to the annihilation of the Roman legions
under Varus in A. D. 9 and the great tempests that helped English
seamen defeat the Spanish Armada furnish additional well-known examples
of the immense importance of weather as a factor in warfare. We need
not, however, look farther back than to the recent world conflict
to find similar examples in profusion. Leaving out of consideration
the indirect effects of the weather upon the progress of the war as
exercised through its control of crops, transportation, and other
features in the economic life of the belligerent and neutral nations,
we need only examine war-time newspapers to see how the armies
themselves were helped or harassed by meteorological conditions at
every turn. The war was a great popular teacher of climatography,
just as it was of geography. The drenching misery of Flemish winters,
as formidable to the soldiers in the trenches as the bullets of the
enemy, became as familiar to the present generation of Americans as did
somewhat similar conditions in Virginia to Americans of the Civil War
period.

The British campaigns in Mesopotamia were as much a conflict with
climate as with human foes. Marches were made when the temperature
stood at 110 degrees Fahrenheit and over. The temperature in the
hospital tents is said to have reached 130 degrees. The disaster
at Kut-el-Amara was due to the rains and floods that prevented
reenforcements from reaching the beleaguered garrison. The failure of
the Dardanelles expedition was partly due to the fact that the extreme
dryness of the country was not realized--as it would have been if
the War Office had called climatologists into council--and totally
inadequate provision was made for the water supply.

The new engines of war brought forth by the recent struggle were
peculiarly susceptible to the effects of weather. The larger guns
and heavy motor trucks were difficult to move over muddy roads. The
aircraft, though they managed to fly in all kinds of weather, suffered
innumerable disasters for which atmospheric conditions--chiefly storms
and fog--were responsible, and their operations were conspicuously
affected by favorable and unfavorable winds. Shells were fired to
unprecedented heights, and their trajectories were modified by unknown
conditions in the upper air. Last but not least, the use of poisonous
gases, especially in the period before gas clouds were largely replaced
by gas shells, was dependent upon the occurrence of appropriate winds;
and a slight miscalculation in this respect sometimes brought disaster
to the troops using the gas.

It is not surprising that professional meteorologists played a part in
the World War, but it is difficult to understand why meteorological
units were not attached to all armies, at least when on active service,
several decades before the year 1914. Meteorologists did, indeed, take
a hand in one earlier conflict, but not as enrolled soldiers. During
the Spanish-American War a special service was organized by the United
States Weather Bureau to protect the American fleet in southern waters
from unpleasant surprises in the shape of West India hurricanes. In
the summer of 1898 a chain of observation stations was established
by the Bureau around the Caribbean Sea. The service then inaugurated
in consequence of the exigencies of war proved so valuable to
shipping in time of peace that it has continued to operate, with some
intermissions, down to the present day.

When the World War broke out, the only country that immediately put
meteorologists, as such, into the field was Germany. The Germans were
fortunate in having a far greater number of trained meteorologists at
their disposal than had their enemies. There were chairs of meteorology
in several German universities and high schools, and the numerous
meteorological observatories and institutes of the Empire had provided
occupation for a large amount of professional talent in this line. One
of the first acts of the army that invaded Belgium was to establish an
aerological service in that country.

The Entente countries were slow in adding meteorological units to their
armies, but their civilian meteorological services were utilized to
the utmost for military purposes from the beginning of the war. They
at first worked under difficulties arising from the cessation of the
customary weather reports from central Europe, but, to offset this
disadvantage, the weather map was expanded in other directions, the
number of daily hours of observation was increased, and eventually the
forecasters in London and Paris acquired much better facilities for
making their predictions than they had enjoyed in time of peace. The
supply of weather information to the public was suspended, and great
precautions were taken to prevent the reports of the Allied services
from being utilized by the enemy. The German meteorologists were
seriously hampered by the lack of reports from the westward. It has
been asserted that such reports were sometimes obtained by radio from
submarines stationed off the coast of Ireland, but such a service, if
it existed, must have been fragmentary and unsatisfactory. That the
Germans made many mistakes in their attempts to infer the atmospheric
conditions over the British Isles from the limited weather map at their
disposal is proved by the fact that their airships frequently crossed
the Channel when, with an ampler knowledge of impending weather, they
would certainly have remained at home. Several Zeppelins came to grief
in the course of these ill-timed raids. One of the interesting routine
duties of the British Meteorological Office during the war was to draw
the weather map for a given moment as the Germans would probably draw
it, with their curtailed set of telegraphic reports, and then predict
the German prediction!

In the spring of 1915 a small meteorological section was organised in
the British Army, and attached to the Royal Engineers. This force was
afterward enlarged, and provided units for service on several battle
fronts. The British also developed a naval meteorological service,
which had existed in embryo before the war, and, eventually, a special
meteorological service for the Royal Air Force. Analogous services were
organized by the French and the Italians.

The United States Army and the United States Navy both established
meteorological services not long after this country entered the war.
The former was attached to the Signal Corps, and was partly officered
and recruited from the Weather Bureau. A training school for army
meteorologists was opened at College Station, Texas. Upward of 300
men were given instruction in this school, and most of them were sent
overseas. The naval meteorological service was headed by the director
of Blue Hill Observatory, and the junior officers received special
training at that institution.

The varied activities carried on by these war-time units were so
different from the traditional duties of meteorologists that they may
be said to mark the advent of a new branch of applied science--Military
Meteorology. They were, moreover, as we shall see, extremely fruitful
of effects upon the science of meteorology in general.

In the principal battle zones the military weather men maintained a
dense network of observation stations, the reports from which, combined
with those received from the regular peace-time weather stations of the
Allied and neutral countries, enabled the forecasters at headquarters
to keep closely in touch with atmospheric changes. Observations of
both surface and upper-air conditions were made at frequent intervals,
and radiotelegraphy was largely used to insure prompt transmission
of the reports. In general, weather maps were drawn four times a day.
Information was distributed locally to the fighting units by telephone
and otherwise.

The vast fleet of aircraft called into being by the war would, of
itself, have imposed upon the military meteorologists the necessity of
paying a great amount of attention to the upper air. Pilot balloons
were sent up so frequently and at so many points that the aviators
generally knew just what winds they would encounter aloft. Special
arrangements were made to follow the progress across the country of the
thundersqualls which constituted a serious danger to the “sausages,” or
observation balloons, as well as to aeroplanes on the ground, and to
hangars.

There was, however, another urgent reason for keeping a close watch
of the winds and other atmospheric conditions at various levels above
the earth’s surface. Experience acquired early in the war proved
that old-fashioned methods of correcting the aim of artillery for
meteorological disturbances were extremely inadequate for modern
guns, the projectiles of which rise to altitudes of from 10,000 to
20,000 feet and encounter conditions quite different from those
prevailing at the surface. The flight of a projectile is affected
by the force and direction of the wind, and the density of the air
through which it passes. Some modern projectiles remain in the air as
long as 70 seconds, and a moderate wind blowing across the path of
such a projectile may easily cause it to fall half a mile away from
the point at which it would strike if fired in still air. One of the
routine duties of the army weather service was to observe the winds
and compute the air-densities at different heights wherever such
information was required by the artillery. In order to facilitate
the application of such data by the gunners ingenious methods were
developed for computing what is known as the “ballistic wind.” This is
a fictitious wind which, if affecting the projectile throughout its
flight, would produce the same total deflective effect and effect on
range as the various winds that the projectile actually encounters.

Meteorological observations were also of great importance in connection
with the new process of locating distant guns known as “sound-ranging.”
This process consists, briefly, in determining the exact instant of
arrival at several points of the sound waves propagated through the
air from the gun that is being located. If sound traveled at a uniform
speed, these observations would show the exact distance of each of the
observing points from the gun, and a simple geometrical construction
would indicate the position of the latter. The speed of sound waves
in the air is, however, affected by both wind and temperature.
Accordingly, allowance had to be made for these varying factors, and
the necessary data were supplied by the meteorological units.

The observation and prediction of winds favoring the use of poisonous
gases by friend or foe was one of the most delicate tasks allotted
to the army meteorologists. The flow of such gases is determined by
the winds close to the surface of the earth, and these are greatly
affected by topography. Local air currents controlled by the slope
of the ground were especially utilized for gas attacks. Strong winds
were unfavorable, because they quickly dissipated the gas cloud. The
meteorologists not only advised their own troops when to use gas, but
also gave warning when the atmospheric conditions were such that gas
was likely to be used by the enemy. The use of gas shells was less
dependent for its success upon the wind than the liberation of gas
clouds, but even when shells were used the wind and weather at the
objective point were factors of importance.

The exigencies of warfare developed several new features of
meteorological practice, the utility of which did not cease with the
war. Thus it became customary to measure the degree of “visibility”
of distant objects, for the benefit of aviators and gunners, and
this element was included in the routine weather reports. Scales of
visibility, ranging from “very bad” to “excellent,” etc., were adopted,
and eventually certain forms of apparatus (“visibility meters”) were
devised for getting fairly precise measurements of this weather factor.

Another novel practice that deserves to be perpetuated was the plan
adopted by the military forecasters of adding to their predictions
a statement as to their probable accuracy; this was expressed on
a numerical scale of “odds,” instead of by use of the vague terms
“probably” and “possibly,” which have generally served the purpose of
the dubious forecaster.

The war brought about many improvements in the instruments and
methods used in sounding the upper air; and the intensive campaign
of pilot-balloon observations carried out at the military stations
provided a body of data for study quite unparalleled in the history of
meteorology.

Lastly, the war revolutionized weather telegraphy in Europe. Before
the war the European forecasters were hampered by exasperating delays
in the collection of reports over the telegraph lines, especially in
the international exchange of observations. Wireless telegraphy had
been extensively used for gathering reports from vessels and supplying
vessels with forecasts, but not for the interchange of meteorological
information on land. The war changed all this. Radiotelegraphic weather
messages became the rule, and the advantages of the new system were
so obvious that the tendency has been to retain it as far as possible
since the war.



CHAPTER XX

MEDICAL AND PHYSIOLOGICAL METEOROLOGY


The starting point in any study of the physiological effects of weather
and climate upon humanity is the remarkable fact that, on the hottest
days of summer and the coldest of winter, in tropical deserts and amid
polar snows, the temperature within the body of a healthy man remains
constant to a fraction of a degree. There are slight temperature
differences between different parts of the body; there is a periodic
daily variation of about half a degree; and there are other slight
changes, due to eating and exercise; but an internal temperature
of about 98.6 degrees F. is maintained with little or no regard to
fluctuations in the temperature of the air.

The body has often been compared to a building in which the temperature
is regulated by a thermostat, but the comparison is not exact. The
thermostat controls the temperature merely by regulating the combustion
of fuel; and with the advent of mild weather we let the fires go out
altogether. In the body the fires are always burning, the briskness
with which they burn--or, to drop the metaphor, the rate at which
bodily heat is generated--depending, above all, upon muscular activity,
but also upon other causes. It is true that we possess a nervous
mechanism, analogous to the thermostat, which tends to adjust the
production of animal heat in such a manner as to offset the cooling
effect of our environment; but this mechanism appears to be less
important in maintaining our constant temperature than another, which
regulates the loss of heat from the body. According to M. J. Rosenau:

“Heat is lost from the body chiefly in two ways; (1) by _heat
transfer_, or loss by radiation, conduction, and convection; (2) by
_evaporation_, chiefly by the evaporation of the water of perspiration.
Pettenkofer and Voit estimated the loss of water by the lungs at
286 grams, and from the skin at from 500 to 1,700 grams daily. This
will give some idea of the effects here concerned. The loss by heat
transfer diminishes as the temperature of the surrounding air rises.
The temperature of the body would rise when the atmospheric temperature
goes above 70 degrees F. were not perspiration then secreted. So long
as the perspiration can evaporate freely the heat production and heat
loss are balanced. With a high humidity evaporation is lessened and the
balance is maintained by rushing blood to the skin, which causes an
elevation of the temperature of the surface, and thus the loss of heat
by radiation, conduction, and convection is facilitated.”

Human sensations of temperature are paradoxical. We talk of being “hot”
and “cold,” as if we belonged to the class of cold-blooded animals--the
fishes and the reptiles--that actually undergo great variations of
temperature, with variations in the temperature of their environment.
We hear people say, for example, that they are most comfortable at a
temperature of 65; yet we know that their temperature is always 98½,
except just at the surface of the body.

The human body is, in fact, a poor thermometer. Our sensations do not
register the temperature of the air, but they do, in a way, register
the cooling power of the air, which depends upon temperature, humidity,
and air movement, and they register, especially, changes in this
cooling power, for within certain limits the body soon adapts itself
to a constant rate of cooling, so as to lose any impression of heat or
cold. When a steady outflow of heat from the body has been set up and
the external cooling power is suddenly increased, we become conscious
of a difference between the temperature at the surface of the body
and the “blood temperature” beneath. The action of the nerves at the
surface and the nerves underneath, under these circumstances, has been
compared to that of a thermo-junction, in which an electric current is
produced by differences of temperature. The rate of evaporation from
the skin, also, has a marked effect upon our sensations of comfort and
discomfort.

The common thermometer was long ago discredited as a means of measuring
atmospheric comfort. Then, for a time, the wet-bulb thermometer had its
day, and its indications were once published on a large scale in this
country as representing the “sensible temperature,” or temperature that
we actually feel. The wet-bulb thermometer is cooled by evaporation
below the air temperature, except when the air is saturated with
moisture, and may therefore give a rough indication of the temperature
acquired by the skin when moistened with perspiration. The temperature
of the skin is not, however, an accurate indication of our feelings
of heat or cold, nor is it a satisfactory guide to the physiological
effects of atmospheric conditions.

Several instruments have been devised for the purpose of measuring the
cooling power of the air, to which the bodily mechanism must respond
in order to maintain a uniform temperature within. One of these was
invented nearly half a century ago by J. W. Osborne. A porous cylinder
was filled with warm water, and an agitator, driven by clockwork, kept
the water in the cylinder at uniform temperature at any given moment.
The rate at which heat was lost from the wet surface of the cylinder
was determined by a thermometer, having its bulb immersed in the water,
and a stop watch. A different plan was adopted by A. Piche, in an
instrument which he called the _deperditometer_, and which was supposed
to imitate more closely the behavior of the body. A porous vessel is
filled with water, the temperature of which is kept constantly at
“blood heat” (98.6 degrees F.) by a gas jet provided with an automatic
regulator. The amount of gas burned in a given time is then supposed to
measure the cooling power of the atmosphere as it affects humanity. J.
R. Milne’s _psuchrainometer_ is constructed on the same principle, but
heat is supplied and measured electrically.

Among many other instruments of this class, one that now enjoys special
favor is the _katathermometer_, devised by Prof. Leonard Hill, in
England. This consists of a pair of large-bulbed spirit thermometers,
one of which has its bulb covered with fine cotton mesh. To use the
instrument, the bulbs are immersed in water at about 150 degrees
F. until the spirit rises to the top of the thermometer tubes. The
excess of water is then jerked off the wet bulb, and the other bulb
is dried. The instruments are finally suspended in the air, and the
rate of cooling from 110 to 100 degrees, or from 100 to 90 degrees,
is taken with a stop watch. The dry-bulb measurements are supposed to
show how fast the human body loses heat at its surface by radiation
and convection, while the wet-bulb measurements also take account of
evaporation.

In order to connect the readings of this device with human sensations,
Hill and his collaborators have used the instrument under a great
variety of atmospheric conditions, both indoors and out, and compared
the readings with independent estimates of comfort and discomfort. On
an ideal summer day the “wet kata” fell from 110 to 100 in 25 seconds,
and the “dry kata” in 85 seconds. Indoors at the seaside in summer,
under comfortable conditions, the readings were 50 seconds and 140
seconds, respectively. A large number of other readings, taken under
different conditions in various parts of the word, have been published.
The katathermometer is now used in both Great Britain and the United
States in the study of ventilation problems, and has acquired a rather
extensive literature. According to its inventor, “the heating and
ventilation of rooms should be arranged so that the wet-bulb falls from
100 to 90 degrees in about one minute, and the dry-bulb in about three
minutes.”

[Illustration: A WEATHER BUREAU KIOSK, IN UNION SQUARE, SAN FRANCISCO.

(_Photograph, U. S. Weather Bureau._)]

[Illustration: THE UNITED STATES WEATHER BUREAU STATION, OBSERVATORY
TYPE, AT PEORIA, ILLINOIS. (_Photograph, U. S. Weather Bureau._)]

Regardless of the merits of these particular instruments, it is
certain that the cooling power of the air--which is quite a different
thing from the temperature of the air--is a very important factor in
determining our comfort and our health. Within certain limits the body
can easily adjust itself to changes in the cooling power of the air;
within wider limits the adjustment is effected with difficulty, and we
experience discomfort and possibly suffer in health; and finally
there are extreme conditions, in either direction, to which adjustment
is not possible; the internal temperature is then either lowered or
raised, as the case may be, and a comparatively small change of this
sort is fatal; i. e., death results by chilling or by heat stroke.

[Illustration:

  Photo U. S. Weather Bureau

THE CENTRAL OFFICE OF THE UNITED STATES WEATHER BUREAU IN WASHINGTON]

One interesting result of recent inquiries on this subject is the
discovery that the bad effects of crowded, “stuffy” rooms are not
generally due to impurities in the air, but to heat, humidity, and
especially lack of air movement. It seems to be now demonstrated
that there is no such thing as “crowd-poisoning,” and that the
bad smells of confined places are no indication that the air is
deleterious. Professor Hill, who has done more than anybody else to
upset traditional ideas with regard to ventilation, tells us that
“the deaths in the Black Hole of Calcutta, the depression, headache,
etc., experienced in close rooms are alike due to heat stagnation; the
victims of the Black Hole died of heat stroke.” Most recent writers
on physiology also discredit the time-honored methods of testing the
purity of the air by measuring the percentage of carbon dioxide it
contains. The amount of this gas normally present in the free air is
about three-hundredths of one per cent, but experiments have shown
that thirty times this amount--a percentage higher than is found in
the worst ventilated rooms--may be breathed for hours together without
detrimental effects. A further departure from old-fashioned views
is seen in the assertion of recent authorities that a deficiency
of oxygen, unless far more pronounced than ever actually occurs in
buildings, mines, etc., where the supply of this gas has been the
subject of so much solicitude, has no physiological significance
whatever. In support of this assertion it is pointed out that at
mountain health resorts the concentration of oxygen out-of-doors is
much less than that found in the worst ventilated rooms at sea-level.
In mines an ample supply of oxygen may be positively dangerous, as
favoring the occurrence of explosions. These were rare before the
enactment of laws requiring a high percentage of oxygen in mine air.

[Illustration: HILL’S KATATHERMOMETER]

Excessive dryness of the skin, which is a common cause of discomfort,
is not very closely related to the humidity of the air. “In winter,”
says Hill, “if there be a wind the rate of evaporation is so
accelerated that the skin feels dry, because in order to check the
loss of body heat the sweat glands are inactive and the blood vessels
of the chilled skin are constricted.” There has been a great deal of
discussion about the dry air of American buildings in winter, and
startling figures have been adduced to show that the air of such
buildings is dryer than that of deserts. So far as measurements of
relative humidity go, this is perfectly true; but, as Dr. G. T.
Palmer, of the New York State Commission on Ventilation, has pointed
out, there is an important difference between dryness and “dryingness.”
The latter depends upon the movement of the air, as well as the
relative humidity. The circulation of the air in a desert is generally
much more active than that of the air in a building with the windows
shut, and therefore much more conducive to rapid evaporation. There are
systems of ventilation in which the air is kept in steady and rapid
motion, and it is probably only in such cases that the air of our
heated houses can be compared to that of a desert. From the European
point of view American buildings are notoriously overheated, but this
is probably due to the fact that our hot summers--much hotter than
those of Europe--have adapted us to a tropical climate.

It is natural to inquire whether the atmospheric conditions that
affect the comfort of man do not also exercise a marked influence
upon his muscular efficiency and his mental powers. This question has
been answered in the affirmative by a number of ingenious writers,
who have sought to establish definite quantitative relations between
certain states of the atmosphere and the output of work in factories,
the grades attained by school children, etc. Thus, Dr. Ellsworth
Huntington, a well-known worker in this field, declares that the most
favorable daily mean temperature for mental activity (the temperature
being measured out-of-doors) is about 40 degrees F., and for physical
activity about 60 degrees F. Contrary to the common opinion, he holds
that our general efficiency is at low ebb in midwinter and fairly high
in summer. Variability in temperature, within certain limits, he finds
to be stimulating; equable temperature the reverse. He has drawn
charts showing the distribution of what he calls “climatic energy”--i.
e., the combination of certain weather factors supposed to control
human efficiency--throughout the world, and other charts showing a
more or less similar distribution of “civilization.” He has also made
an ambitious attempt to interpret the history of mankind in terms of
weather and climate.

Another fruitful worker along similar lines is Dr. Griffith Taylor,
of Australia, who has made interesting studies of the control of
settlement in his own country and elsewhere by temperature and
humidity, and has introduced some novel graphic methods (“climographs”)
for comparing climates with respect to their effects on humanity.

There has, in short, arisen a new school of climatologists whose aim
is to develop exact mathematical formulæ whereby we shall be able
to adjust the economic arrangements of mankind on an intelligent
basis as regards climate. The success of their efforts is a question
for the future to decide, but there is no doubt that their work is
profoundly suggestive. These undertakings, it may be noted, bear a
striking analogy to those of the present generation of agricultural
meteorologists, who are applying climatic statistics to the problem of
selecting crops and to the improvement of agricultural methods.

A certain number of specialists are engaged in studying the
physiological effects of sunlight and other special kinds of solar
radiation, the distribution of which varies greatly from place to
place and from time to time, especially on account of differences in
the selective absorption of such rays by the atmosphere. The chemical
action of sunshine that causes sunburn--even at very low temperatures,
as, for example, on high mountains--may have far-reaching effects on
the human organism (as it certainly has on plants), and there is great
need of collecting more data of “photochemical climate” than we now
possess, in order that this subject may be thoroughly investigated. One
of the few institutions in the world at which a large amount of work
has been done in this line is the private observatory of Dr. C. Dorno,
at Davos, the well-known health resort in the Alps. Dorno’s studies
throw a good deal of light upon the therapeutic effects of sunshine in
a mountain climate.

Many forms of dust in the atmosphere are capable of producing
pronounced physiological and pathological effects. There is a long
list of “dusty trades” in which the production of excessive dust has
notoriously evil effects upon the health of workmen, leading especially
to pulmonary diseases; sometimes to various kinds of poisoning. These
harmful dusts are by no means confined to factories, mines, quarries,
and the like. The air of the average city street abounds in them. Dr.
J. G. Ogden states that 61 per cent of the dust found in the air of the
New York subways consists of jagged splinters of steel, resulting from
the wearing away of brake shoes, wheels, and rails.

The amount of danger to human health incurred through the presence
of disease germs in the atmosphere has been the subject of much
controversy. The present tendency is to regard this danger as very
slight, under ordinary conditions. Thus, Dr. F. S. Lee writes:

“Evidence that disease germs pass through the air from room to room
of a house or from a hospital to its immediate surroundings always
breaks down when examined critically. It is indeed not rare now to
treat cases of different infectious diseases in the same hospital
ward. The one place of possible danger is in the immediate vicinity
of a person suffering from a disease affecting the air passages, the
mouth, throat, or lungs, such as a ‘cold’ or tuberculosis. Such a
person may give out the characteristic microbe for a distance of a few
feet from his body, not in quiet expiration, for simple expired air is
sterile, but attached to droplets that may be expelled in coughing,
sneezing, or forcible speaking. In this manner infection may, and
probably does, occur, the evidence being perhaps strongest in the case
of tuberculosis. But apart from this source there appears to be little
danger of contracting an infectious disease from germs that float in
the air.”

In regard to sewer gas, which still inspires so much dread in the
popular mind, Dr. Lee says:

“Workmen in sewers are notoriously strong, vigorous, healthy men,
with a low death rate among them. The specter of an invisible monster
entering our homes surreptitiously from our plumbing pipes and sapping
our lives and the lives of our children must be laid aside; we need no
longer leave saucers of so-called ‘chlorides’ standing about our floors
to neutralize in an impossible manner mysterious effluvia that do not
exist; and when we return to our town houses in the autumn we may enter
them with no fears that we are risking our lives by coming into a
toxic, germ-infected, sewer gas-laden, deadly atmosphere.”

Present-day knowledge on the subject of infectious diseases discredits
many ideas that once prevailed with regard to the effects of tropical
climates on health. The remarkable results accomplished by vigorous
sanitary measures in such places as Havana, the Isthmus of Panama and
Guayaquil have aroused hopes--perhaps too sanguine--that eventually
all parts of the tropics will be made healthful for the white race. In
the Canal Zone the death rate of the large population of American men,
women, and children is not higher than prevails in many cities of the
United States; whereas, a generation ago, when the French were at work
on the canal, the “climate” of this region was regarded as one of the
most unhealthful in the world. Some authorities go so far as to assert
that the deterioration in the general health and efficiency of white
people in the tropics, so far as it actually occurs, is due entirely
to preventable diseases. It would seem more rational, however, to
assume that there are climates both in and out of the tropics which,
on account of their heat, humidity, and other purely physical factors,
are not so suitable for habitation for any race of humanity as others.
How far acclimatization can go toward offsetting the effects of these
atmospheric conditions is problematical.

The changes in the barometer that occur from day to day in regions
where these changes are most pronounced are, on an average,
not greater than those encountered in going from the bottom to
the top of a good-sized hill, and are probably not directly of
physiological importance. Certain European investigators, however,
ascribe pathological effects to the minute and rapid barometric
fluctuations--too small to be detected with an ordinary barometer--that
occur, for example, when the foehn wind is blowing. Whether this is
the cause, or a contributory cause, of “foehn-sickness,” of which one
hears in Switzerland, is still uncertain.

The physiological effects of a rarefied atmosphere, as experienced
in mountain climbing, ballooning, and aviation, are not yet well
understood, despite the large amount of study that has been devoted to
this subject. Recent views are thus summarized by Rosenau:

“The symptoms produced by a marked diminution in atmospheric pressure
vary with circumstances. The effects are increased by cold, active
muscular exertion, or improper clothing. The noticeable symptoms are
increased rapidity of respiration and acceleration of the circulation,
noises in the head and dizziness, impairment of the senses of sight,
hearing, and touch, dullness of the intellectual faculties, and a
strong desire to sleep. Sudden changes to a rarefied atmosphere cause
syncope, weakness, dyspnœa, dizziness, and nausea. These threatening
symptoms go by the name of ‘mountain sickness,’ Bert and Journet
believe this condition is due to lack of oxygen, and the symptoms may,
in fact, be relieved by adding oxygen to the air inspired. Kronecker
concludes that mountain sickness is caused by a congestion of the
lungs, impeding the flow of blood through them. Mosso and his followers
attribute the physical disturbances of a reduced atmospheric pressure
to the fact that the blood loses carbon dioxide more quickly than it
loses oxygen, and they ascribe mountain sickness to this decrease of
carbon dioxide in the blood. Cohnheim believes there is a concentration
of the blood at high altitudes; in fact, insignificant increases have
been found by competent observers.”

Divers and workers in caissons are subjected to high barometric
pressure, amounting, at the maximum, to about 4½ atmospheres.
According to Rosenau:

“The physiological effects of an increased atmospheric pressure
are mainly due to an increase in the amount of atmospheric gases
(especially nitrogen) which are taken up by the blood, and also to an
increase in the chemical absorption of oxygen by the blood. The serious
consequences usually result from too rapid decompression. As the
pressure is released gas bubbles form. Gradual decompression gives a
chance for the gas to escape from the lungs and be expelled without the
production of bubbles.”

The health and comfort of many people seem to be affected, in a
rather striking way, by the passage of the barometric depressions and
areas of high pressure that alternate at intervals of a few days in
the temperate zones. These effects should not be ascribed to changes
of pressure, but rather to the accompanying changes in the other
meteorological conditions. The late Dr. Weir Mitchell, who was a
pioneer student of such phenomena, wrote of “a neuralgic belt, within
which, as it sweeps along in advance of the storm, prevail in the hurt
and maimed limbs of men, in the tender nerves and rheumatic joints,
renewed torments called into existence by the stir and perturbation of
the elements.” Victims of neuralgia and rheumatism are probably quite
justified in regarding themselves as human barometers, capable of
predicting with considerable accuracy the advent of stormy and rainy
weather.

The fluctuations of temperature, humidity, and wind that attend
the passage of barometric highs and lows would seem, in virtue
of their effects on the heat-regulating mechanism of the body and
consequent reactions upon the nervous system in general, to supply
an ample explanation of the unpleasant symptoms above noted in the
case of sensitive people; conditions to which the collective name of
“cyclonopathy” has been given by European investigators, and which are
extensively discussed in the works of Hellpach, Frankenhäuser, and
Berliner. Some authorities have, however, invoked in this connection
the possible effects of atmospheric electricity, and pointed to the
extreme sensitiveness of many persons to the approach of thunderstorms;
a condition which Dr. G. M. Beard named “astraphobia.” It is stated
that the passage of a low-pressure area favors the emission of
radioactive emanations from the ground, that the ionization of the
atmosphere, and hence its electrical conductivity, is thus increased,
and that the electric charge of the body is carried away more rapidly
than usual. Here we enter upon a debatable subject, but one that
thoroughly merits investigation. The human organism is the seat of
various electrical phenomena, and these certainly cannot be independent
of changes in the electrical state of the atmosphere.

Apart from possible direct effects of atmospheric electricity upon
the human system, it has been suggested that electrical changes in
the atmosphere affect the rate of reproduction of bacteria, and may
therefore have some influence on the spread of infectious diseases.

The weather has many subtle influences upon the human mind, producing
moods of cheerfulness and depression, and manifesting themselves in
the records of the behavior of school children, in statistics of
crime, insanity, suicide, drunkenness, etc. An interesting account of
these manifestations is given by Dr. E. G. Dexter in his book “Weather
Influences” (New York, 1904).

Finally, the aspect of meteorology that has thus far acquired the most
definite shape in medical circles and given rise to the most coherent
body of literature is Medical Climatology, which is designed to be
applied in the climatic treatment of disease (climatotherapy). Thus
many compilations have been made of the climatic statistics of health
resorts, and these resorts have been classified with respect to their
supposed climatic effects upon various diseases. From the point of view
of the physical climatologist, the statistics found in such books seem,
in general, both meager and ill adapted to bring out important features
of the climates discussed; to say nothing of the fact that the whole
subject of climatotherapy is fraught with controversy--whereof the
history of the treatment of tuberculosis furnishes a shining example!



CHAPTER XXI

WEATHER-MAKING


Meteorologists, in their candid moments, have been heard to express
disappointment over the amount of progress made in the art of weather
forecasting during the past half-century. “Shall we ever,” they ask,
“be able to predict the weather with mathematical certainty, as the
astronomer predicts an eclipse of the sun or moon?”

Perhaps even within the meteorological fold there are unorthodox
optimists who would answer such a question thus: “Yes, because some day
we shall _control_ the weather. It is inconceivable that man, who is
every day achieving new miracles in the conquest of nature, should not
eventually find a way of regulating the rainfall and sunshine that are
of such vital importance to his crops, the winds that must be reckoned
with in his voyages by sea and air, and the various other elements of
weather that have so much to do with his happiness and welfare. The
attainment of this object is so tremendously desirable that it cannot
forever baffle human ingenuity.”

In support of such a bold assertion it might be pointed out that we
already control the weather to a certain limited extent. When the
horticulturist burns orchard heaters to protect his fruit from frost
he certainly alters the weather for a few hours over a small area of
the earth. If there were any practical justification of the process,
the temperature of the air over an entire State, for example, could be
raised throughout the winter, with appreciable effects on agriculture.
The difference between heating a single orchard for a night and heating
a State for a season is one of degree, and not of principle.

The climate of a city is, through causes dependent upon man, materially
different from the natural climate of the surrounding country. Every
dwelling provided with heating arrangements enjoys an artificial summer
amid the blasts of winter. Local control of the winds is exemplified
in the planting of thousands of miles of trees as windbreaks in the
prairie regions of our Middle West. By moderating the winds this
process has a marked effect on temperature and evaporation and is so
beneficial to crops that in the aggregate it furnishes a striking
example of successful “weather-making” by mankind. Analogous methods,
perfectly feasible with means already at our disposal, would change the
whole climatic aspect of large areas of the earth’s surface.

The question “Can we make it rain?” may be answered in the affirmative
by those who are neither impostors nor victims of self-delusion. The
deposit of spray from the spout of a teakettle might, without much
stretching of terms, be described as a miniature artificial rainstorm;
but much bigger showers, in nowise different from those occurring in
nature, can also be produced artificially. Huge clouds have often been
observed to form over forest fires and other great conflagrations.
These clouds, composed of water drops, tower far above the smoke cloud,
and are identical in character and mode of origin with the cumulus and
cumulo-nimbus clouds formed by currents of moist air rising from the
heated ground on a summer day. There are several well-authenticated
cases in which rain has been seen to fall from such clouds, and these
showers have sometimes been so heavy as to extinguish the fires
that generated them. Hence, given favorable conditions of humidity,
temperature and wind, mankind can certainly produce a rainstorm (and
perhaps a thunderstorm into the bargain) by the relatively simple
process of building a big fire.

Unfortunately the vast majority of methods whereby man has attempted
to regulate the weather have no such rational foundation as those we
have just mentioned. Some are wholly superstitious, others are purely
empirical, and yet others are based upon ideas that their promoters
suppose or pretend to be scientific, but that are actually fallacious.

In the history of superstitious practices weather-making plays a
prominent part. Sir J. G. Frazer, in that great storehouse of myth and
folklore, “The Golden Bough,” says: “Of the things which the public
magician sets himself to do for the good of the tribe, one of the
chief is to control the weather and especially to insure an adequate
fall of rain. In savage communities the rain-maker is a very important
personage; and often a special class of magicians exists for the
purpose of regulating the heavenly water supply.” Frazer devotes ninety
pages of his work to a rapid survey of the superstitious methods of
controlling the weather that have found credence among the various
races of mankind. These range all the way from the most complicated
ceremonies to the summary expedient of throwing a passing stranger into
a river to bring rain.

The sailor who whistles or scratches the mast to raise a wind is merely
keeping up a quaint custom, in the efficacy of which he may or may
not put some lingering faith, but which the world at large long since
ceased to take seriously. When, however, a vessel master attempts to
disperse a waterspout by firing a cannon at it, he is doing what nine
educated persons out of ten would probably do under the circumstances.
Yet one process is no more futile than the other, and both are based
on superstition. Ages ago sailors sought to frighten waterspouts away
by pointing knives at them, or by shouts and the clashing of swords,
and the use of cannon originally embodied the same idea of terrifying
the watery monster. It is our purpose in the present chapter to
describe especially several processes of weather-making which, while
not obviously chimerical from the point of view of the layman, have
been more or less positively discredited through the scrutiny of men of
science.

The efforts of modern weather-makers have been directed especially
to two objects; viz., the production of rain and the prevention of
hailstorms. In the United States a certain amount of ingenuity has
also been devoted to the task of dispelling tornadoes. Some years
ago a device for the latter purpose was patented, consisting of a
box, containing explosives, mounted on a pole and erected a mile or
so to the southwestward of the village to be protected from these
unwelcome visitors. The force of the wind was expected to detonate the
explosives by driving a movable board against percussion caps. The
inventor believed that a violent explosion would disperse the passing
tornado funnel. Apart from the fact that a single installation of
this character, or even several of them, would seldom happen to be
at exactly the right spot to explode close to the relatively small
vortex of a tornado, the effect of the explosion, even in the very
heart of the storm, would certainly be negligible. The energy that
keeps the tornado in action is supplied continuously from a level far
above the earth, while the disturbance due to the explosion would be
only momentary. Above all, the energy developed in any discharge of
explosives that the community could afford to pay for would be quite
insignificant compared with that which prodigal nature supplies to the
tornado.

The same disproportion between the giant forces at work in the
atmosphere and the pygmy forces at the disposal of mankind is a point
that is overlooked in most attempts at weather-making.

The widespread belief that rain can be produced by explosions
rises so far above the level of ordinary popular delusions that it
has sometimes led to large expenditures of money on the part of
drought-ridden communities and even of national governments. Perhaps
the most remarkable example of official confidence in the efficacy of
this process was that furnished some years ago by the Volksraad, or
legislative assembly, of the Transvaal, which passed a law forbidding
the bombardment of the clouds to produce rain, on the ground that the
rain-makers were thwarting the will of the Almighty!

One manifestation of the belief in question is found in the common
assertion that rain is the usual sequel of battles. This idea
originated, however, long before the invention of gunpowder. It
is mentioned by Plutarch and other writers of antiquity. Whatever
superstition or crude process of reasoning may have first given support
to this notion in the popular mind, the explanation now commonly
advanced is that the condensation of moisture is promoted by the
concussion due to cannonading, or that the drops already condensed and
constituting the clouds are jostled together by the same disturbance,
with the result that they coalesce and fall as rain. There is no ground
for such assumptions. As was once pointed out by the late Professor
Simon Newcomb, the effect of a violent explosion upon a body of moist
air a quarter of a mile distant is about the same as that which the
clapping of one’s hands would produce upon the moist air of the room
in which the experiment is performed. Again, if we stand in the steam
escaping from a teakettle and clap our hands we shall not produce a
shower, though we jostle the water drops much more than the explosion
does at a distance of a quarter of a mile.

In recent years another explanation has been offered for the alleged
production of rain by explosions; viz., that the smoke and gases
arising from an explosion increase condensation by increasing the
number of “nuclei” in the atmosphere. As we have seen, however, in
considering the natural formation of rain, the number of condensation
nuclei normally present in the atmosphere is so great that it must be
diminished, rather than increased, before drops as large as raindrops
can be formed. Moreover, the nuclei required for the condensation of
water vapor, including molecules of highly hygroscopic gases, are given
forth in abundance by great manufacturing centers, yet these places do
not have a heavier rainfall than the surrounding country. Pittsburgh,
for example, is actually one of the driest places in Pennsylvania.

One obvious reason why rain often follows a battle is that battles are
frequently fought in regions where rain normally occurs every two or
three days, on an average, whether in peace or war. In northern France,
for example, where the battles of the World War were plenteously
interspersed with showers, meteorological records show that the average
number of rainy days per annum is upward of 150. The drenching rains
that made “Virginia mud” a byword during the American Civil War gave
great currency to the belief in “rain after battles,” Here, again, we
have accurate weather records to help us dispel a fallacy. Thus at
Richmond rain falls on 122 days in an average year, at Lynchburg on 124
days, and at Petersburg on 105 days.

There is, however, a particular reason why rain is rather more likely
to occur soon after a battle than shortly before one; viz., the fact
that intervals of fair weather, with consequent dry roads, are used
by commanders in carrying out the movement of troops that precede an
engagement. By the time such arrangements, often occupying several
days, are completed, the “spell” of fine weather is likely to be over,
and a rainstorm is due in accordance with the normal program of nature.

The most famous undertaking in the history of rain-making--and one
which has had an incalculable effect in fostering the credulity of the
public with respect to similar enterprises--was that carried out by
General Robert Dyrenforth on behalf of the United States Government
in 1891. Congress voted appropriations amounting to $9,000 for these
experiments, and Dyrenforth was appointed a “special agent” of the
Department of Agriculture to direct them. After some preliminary trials
in the suburbs of Washington, the experimenters proceeded to a ranch
near Midland, Texas. Here a few balloons filled with a mixture of
oxygen and hydrogen, as well as several sticks of dynamite carried
up by kites, were exploded in the air, but the only explosions of
considerable magnitude were set off on the ground. The experiments
continued over a period of three weeks and in some cases showers fell
within a few hours after an explosion, but, in spite of the somewhat
favorable tone of the official report, the consensus of scientific
opinion was that the undertaking was a failure, while the views of
the public at large were divided. The attitude of the Government is
sufficiently indicated by the fact that it has never since undertaken
experiments in this line. The one tangible outcome of this affair was
that a crop of private rain-makers sprang up all over the country, and
to this day the example set by the official experimenters is cited
in support of every sort of harebrained scheme for juggling with the
weather.

In 1911 and 1912 the late C. W. Post, of breakfast-food fame, expended
many thousand pounds of dynamite in efforts to produce rain in Texas
and Michigan. Showers undoubtedly occurred in conjunction with Post’s
experiments, but conditions favoring their occurrence were plainly
indicated on the current daily weather maps, and they had been duly
forecast by the Weather Bureau.

The professional rain-maker does not generally resort to the expensive
process of bombarding the clouds. His methods most frequently involve
the use of chemicals, and the details are shrouded in mystery. For
example, about a quarter of a century ago one Frank Melbourne, known as
“the Australian rain-maker,” enjoyed great celebrity and coined money
by his exploits in this field. His plan was to shut himself up in a
barn, freight car, or other structure, and manipulate his chemicals
and electric batteries for hours or days. Naturally rain sometimes came
after these operations, but as often it did not.

Several of the methods that have been suggested from time to time
for producing rain are sufficiently discredited by the fact that the
expense of putting them into execution would more than offset any
benefits derived from the rain, if the experiments proved successful.
Thus one genius, observing the deposit of water on the outside of an
ice pitcher in a warm room, proposed to set up a barrier packed with
ice in the path of moisture-bearing winds. Another plan occasionally
suggested is to sprinkle the atmosphere aloft with liquid air or
liquid carbon dioxide, in order to lower the temperature, by the rapid
evaporation of these substances, below the point of condensation. A
third proposal is to create a local updraft of air by means of powerful
fans or blowers, thus imitating the convectional process by which
clouds and rain are formed in nature.

Lastly, various plans have been suggested for altering the electrical
condition of the upper atmosphere--for example, by electrical
discharges from balloons--on the assumption that rainfall might thus be
promoted. This assumption, however, is not consistent with known facts
as to the relation between atmospheric electricity and the condensation
of atmospheric water vapor.

While in America a vast amount of money has been wasted on futile
experiments in rain-making, far more has been spent in Europe on
schemes for averting hailstorms. Methods of accomplishing this purpose
have varied from age to age. In antiquity it was the custom to shoot
arrows or hurl javelins toward the gathering clouds in the hope of
frightening them away. In the Middle Ages ecclesiastical or occult
agencies were invoked; “hail crosses” (such as are still seen in the
Tyrol) were erected; and the ringing of church bells was considered
efficacious against both hail and lightning, as is shown by the
inscriptions found on many old bells.

The custom of firing cannon at the clouds to avert hail began centuries
ago in Styria and northern Italy, and it was well established in France
before the Revolution. Toward the end of the eighteenth century,
however, another method of hail protection was introduced in France,
whence it spread over the rest of Europe. This consisted in setting up
tall, metal-tipped poles, imitated from lightning rods. It was supposed
that these poles, which were known as _paragrêles_, would draw the
electric charge from the clouds and thereby (though nobody could say
why) would prevent the formation of hailstones. This device, though
reported on unfavorably by the French Academy of Sciences, gained
great popularity. One of its advocates, writing in 1827, states that
more than a million _paragrêles_ were at that time in use in France,
Switzerland, Italy, Austria, Bavaria, and the Rhine country. The vogue
enjoyed by these contrivances is said to have come to a sudden end when
a tremendous hailstorm not only devastated the fields and vineyards
they were supposed to protect, but also knocked down a great number of
the rods themselves.

In recent times both the hail cannon and the _paragrêle_ have been
revived. The new era of “hail-shooting,” as the process of cannonading
the hail clouds is called, dates from the year 1896, when a number of
cannon of a new type were installed in the vine-growing district of
Windisch-Feistritz, in Styria. The success claimed for them in this
region led to their introduction on a vast scale over the greater
part of southern and central Europe. The cannon employed were small
mortars, to the muzzles of which were attached sheet-iron funnels.
No projectile was used, but the explosion of the charge sent aloft a
curious whirling ring of smoke and gas, powerful enough to splinter
sticks and kill small birds several hundred feet from the cannon. By
the year 1900 at least 10,000 hail cannon were in use in Italy alone.
Several modifications of the device were introduced, such as the use of
acetylene in place of gunpowder; and eventually certain forms of rocket
and bomb were adopted, for concentrating the effects of the explosion
at as high a level as possible.

About the year 1899 a new form of hail rod was introduced in France,
and this has become the favorite means of protection against hail in
that country. It is essentially a very large lightning rod of pure
copper, grounded by means of a broad copper conductor. Such rods have
been installed, in some cases, on church steeples and other tall
edifices, including the Eiffel Tower, in Paris, and in other cases
on tall steel towers erected for this purpose. This device is called
fantastically an “electric Niagara,” because, according to the claims
of its promoters, it draws down “torrents” of electricity from the
clouds. Hundreds of these “Niagaras” have been constructed in France.
Some of them are set up in rows, or so-called _barrages_, across
the habitual paths of hailstorms. The French Government was induced
to appoint a “Comité de Défense contre la Grêle” (Hail-protection
Committee), which before the war had made elaborate plans for
“protecting” not only the whole of France, but also Algeria and Tunis,
with these devices. Similar rods have been erected in Argentina, and
plans for introducing them in South Africa were near consummation at
the time the World War broke out.

In order to understand the extraordinary hold that the various
hail-protecting devices have taken upon the minds of European
cultivators it should be remembered that the intensive cultivation
of the soil is the rule over the greater part of Europe, so that a
hailstorm of relatively small extent often does enormous damage.
Vineyards are especially subject to injury from this cause, and many
of the richest vine-growing districts of the Old World are notoriously
afflicted with hailstorms.

Scientific commissions appointed by the Austrian and Italian
governments conducted long series of tests of the methods of bombarding
the clouds with mortars, bombs, and rockets, and declared them to
be of no value. The erection of hail rods, though it has received a
certain amount of official encouragement in France, is also strongly
discountenanced by the majority of scientific men, as well as by
a large proportion of intelligent agriculturists. Reports on the
actual operation of the rods support conflicting opinions--as might
be expected from the fact that the hailstorm is a decidedly erratic
phenomenon. Thus, some observers claim that the storm clouds change
conspicuously in appearance as they approach a “Niagara,” and if they
shed hail upon the spot it is in a soft and harmless form. Others deny
the accuracy of these observations, and point to the stubborn fact that
ordinary hail has fallen on several of the rods themselves, including
the one on the Eiffel Tower. In the suburbs of Clermont-Ferrand a
“Niagara” is installed on an iron tower, 100 feet high. This rod was
pelted with hail twice in 1912 and four times in 1913, and in one case
the hailstones attained the size of hen’s eggs! Nobody has ever offered
any plausible scientific hypothesis to explain why these rods should
have an effect upon hail, even if they are able, as seems unlikely, to
reduce the electrical charge of the clouds; since the formation of hail
is due to movements of the air, which, in turn, are the cause and not
the result of the charge in question.

Fortunately for the farmer and the horticulturist--especially in
Europe--a method of averting the losses due to hailstorms is available
in the shape of insurance, and its cost is decidedly less than that
entailed in systematic hail-shooting or in the general erection of hail
rods. Hailstorm insurance has been extensively practiced in the Old
World since the end of the eighteenth century. In some countries it has
been conducted or subsidized by the government. Generally each country
is divided into a number of zones, according to the recorded frequency
of hailstorms, and the premiums vary proportionately. Premiums also
vary for different crops, since some are better able to withstand the
effects of hail than others. The amount of insurance of this kind
carried in Germany, alone, shortly before the World War, was more than
$800,000,000.

Hailstorm insurance is fairly common in the United States, especially
in the Middle West, but still lacks an adequate statistical basis
in the shape of detailed records of hail frequency. In 1919 growing
crops in this country were insured against hail to the extent of
$559,134,000. Much information on this subject will be found in V. N.
Valgren’s “Hail Insurance on Farm Crops in the United States” (U. S.
Dept. of Agriculture, Bulletin 912), published in 1920.

Besides the weather-making schemes already noted, mention should
be made of certain more ambitious projects of this character that
have been bruited from time to time, and that have found plenty of
credulous supporters. In the year 1845 an American meteorologist of
undoubted ability, but much inclined to the riding of hobbies--viz.,
James P. Espy--proposed the building of great fires in the western
part of the United States in order to regulate the winds and rainfall
to the eastward. The fires were to extend in a line of six or seven
hundred miles from north to south, and were to be set off once a week
throughout the summer. Another genius, of less celebrity, proposed to
destroy blizzards by means of a line of coal stoves along the northern
boundary of the country. A favorite idea of those who aspire to produce
wholesale changes of climate is to alter the course of ocean currents
for this purpose. One early plan contemplated the damming of the Strait
of Belle Isle in order to improve the climate of New England and the
Canadian provinces; while, a few years since, a proposal to build an
immense jetty eastward from Newfoundland for the purpose of “protecting
the warm north-flowing Gulf Stream from the onslaughts of the ice-cold,
south-flowing Labrador Current” actually received, serious attention
from the Congress of the United States.



CHAPTER XXII

ATMOSPHERIC BYWAYS


1. IGNIS FATUUS, OR WILL-O’-THE-WISP

Will-o’-the-wisp is proverbially elusive. It has thus far escaped the
fate of the rainbow, deplored by Keats. We do not know its woof and
texture, and it is not given in the dull catalogue of common things.

A strong argument in favor of the reality of this phenomenon is found
in the great number of names that have been applied to it. There are
forty or fifty in the British dialects alone. A myth generally carries
its nomenclature with it, as it spreads from one community to another,
while a fact of nature may give rise to a variety of local names.

It is certain, however, that a great many different phenomena have
been described as will-o’-the-wisp. Some of these are: (1) The
phosphorescence of decaying wood (“fox fire”) and other vegetable
matter. This is due to luminous fungi. According to H. Molisch there
are some forty-five species of fungi, including twenty species of
bacteria, that have the property of luminosity. Sometimes the ground
under a forest is illuminated on all sides with a soft, white light
from decaying leaves. (2) Fireflies, including glowworms (the wingless
females of the firefly and the larvæ). (3) Luminous birds and animals.
Their luminosity is supposed to be due to parasitic fungi. Certain
species of skunk have been described as giving off in the darkness a
continuous flame, the head being fiery red, which blends into a bright
blue at the tail. (4) Ball lightning. (5) St. Elmo’s fire. (6) Moving
lanterns, distant lights of houses, and other lights due to human
agency. (7) Burning gas ascending from marshes, stagnant pools, and the
like. Marsh gas and other inflammable gases commonly rise from such
places, and are often ignited by man, or by lightning, etc. Such fires
are sometimes seen by day as well as by night. (8) Burning naphtha
springs.

Excluding the numerous reported cases of will-o’-the-wisp in which the
phenomenon may be plausibly identified with one of those mentioned
above, there remain several cases, some of them reported by very
careful observers, which appear to belong to a different category. The
reports in question differ somewhat in details, but yield the following
composite description:

Small luminous bodies, “about as large as your fist,” or “the size of
a candle flame,” are seen hovering a few feet above the ground; not
only over marshes and pools, but also over dry land. Sometimes they
are stationary; at other times they appear to drift with the wind, or
even to move independently. They appear and disappear, after the manner
of fireflies. They do not set fire to objects with which they come in
contact, and are believed to be without sensible heat. Their color
is most often described as bluish, but may be yellow, purple, green,
etc.; rarely pure white. They are without odor and without smoke.
Traditionally they are associated with graveyards, but in very few of
the cases heretofore recorded were they actually seen in such places.
The popular idea that they flee from the traveler who tries to approach
them and follow him when he seeks to avoid them is also unsupported by
the evidence thus far adduced.

One of the most circumstantial accounts of these objects is that
published in the Belgian journal “Ciel et Terre” for July-August,
1920, by a retired army surgeon, Jules Rossignol, who observed them
repeatedly in the autumn of 1908 in and about some marshy woods near
Grupont. They were generally seen to rise from the ground, at first
in the shape of little white clouds, which changed to luminous globes
on attaining an altitude of a dozen yards, and returned by a circular
path toward the ground. They lasted from one to several minutes before
disappearing in the air.

It is astonishing that the phenomenon of _ignis fatuus_, though
reported from so many parts of the world, has not yet been made the
subject of direct scientific examination. Nobody has ever studied
its light with the spectroscope, for example. Chemists have, indeed,
attempted to reproduce the phenomenon, yet the chemical explanations
of it that have appeared in reference books down to a recent date
are quite untenable. It has sometimes been attributed to marsh gas
(methane, CH4), and sometimes to phosphureted hydrogen (phosphine,
PH3). But marsh gas, besides not being spontaneously combustible,
diffuses too rapidly in the air to produce the effects described,
while phosphureted hydrogen, though it takes fire spontaneously in the
air, produces thick wreaths of smoke when burning and has a powerful
odor--features never reported in connection with will-o’-the-wisp.

At least two more plausible explanations of _ignis fatuus_ have been
offered in the last few years. Mr. F. Sanford (“Scientific Monthly,”
Oct., 1919) believes that it is due to “swarms of luminous bacteria
which are carried up from the bottom of the marsh by rising bubbles of
gas.” A Belgian chemist, M. Léon Dumas (“La Nature,” Dec. 11, 1909),
claims to have produced little luminous clouds, corresponding to the
traditional descriptions of will-o’-the-wisp, by combining the two
gases sulphureted hydrogen and phosphine. Both these substances are
produced in the decay of animal matter, especially of the brain and
spinal cord. The body of an animal, buried in some wet place, would
accumulate the two gases under pressure in the skull and spinal canal,
and their escape, simultaneously, would fulfill the conditions of M.
Dumas’ experiments.

(At the request of the present writer, these experiments were repeated
at the Bureau of Standards, in Washington, with only partial success.
Further trials with these and other gases due to putrefaction are
desirable.)


2. THE RAIN TREE

From time to time the newspapers publish accounts of a wonderful tree,
said to grow wild in Peru or elsewhere, from the leaves of which falls
a continuous shower of rain, even in the driest weather. The writers
generally urge the introduction of this tree in regions where the
rainfall is deficient, and a so-called “rain tree” has actually been
sold for this purpose by nurserymen.

The story of this tree is very old. Early voyagers reported finding it
in the East Indies, Guinea, Brazil, and especially the island of Ferro,
in the Canaries. Nowadays the name “rain tree” is applied especially
to a magnificent tree of tropical America generally known to botanists
as _Pithecolobium_ (or _Enterolobium_) _saman_. One of its common names
is “guango.”

[Illustration: THE RAIN TREE

From “Voyageurs anciens et modernes”

(_By E. T. Charton_)

A legendary scene in the island of Ferro]

That many plants spontaneously exude moisture under suitable conditions
is well known. The phenomenon is called “guttation.” The moisture drawn
up from the roots is usually transpired from the leaves in the form
of invisible water vapor; i. e., it is evaporated on passing into the
air. If, however, the humidity of the surrounding air is sufficiently
high, or its temperature sufficiently low, to check evaporation, the
water will collect on the surface of the plant in liquid form, and may
ultimately trickle to the ground in considerable quantities. Guttation
occurs chiefly at night, or in cloudy or foggy weather. In a very dry
climate it does not occur at all; and for this reason, even if the
so-called rain tree could be successfully introduced in such a climate,
it would not help solve the problem of irrigation.

The dripping of moisture deposited on plants by drifting fog is
another common process that may have contributed to the legend of
the rain tree. A classic example of this process--technically called
“fog drip”--is that described by Dr. R. Marloth, who has made actual
measurements of the abundant moisture captured by the vegetation of
Table Mountain, South Africa, from the driving clouds of the southeast
trade winds during the nearly rainless summer months. Mr. Madison
Grant, writing of a similar phenomenon witnessed in the redwood forests
of California, tells us that “these forests are sometimes so wet that
the dripping from the high crowns is like a thin rain, and in summer it
is oftentimes hard to tell whether it is raining or not, so saturated
with moisture are the foliage and the trunks when the fog darkens the
forest.”

A copious production of “honeydew” by plant lice, scale insects, etc.,
may be at the bottom of some of the rain-tree stories. F. E. Lutz, in
his “Field Book of Insects,” writes of “weeping trees,” which drip
fluid of insect origin, and he says of the honeydew secreted by the
pear psylla (_Psylla pyricola_): “When the psyllas are numerous the
leaves and fruit become coated with this sticky substance and it even
drops from them like rain and runs down the trunk.”

The following account of the Peruvian rain tree, quoted from the
traveler Spruce, was published in “Nature” of Feb. 28, 1878, by Prof.
Thiselton Dyer:

“The Tamia-caspi, or rain tree of the eastern Peruvian Andes, is not
a myth, but a fact, although not exactly in the way popular rumor has
presented it. I first witnessed the phenomenon in September, 1855,
when residing at Tarapoto. I had gone one morning at daybreak, with
two assistants, into the adjacent wooded hills to botanize. A little
after seven o’clock we came under a lowish spreading tree, from which
with a perfectly clear sky over-head a smart rain was falling. A glance
upward showed a multitude of cicadas, sucking the juices of the tender
young branches and leaves and squirting forth slender streams of limpid
fluid.”


3. DEW PONDS

The Down country of southern England is one of the few places in the
world where the people go to the hilltops to seek water in dry weather.
On the summits of the Downs are found many artificial shallow ponds,
most of them very old. Some, indeed, date back to prehistoric times.
The bottom of these ponds consists of a layer of puddled chalk or clay
and is impervious to water, so that there is no loss by seepage. As
the ponds are not fed by springs or surface drainage, and as lack of
rain does not cause them to dry up, it is popularly believed that their
maintenance depends upon dew. Hence they are called “dew ponds.”

Kipling mentions them in his poetical description of Sussex:

  We have no waters to delight
    Our broad and brookless vales--
  Only the dew pond on the height
    Unfed, that never fails.

The leading authority on dew ponds is Mr. Edward A. Martin, who has
written a book about them. Mr. Martin’s experiments have demonstrated
that dew can make no important contribution to the water supply of
these ponds. The rainfall on the hilltops is somewhat higher than
in the valleys, and the greater part of the water in the ponds is
undoubtedly derived from this source. The real key to the mystery,
however, is found in the wet fogs that drift in from the sea. The
process of “fog drip,” which we have mentioned in connection with the
rain tree, supplies the deficiencies of the rainfall, and the name
“mist ponds,” occasionally applied to these bodies of water, is more
appropriate than “dew ponds.”

It remains, however, an interesting paradox that, in time of drought,
the farmers of the Downs drive their cattle to the hilltops to be
watered and send their carts uphill to procure water for household
use in the valleys below. Was it, perhaps, in this topsy-turvy
region--where uplands are called “downs”--that Jack and Jill went “up
the hill” on their ill-starred water quest?


4. BLOWING WELLS

Wells that predict weather changes are local curiosities in many parts
of the world. Such wells are not uncommon in the United States. If the
well is open at the top, its manifestations consist of occasional
disturbance of the water and the discharge of numerous bubbles. If it
is covered, a strong current of air is, at times, emitted from any
small orifice in the cover. This may be strong enough to lift and blow
away light objects placed over the aperture. Its emission is frequently
accompanied by a loud whistling or roaring sound. Such occurrences
are supposed to betoken an approaching storm. These wells are called
“blowing wells”; sometimes “weather wells” or “barometer wells.”

In certain cases an indraft of air is sometimes observed; i. e., the
well alternately “sucks” and “blows.”

As a rule these phenomena correspond to fluctuations in barometric
pressure, and therefore are, in a rough way, indicative of changes in
weather. It is obvious that a body of air inclosed in the earth and
communicating by one or a few small openings with the air above will
set up outdrafts and indrafts in adjusting its tension to that of
the latter. The amount of air contained in the well itself would not
suffice to produce the violent effects observed and it is therefore
assumed that the typical blowing well taps a subterranean reservoir of
air, probably filling the interstices of sand and gravel beds. When
the pressure of the external air is diminished, some of the imprisoned
air escapes. For a given body of inclosed air, the smaller the channel
or channels by which it emerges, the stronger the outdraft. When the
barometric pressure outside increases, the current of air flows in the
reverse direction. In winter the indraft of cold air in such a well
sometimes causes the water to freeze, even at a depth of 100 feet or
more below the surface of the ground, and is therefore a source of
inconvenience to the owner.

Various other circumstances may give rise to the bubbling and blowing
of wells. Carbon dioxide and other gases dissolved in the well water
account for the bubbling of many wells, and this process is more active
at times of low barometric pressure, because at a given temperature,
the amount of gas that the water can hold in solution varies with the
pressure.

Another cause of the blowing of a well is, in some cases, a sudden rise
in the water level (“water table”) in the surrounding ground, as after
a heavy rainstorm. If the ground is overlain by an impervious stratum,
the air imprisoned between this stratum and the surface of the ground
water over an extensive area will escape with violence through any
available channel, such as that supplied by the well.

Lastly, cases have been described in which subterranean air currents
arise from the friction of rapidly flowing underground streams, setting
up permanent indrafts or outdrafts through wells communicating with
such streams.


5. CURIOUS SHOWERS.

Showers of blood, sulphur, manna, frogs, fishes, and what not figure
in all the old chronicles, and are still frequently reported. Many
occurrences of this kind are recorded in Camille Flammarion’s book “The
Atmosphere,” Dr. E. E. Free’s “Movement of Soil Material by the Wind”
(U. S. Bureau of Soils, Bulletin 68), and Mr. W. L. McAtee’s article
“Showers of Organic Matter” in the “Monthly Weather Review” for May,
1917.

The power of the wind to whirl objects aloft is a matter of familiar
observation. McAtee tells of seeing a silk hat lifted from its owner’s
head and blown over a ten-story building in the city of Washington.
The vortex of a tornado or a waterspout furnishes the most favorable
skyward route for things that belong on _terra firma_. Objects weighing
scores or even hundreds of pounds are lifted by these whirls. Within
a mile or so of a tornado a shower of cart wheels or cook stoves
would not necessarily constitute a “prodigy.” A chicken coop weighing
75 pounds has been carried four miles and a church spire seventeen
miles. Oersted tells of a waterspout at Christiansö, on the Baltic,
that emptied the harbor to such an extent that the greater part of the
bottom was uncovered, while McAtee says that “waterspouts have been
observed to accomplish the comparatively insignificant feat of emptying
fish ponds and scattering their occupants.”

There is, in fact, no mystery about the way in which terrestrial
objects of many sorts get into the air; nor, considering the force
of the winds and their occasional strong vertical components, is it
strange that such objects sometimes travel a long way from home before
they return to earth.

There are, however, a great many cases of reported showers in which the
objects did not really fall, as supposed. McAtee gives the following
account of these spurious showers in the “Monthly Weather Review”:

“_Insect larvæ._--The rains of insect larvæ that have been investigated
have proved to be merely the appearance in large numbers on the
surface of the ground or upon snow of the larvæ of soldier beetles
(_Telephorus_), or sometimes caterpillars, which have been driven from
their hibernating quarters by the saturation of the soil by heavy rains
or melting snow.

“_Ants._--Accounts of showers of ants have usually been founded on
incursions of large numbers of winged ants, which of course needs no
assistance from the elements to follow out their habit of swarming
forth periodically in immense numbers.

“_Honey; sugar._--Showers of honey and of sugar are popular names for
what scientists know are exudations of certain plants, or of plant lice
which feed on a great variety of plants and whose product is often
known also as honeydew.

“_Grains._--Showers of grain, usually considered miraculous, have in
most cases been determined to be merely the accumulation by washing
during heavy rains of either the seeds or root tubercles of plants of
the immediate neighborhood.

“_Manna._--An account of manna ‘rains’ certainly pertains to the
discussion of showers of vegetable matter, for the substance manna
consists of lichens of the genus _Lecanora_ but in none of the numerous
recorded instances of manna ‘rains’ is there any direct evidence that
the substance really fell from the sky. These lichens form small,
round bodies that are easily blown over the surface of the ground and
accumulate in depressions; they are very buoyant also and hence easily
drifted into masses during the run-off of rain water. Manna ‘rains’
have not occurred except in countries where these lichens are common,
and as for statements of their falling down upon roofs or upon people,
or for any other proofs that they really rained down, I have seen none.

“_Blood rains._--The most frequently reported showers that are
spurious, at least in name, are the so-called blood rains. In all
times the phenomena going under this name have frightened the people
and have been taken as portents of terrific calamities. One of the
famous plagues of Egypt was a bloody rain which prevailed throughout
the whole land, continuing three days and three nights. Homer and
Virgil both allude to blood rains, and, in fact, the general subject of
preternatural rains was a favorite with the older writers.

“But scientific investigation has done away with the element of
mystery in these phenomena and has explained, with the others, the
rains of blood. Some blood rains have been found to be the meconial
fluid ejected by large numbers of certain lepidoptera simultaneously
emerging from their chrysalides; other red rains are due to the rapid
multiplication in rain pools of algæ and of rotifers containing red
coloring matter; “red snow” results from the presence of similar
organisms. But in no case have they rained down, except in the sense
that their spores or eggs have at some time been transported, probably
by the wind. The precipitation of moisture furnishes favorable
conditions for their rapid development and multiplication.”

Most of the reported showers of blood, however, have probably been
rainstorms in which the rain was colored with reddish dust. The
occurrence of such dust in the atmosphere is very common in some parts
of the world, as we have stated in a previous chapter. It has been
asserted that rain which fell at Oppido Mamertina, Italy, May 15, 1890,
actually contained blood, believed to be from birds.

Showers of supposed “sulphur” are due to pollen, chiefly from pine
trees. The air in the vicinity of pine forests is sometimes filled with
clouds of this material and the wind carries it for many miles. It is
reported that a pollen shower at Pictou, Nova Scotia, in June, 1841,
was so heavy that bucketfuls were swept up on a ship.

In the case of alleged showers of “paper” the material has been found
to be the crusts of dried algæ, which form on the surface of the ground
exposed by the evaporation of the water of shallow ponds.


6. THE YEAR WITHOUT A SUMMER

The Weather Bureau is a bureau of information, and one of the ways
in which it strives to give a good account of itself is by answering
endless questions about “the year without a summer.” This title has
been given to the year 1816.

Blodget, in his “Climatology of the United States,” tells us that all
the summers from 1811 to 1817 were cold in this country, and that in
every month of the summers of both 1812 and 1816 snows and frosts
occurred in the Northern States. It is the latter summer, however,
that has lived in popular tradition. The year 1816 is known further as
“poverty year,” or “eighteen hundred and froze to death.” It acquired
the name of “mackerel year” in New Hampshire, where people ate mackerel
as a substitute for pork, little of which was fattened on account of
the extreme scarcity of corn. Western Europe, also, had a cold summer
in 1816, and the year as a whole seems to have been a cold one over a
great part of the world.

Sources of information about the cold summer in this country, besides
Blodget’s book above mentioned, include Perley’s “Historic Storms
of New England,” which devotes a whole chapter to the subject, and
Charles Peirce’s “Weather in Philadelphia.” Peirce tells us that
at Philadelphia “there was ice during every month of the year, not
excepting June, July, and August, There was scarcely a vegetable
came to perfection north and east of the Potomac.” According to the
“Monthly Weather Review,” citing the recollections of James Winchester,
of Vermont: “It is said that in June of that year snow fell to the
depth of three inches in New York, Pennsylvania, and New Jersey on the
17th; five inches in all the New England States, except three inches
in Vermont. There was snow and ice in every month of this year. The
storm of June 17 was as severe as any that ever occurred in the depth
of winter; it began about noon, increasing in fury until night, by
which time the roads were impassable by reason of snowdrifts; many were
bewildered in the blinding storm and frozen to death.... There was
a heavy snowstorm August 30th.... The year 1816 had neither spring,
summer, nor autumn. The only crop of corn raised in that part of
Vermont that summer was saved by keeping bonfires burning around the
cornfield night and day.”

At the time of its occurrence the frigid weather of the summer of 1816
was popularly attributed to sun spots, which were big enough to be seen
with the naked eye in May and June. A present-day hypothesis on the
subject has been mentioned in our chapter on atmospheric dust. The dust
cloud from the eruption of Tomboro, in 1815, was so vast that for three
days there was darkness at a distance of 300 miles from the volcano.

A proximate cause of the cold summer is perhaps to be sought in an
unusual intensity and extent of the area of low barometric pressure
which is more or less permanently located in the vicinity of Iceland
and, as one of the principal atmospheric “centers of action,” has a
great deal to say about the weather of the countries adjacent to the
North Atlantic. Dr. C. F. Brooks has called attention to the fact that
the Arctic navigator, Scoresby, found unusually mild and open weather
that summer in the seas east of Greenland. This would be explained
by strong southerly winds, forming part of the “counter-clockwise”
circulation around the Iceland low; and if the same pressure system
extended its influence to our shores, persistent cold northwest winds
might be expected to result over the northeastern United States.


7. INDIAN SUMMER AND THE ICE SAINTS

Indian summer _weather_ is an undeniable fact. Every inhabitant of
the northern United States and southern Canada is familiar with the
mild, calm, hazy state of the atmosphere that frequently occurs in the
autumn, sometimes following a brief period of unseasonable cold known
as “squaw winter.” It is, however, one thing to recognize the existence
of a certain type of weather as characteristic of our autumns, and
quite another to admit that one definite spell of such weather occurs
more or less regularly from year to year. One true summer, and only
one, comes to pass each year, and occupies an approximately fixed place
in the calendar. Even the so-called “year without a summer,” which we
have just described, was merely a year in which the regular annual
rise of the temperature curve was less marked than usual. Indian
summer, on the contrary, has never been tied down to a particular part
of a particular month. In his notes on the meteorological conditions
at Concord, Massachusetts, during the ten years, 1851-1860, Thoreau
records the occurrence of Indian summer weather on dates all the way
from September 27 to December 13; a range of 77 days.

The belief in the definite occurrence, year after year, of what has
sometimes been called the “after-summer” is not peculiar to America. It
prevails also in Europe, where this supposed period of renewed warmth
has been assigned to certain dates, owing in part to its association
with the names of particular saints in the calendar. These dates vary
widely, however, from one region to another, ranging from August 15
(Julian calendar), the beginning of the “young women’s summer” of
Russia, to November 15, St. Martin’s day, a date popularly associated
with after-summer in Germany, Holland, France, Italy, and sometimes
England.

The supposed tendency of particular types of weather to occur at
about the same period every year, independently of and often in sharp
contrast to the regular march of the seasons, has been described by
R. Abercromby under the name of “recurrence,” and there is a large
literature on the subject; especially in connection with periods of
unseasonable temperature. While Indian summer is the most discussed
example of recurrence in the American weather calendar, in the Old
World more attention, both popular and scientific, has been devoted
to a frosty period supposed to recur in May. With the elaboration of
the ecclesiastical calendar, the frosts in question became definitely
associated with the days dedicated to Saints Mamertus, Pancras,
and Servatius (May 11, 12, 13), or, in south-central Europe, Saints
Pancras, Servatius, and Boniface (May 12, 13, 14), hence known as the
Ice Saints. These saints and their days are called in French _saints de
glace_, and in German _Eisheiligen_, _Eismänner_, or _gestrenge Herren_.

Yet other examples of the elusive phenomenon of recurrence are the
“January thaw” of New England, the April “blackthorn winter” of
England, and the June “sheep-cold” (_Schafkälte_) of Germany, dangerous
to newly shorn sheep.

In the middle of the last century the cold weather of the Ice Saints
was variously ascribed to the melting of the ice and snow of high
latitudes, the passage of periodic meteor showers between the earth and
the sun, and other far-reaching terrestrial or cosmical causes. FitzRoy
believed that the liberation of latent heat in autumn during the
formation of ice in the circumpolar regions was accountable for Indian
summer. A review of the whole body of literature concerning supposed
recurrent irregularities in the annual march of temperature will be
found in the “Monthly Weather Review” (Washington) for August, 1919.

Whether recurrence, in Abercromby’s sense of the term, is a
real phenomenon is still an unsettled question. Many periods of
unseasonable weather occur in the course of each year, and it is easy
for the uncritical observer to identify one of them with the Ice
Saints, another with Indian summer, and so on. About the best that
meteorologists can do at present is to explain each particular instance
of such weather by reference to barometric and other conditions shown
on the daily weather map.



GLOSSARY


The vast vocabulary of meteorology is very inadequately represented
in ordinary dictionaries, and has never been made the subject of a
comprehensive special glossary. The writer of this book has been
gathering material toward such a glossary for some years, and from the
material now in hand it appears that an approximately complete English
meteorological dictionary, embracing both scientific and nonscientific
terms relating to the atmosphere and its phenomena, would contain
upward of fifteen thousand definitions.

From this statement it will be evident that the brief glossary herewith
appended is of the most fragmentary character. It includes only a
selection of the _meteorological_ terms found in the present book. It
does not, in general, include terms pertaining primarily to physics,
chemistry, astronomy, physiology, etc., even though they figure to some
extent in meteorology, as all such terms used in the book are more
or less satisfactorily defined in the latest editions of the large
American dictionaries.

    _Absolute Extremes._--The highest and lowest values of a
    meteorological element (especially temperature) that have ever
    been recorded at a station; known, respectively, as the _absolute
    maximum_ and the _absolute minimum_. (The term is sometimes
    improperly applied to the highest and lowest values for a specified
    year.)

    _Aeroclinoscope._--A semaphore formerly used in Holland for
    displaying weather signals.

    _Aerology._--The branch of meteorology dealing with the “free”
    atmosphere; i. e., all parts of the atmosphere not near the earth’s
    surface. Aerological investigations are made with kites and
    balloons, and also include observations of clouds, meteor trails,
    the aurora, etc.

    _Afterglow._--1. The glow in the western sky after sunset. 2. A
    renewal of rosy light on mountain peaks after the first sunset
    illumination has faded; also called _recoloration_. This is one
    stage of the _Alpenglow_.

    _After-summer._--A renewal of mild weather in the autumn; called
    Indian summer in America, St. Martin’s summer, etc., in Europe.

    _Alpenglow._--Successive appearances and disappearances of rosy
    light sometimes seen on mountain peaks in clear weather after
    sunset or before sunrise.

    _Altimeter._--A barometer used for measuring altitude.

    _Alto-cumulus_; _Alto-stratus_.--Forms of cloud. (See Chapter VI.)

    _Anemogram._--The record traced by a self-registering anemometer.

    _Anemometer._--An instrument for measuring the force or speed of
    the wind.

    _Aneroid Barometer._--A barometer consisting of a thin-walled metal
    vacuum-box, which changes its shape with changes of atmospheric
    pressure. The movements of the box are communicated, by levers, to
    an index or (in the barograph) to a recording pen.

    _Anthelion._--A rare species of halo, consisting of a brilliant,
    usually white image of the sun opposite the latter in azimuth.
    (This term has also been applied to the _glory_, q. v.)

    _Anticrepuscular Rays._--The continuation of the crepuscular rays
    converging toward a point in the sky opposite to the sun.

    _Anticyclone._--An area of high barometric pressure and its
    attendant system of winds. (Cf. _cyclone_.)

    _Antitrades._--Term formerly applied to the prevailing westerly
    winds of middle latitudes, but now more frequently applied to the
    westerly return-currents lying over the trade winds. Some writers
    prefer to call the former the _antitrades_ and the latter the
    _countertrades_.

    _Antitwilight Arch._--The pink or purplish zone of illumination
    bordering the shadow of the earth (_dark segment_) in the part of
    the sky opposite the sun after sunset and before sunrise.

    _Arcs of Lowitz._--A pair of rare halo phenomena. These arcs are
    directed obliquely downward from the parhelia of 22 degrees on
    either side of the sun toward the halo of 22 degrees.

    _Astraphobia._--A pathological condition experienced by certain
    persons before and during thunderstorms.

    _Atmometer._--An instrument for measuring evaporation; also called
    _atmidometer_, _evaporimeter_, etc.

    _Aureole._--(See _corona_. 1).

    _Aurora._--A luminous phenomenon due to electrical discharges
    in the atmosphere; probably confined to the tenuous air of high
    altitudes. It is most commonly seen in sub-Arctic and sub-Antarctic
    latitudes. Called _aurora borealis_ or _aurora australis_,
    according to the hemisphere in which it occurs. Observations with
    the spectroscope seem to indicate that a faint “permanent aurora”
    is a normal feature of the sky in all parts of the world.

    “_Backstays of the Sun._”--A sailor’s name for crepuscular rays
    extending downward from the sun.

    _Baguio._--The name current in the Philippines for a tropical
    cyclone.

    _Ballistic Wind._--A military term applied to a fictitious wind
    which, if affecting a projectile throughout its flight, would
    produce the same total effect in deflecting it from its course
    and altering its range as do the various winds that it actually
    encounters.

    _Ballon-sonde._--A sounding-balloon.

    _Bar._--A unit of pressure equal to 1,000,000 dynes per square
    centimeter. A bar = 100 _centibars_ = 1,000 _millibars_. A
    barometric pressure of one bar is sometimes called a “C. G. S.
    atmosphere,” and is equivalent to a pressure of 29.531 inches of
    mercury at 32 degree F. and in latitude 45 degrees.

    _Barisal Gun._--Same as _brontide_.

    _Barocyclonometer._--One of several instruments that have been
    devised for locating tropical hurricanes without the aid of a
    weather map.

    _Barograph._--A self-registering barometer.

    _Barometer._--An instrument for measuring the pressure of the
    atmosphere. The two principal types are the _mercurial_ and the
    _aneroid_. The _microbarometer_ is used to show minute changes
    of pressure. Certain forms of hygroscope are popularly miscalled
    “barometers.”

    _Barometer Well._--Same as _blowing well_.

    _Barometric Tendency._--The change of barometric pressure within
    a specified time (usually three hours) before one of the regular
    observations.

    _Beaufort Scale._--A scale of wind force, originally devised for
    use at sea, but now used also on land. The scale runs from 0 = calm
    to = hurricane. Many other scales are similarly employed in the
    noninstrumental observation of wind force.

    _Bioclimatic Law._--A phenological law, announced by Dr. A. D.
    Hopkins, according to which periodical events of plant and animal
    life advance over the United States at the rate of 1 degree of
    latitude, 5 degrees of longitude, and 400 feet of altitude every
    four days--northward, eastward, and up-ward in spring, and
    southward, westward, and downward in autumn.

    _Bishop’s Ring._--A large corona due to fine dust in the
    atmosphere. It has been seen after certain great volcanic
    eruptions, especially that of Krakatoa, in 1883.

    _Blizzard._--A violent, intensely cold wind, laden with snow.

    _Blowing Well._--A well which emits a strong current of air
    from any small opening in its cover during a fall of barometric
    pressure. During a rise of barometric pressure such wells are
    sometimes observed to “suck.” Wells that are thus responsive to
    barometric changes are sometimes called “barometer wells” or
    “weather wells.”

    _Bora._--A cold wind of the northern Adriatic, blowing down from
    the high plateaus to the northward. Also, a similar wind on the
    northeastern coast of the Black Sea.

    _Brave West Winds._--The boisterous westerly winds blowing over the
    ocean between latitudes 40 and 50 degrees S. This region is known
    as the “roaring forties.”

    _Bright Segment._--The broad band of golden light that, in clear
    weather, borders the western horizon just after sunset and the
    eastern just before sunrise.

    _Brontide._--A sound resembling a distant muffled detonation,
    usually indefinite as to direction. Brontides are rather common
    in certain parts of the world. They are called _mistpoeffers_ on
    the Belgian coast, _Barisal guns_ in the Ganges delta, _bulldag_,
    _desert sounds_, or _Hanley’s guns_ in parts of Australia,
    _gouffre_ in Haiti, _Moodus noises_ at Moodus, Connecticut,
    _Nebelzerteiler_, _Seedonner_, _Seeschiessen_, etc., in Germany,
    _baturlio_, _boniti_, _bombiti_, etc., in Italy. These sounds are
    probably of subterranean origin in most cases.

    _Bump._--An upward jolt experienced by an aviator, as if running
    over an obstruction. A bump may be caused by any condition that
    suddenly increases the lift of the machine, but is perhaps most
    frequently due to rising air currents. Air in which bumps are
    experienced is said to be “bumpy.” (Cf. _hole in the air_.)

    _Callina._--A Spanish name for dry fog.

    _Calms of Cancer_; _Calms of Capricorn_.--The belts of high
    pressure lying north of the northeast trade winds and south of the
    southeast trade winds, respectively.

    _Center of Action._--Any one of several large areas of high and low
    barometric pressure, changing little in location, and persisting
    through a season or through the whole year; e. g., the Iceland
    low, the Siberian winter high, etc. Changes in the intensity and
    positions of these pressure systems are associated with widespread
    weather changes.

    _Ceraunograph._--A self-registering thunderstorm recorder.

    _Chinook_, or _Chinook Wind_.--A foehn blowing down the eastern
    slopes of the Rocky Mountains over the adjacent plains, in the
    United States and Canada. In winter, this warm, dry wind causes
    snow to disappear with remarkable rapidity, and hence it has been
    nicknamed the “snow-eater.” (Cf. _foehn_.) The “wet chinook” is a
    wind of a different character, blowing from the Pacific Ocean over
    the northwestern United States.

    _Circumscribed Halo._--A halo formed by the junction of the upper
    and lower tangent arcs of the halo of 22 degrees, when the luminary
    is about 40 degrees or more above the horizon. As the altitude of
    the luminary increases, the circumscribed halo gradually assumes an
    elliptical form and finally merges into the halo of 22 degrees.

    _Circumzenithal Arc._--A rainbow-tinted halo, often very bright,
    convex to the luminary and 46 degrees or a little more above it. It
    is sometimes called the _upper quasi-tangent arc of the halo of 46
    degrees_, but the circumzenithal arc and the halo of 46 degrees are
    rarely seen at the same time.

    _Cirro-cumulus_; _Cirro-stratus_; _Cirrus_.--Forms of cloud. (See
    Chapter VI.)

    _Cistern._--The cup, containing mercury, at the base of a mercurial
    barometer.

    _Climatography._--1. Descriptive and statistical climatology. 2. An
    account of the climate of a particular place or region.

    _Climatology._--1. The science of climate. 2. A body of knowledge
    concerning the climate of any place or region; as, “the climatology
    of Panama.”

    _Climograph._--A diagram introduced by Dr. Griffith Taylor,
    of Australia, for showing the mean monthly values of wet-bulb
    temperature and relative humidity at any place, and for comparing
    such data as recorded at different places throughout the world;
    especially with reference to the effects of climate on mankind.
    Other pairs of elements can be used in constructing climographs:
    e. g., the dry-bulb temperature and the relative humidity.

    _Cloud-banner._--A bannerlike cloud streaming off from a mountain
    peak.

    _Cloud-burst._--A sudden and extremely heavy downpour of rain;
    especially one in which the water falls in a continuous stream
    rather than in drops. The term has been most commonly applied to
    downpours in mountainous regions.

    _Cloud-cap._--A caplike cloud crowning (1) a mountain summit, or
    (2) another cloud, especially a mass of cumulo-nimbus.

    _Col._--The neck of low pressure between two anticyclones; also
    called a _saddle_.

    _Cold Wave._--A rapid and marked fall of temperature during the
    cold season of the year. The United States Weather Bureau applies
    this term to a fall of temperature in 24 hours equaling or
    exceeding a specified number of degrees and reaching a specified
    temperature or lower; the specifications varying for different
    parts of the country and for different periods of the year.

    _Collector._--A device used in measurements of atmospheric
    electricity for determining the potential gradient.

    _Continental Climate._--The type of climate characteristic of
    the interior of a continent. As compared with a marine climate,
    a continental climate has a large annual and daily range of
    temperature.

    _Corona._--1. A colored luminous circular area formed, by
    diffraction; around the sun, moon, or other source of light seen
    through clouds or dust haze. Coronas invariably show a brownish-red
    inner ring, which, together with the bluish-white inner field
    between the ring and the luminary, forms the so-called _aureole_.
    Most frequently the aureole alone is visible. Well developed
    coronas show one or more series of spectral colors outside the
    aureole. 2. A luminous circle formed by the apparent convergence
    of auroral beams about the place in the sky toward which the
    dipping-needle points.

    _Corposant._--(See _St. Elmo’s fire_.)

    _Countertrades._--(See _antitrades_.)

    _Crepuscular Rays._--Beams of light radiating from the sun, seen
    both before and after sunrise and sunset. The beams are made
    visible by the presence of water-drops or dust in the atmosphere,
    and the intervening dark spaces are the shadows of clouds. The
    beams are actually parallel; their apparent divergence is the
    result of perspective.

    _Critical Period._--A period in the growth of a plant when it is
    especially susceptible to the effects of atmospheric conditions.

    _Cumulo-nimbus_; _Cumulus_.--Forms of cloud. (See Chapter VI.)

    _Cyclone._--An area of low barometric pressure with its attendant
    system of winds. The cyclones of the region within the tropics
    (_tropical cyclones_) are violent storms; those of higher latitudes
    (_extra-tropical cyclones_) may be stormy or otherwise. Tropical
    cyclones are also called _hurricanes_, _typhoons_ or _baguios_.
    Extra-tropical cyclones are commonly known as _lows_ or _barometric
    depressions_.

    _Cyclonopathy._--The abnormal sensitiveness of certain persons to
    the weather changes attending the passage of barometric depressions.

    _Cyclonoscope._--A pasteboard dial formerly used in the West Indies
    for locating cyclones.

    _Dark Segment._--The shadow of the earth which, in clear weather,
    rises from the eastern horizon at sunset and sinks below the
    western horizon at sunrise.

    _Deperditometer._--An instrument devised by A. Piche for measuring
    the cooling power of the atmosphere, with reference to its
    physiological effects.

    _Depression._--A cyclonic area, or low.

    _Desert Sounds._--(See _brontide_.)

    _Devil._--The name applied to a dust whirlwind in India. The term
    is also current in South Africa.

    _Dew._--Atmospheric moisture condensed, in liquid form, upon
    objects cooler than the air, especially at night.

    _Dew-point._--The temperature at which, under ordinary conditions,
    condensation of water vapor begins in a cooling mass of air. It
    varies with the absolute humidity.

    _Dew Pond._--The name applied in southern England to certain
    artificial ponds on the uplands. They contain water in the driest
    weather, and are popularly supposed to be fed by dew.

    “_Doctor._”--A colloquial name for the sea breeze in tropical
    climates. The name is sometimes applied to other cool, invigorating
    breezes.

    _Doldrums._--The equatorial belt of calms or light, variable winds,
    lying between the two trade-wind belts.

    _Drought._--A protracted period of dry weather. In the United
    States a drought has been defined as a period of thirty or more
    consecutive days during which precipitation to the amount of 0.25
    inch does not occur in twenty-four hours. Other quantitative
    definitions have been used in other countries.

    _Dry Fog._--A haze due to the presence of dust or smoke in the air.

    _Dust-counter._--An instrument for determining approximately the
    number of dust particles or condensation nuclei per unit volume in
    a sample of air.

    _Dynamic Meteorology._--The branch of meteorology that treats
    of the motions of the atmosphere and their relations to other
    meteorological phenomena.

    _Earth-air Current._--The electrical current that passes between
    the earth and the air on account of their difference of potential.

    _Eddy._--A more or less fully developed vortex in the atmosphere,
    constituting a local irregularity in a wind. All winds near the
    earth’s surface contain eddies, which at any given place, produce
    “gusts” and “lulls.” Air containing numerous eddies is said to be
    “turbulent.”

    _Electric Niagara._--(See _hail rod_.)

    _Evaporimeter._--(See _atmometer_.)

    _Eye of the Storm._--A calm region at the center of a tropical
    cyclone, or a break in the clouds marking its location.

    _Fall Wind._--A wind blowing down a mountain-side; or any wind
    having a strong downward component. Fall Winds include the foehn,
    mistral, bora, etc.

    _False Cirrus._--Cirruslike clouds at the summit of a thundercloud;
    probably identical in structure with true cirrus, or cirro-stratus.
    Sometimes more appropriately called “thunderstorm cirrus.”

    _Fata Morgana._--A complex form of mirage, characterized by marked
    distortion of images.

    _Festoon Cloud._--Mammato-cumulus.

    _Flashing Arcs._--Visible atmospheric sound waves, or explosion
    waves.

    _Flat._--Featureless; said of weather maps.

    _Foehn._--A dry fall wind warm for the season, characteristic
    of many mountainous regions. The air is cooled dynamically in
    ascending the mountains, but this leads to condensation, which
    checks the fall in temperature through the liberation of latent
    heat. The wind deposits its moisture as rain or snow. In descending
    the opposite slope it is strongly heated dynamically and arrives in
    the valleys beyond as a warm and very dry wind. Some writers apply
    this term to any wind that is dynamically heated by descent; e. g.,
    the sinking air of an anticyclone.

    _Foehn-sickness._--Headache, lassitude, depression, etc.,
    attributed, in the Alpine valleys, to the blowing of the foehn.

    _Foehn-wall_ (German: _Föhnmauer_).--A wall of cloud that forms
    along the crest of a mountain ridge over which the foehn is blowing.

    _Fog._--A cloud at or near the earth’s surface. A fog and a
    cloud are identical in structure, though the former is due to
    thermal conditions of the earth’s surface, while the latter is
    most frequently clue to the dynamic cooling of ascending air. In
    ordinary speech, the term “fog” generally implies an obscurity of
    the atmosphere sufficiently great to interfere with navigation or
    locomotion. (Cf. _dry fog_.)

    _Fogbow._--A rainbow, colorless or nearly so, formed in a fog.

    _Fog-drip._--Moisture that is deposited on terrestrial objects by
    fog, and drips from them to the ground.

    _Fracto-cumulus_; _Fracto-nimbus_; _Fracto-stratus_.--Forms of
    cloud. (See Chapter VI.)

    _Freeze._--1. Freezing of plants without deposit of hoarfrost.
    2. Freezing temperatures prevailing generally over a region; not
    exclusively nocturnal and not confined to the air close to the
    earth’s surface. (Cf. _frost_.)

    _Frost._--1. The act or state of freezing. In America, a “frost”
    generally means the occurrence, near the beginning or end of
    the growing season, of nocturnal temperatures low enough to be
    injurious to vegetation; distinguished from a “freeze,” which is
    more general and severe. The Weather Bureau classifies frosts,
    according to their effects, as “light,” “heavy,” and “killing.” 2.
    Atmospheric moisture condensed upon terrestrial objects in the form
    of ice; sometimes frozen dew. Also called _hoarfrost_.

    _Frost-smoke._--Frozen fog rising from the water.

    _Fulgurite._--A glassy tube formed in sandy soil or in rock by the
    passage of lightning.

    _Garúa._--A wet fog of the west coast of South America.

    _Geocoronium._--The name applied by Dr. A. Wegener to a
    hypothetical atmospheric gas, supposed to be much lighter than any
    gas now known to chemists.

    _Glaze._--Term applied by the U. S. Weather Bureau to a smooth
    coating of ice on terrestrial objects due to the freezing of rain;
    often popularly called “sleet.” In Great Britain such a deposit is
    called _glazed frost_. A deposit of glaze on an extensive scale
    constitutes an “ice storm.”

    _Glory._--A series of concentric colored rings seen around the
    shadow of the observer, or of his head only, cast upon a cloud or
    fog bank. It is due to the diffraction of reflected light.

    _Gouffre._--(See _brontide_.)

    _Gradient._--Change of value of a meteorological element per unit
    of distance. The gradients commonly discussed in meteorology are
    the horizontal gradient of barometric pressure, the vertical
    gradient of temperature, and the vertical gradient of electric
    potential. British meteorologists now prefer the term _lapse-rate_
    to _vertical gradient_.

    _Graupel._--A kind of granular snow, sometimes called _soft hail_.

    _Green Flash._--A bright green coloration of the upper edge of the
    sun’s disk, sometimes seen when the rest of the disk is below the
    horizon, at sunrise or sunset.

    _Growing Season._--In agricultural meteorology, the interval
    between the last killing frost in spring and the first killing
    frost in autumn.

    _Gust._--A sudden brief increase in the force of the wind. Most
    winds near the earth’s surface are made up of alternate gusts and
    _lulls_, the majority of which are too brief to be registered by an
    ordinary anemometer.

    _Hail._--Balls or irregular lumps of ice, often of considerable
    size, having a complex structure; large hailstones generally
    have a snowlike center, surrounded by layers of ice, which may
    be alternately clear and cloudy. Hail falls al-most exclusively
    in connection with thunderstorms. For so-called “soft hail” see
    _graupel_. (Cf. _sleet_.)

    _Hail Rod._--A device analogous to a lightning rod, supposed to
    have the property of averting the fall of hail. Hail rods have been
    especially popular in France, where they are called _paragrêles_.
    Large hail rods of recent construction are known as “electric
    Niagaras.”

    _Hail-shooting._--Bombarding the clouds to prevent the fall of hail.

    _Halo._--A generic name for a large group of optical phenomena
    caused by ice crystals in the atmosphere. The commonest of these
    phenomena is the _halo of 22 degrees_ (i. e., of 22 degrees radius)
    surrounding the sun or moon. The _halo of 46 degrees_ and the rare
    _halo of 90 degrees_, or _halo of Hevelius_, also surround the
    luminary. Other forms of halo are the _tangent arcs_, _parhelia_
    (_or paraselenæ_), _parhelic_ (or _paraselenic_) _circle_,
    _anthelion_, etc.

    _Harmattan._--A dry, dusty wind of the west coast of Africa,
    blowing from the deserts.

    _Haze._--A lack of transparency in the atmosphere; sometimes due
    to irregularities in the density of the air (_optical haze_),
    sometimes to dust (_dust haze_, which when dense constitutes _dry
    fog_), sometimes to fine particles of water or ice (grading into
    true _fog_).

    _Helm and Bar._--A pair of clouds seen when the “helm wind” is
    blowing over Crossfell, an English mountain; the “helm” capping the
    mountain and the “bar” lying to leeward of it.

    _High._--An area of high barometric pressure; an anticyclone.

    _Hoarfrost._--(See _frost_.)

    _Hole in the Air._--A colloquial name for any condition in the
    atmosphere that suddenly decreases the lift of an aeroplane. (Cf.
    _bump_.)

    _Horse Latitudes._--The regions of calms and variable winds
    coinciding with the subtropical high-pressure belts lying on the
    poleward sides of the trade winds. (The term has generally been
    applied only to the northern of these two regions, in the North
    Atlantic Ocean, or to the portion of it near Bermuda.)

    _Hot Wave._--A period of abnormally high temperatures. It has
    sometimes been defined, in the United States, as a period of
    three or more consecutive days during each of which the maximum
    temperature is 90 degrees F. or over.

    _Hot Wind._--A hot, parching wind characteristic of certain
    continental interiors; especially Australia, northern India and the
    prairie region of the United States.

    _Humidity._--The degree to which the air is charged with water
    vapor; viz., the actual amount of water vapor present (_absolute
    humidity_, which may be expressed in terms of weight per unit
    volume or as vapor pressure), or the ratio which this amount
    bears to the maximum amount the air can contain at the prevailing
    temperature (_relative humidity_, expressed in percentage).

    _Hurricane._--A tropical cyclone; especially one of the West Indies
    region. (A cyclone originating in this region and passing northward
    into the temperate zone is still called a “West India hurricane,”
    even after it has assumed the character of an extratropical
    cyclone, and, if sufficiently severe, justifies the display of
    “hurricane warnings” at ports of the United States. “Hurricane”
    is also the designation of the highest wind force on the Beaufort
    scale, and is thus applied to any wind exceeding about seventy-five
    miles an hour.)

    _Hygrograph._--A self-registering hygrometer.

    _Hygrometer._--Any instrument for measuring the humidity of the air.

    _Hygroscope._--A device that gives a rough indication of the
    relative humidity of the air. Most hygroscopes are mere toys.

    _Ice Rain._--1. A rain that causes a deposit of glaze. 2. Falling
    pellets of clear ice (called _sleet_ by the U. S. Weather Bureau).

    _Ice Saints._--A period of cold weather popularly reputed in Europe
    to occur yearly about May 11-13 (or, in south-central Europe, May
    12-14); also, the saints whose days in the ecclesiastical calendar
    fall on these dates.

    _Ice Storm._--(See _glaze_.)

    _Ignis Fatuus._--Will-o’-the-wisp. (See Chapter XXII.)

    _Indian Summer._--A period of mild, calm, hazy weather occurring in
    autumn or early winter, especially in the United States and Canada;
    popularly regarded as a definite event in the calendar, but weather
    of this type is really of irregular and intermittent occurrence.
    (Cf. _St. Martin’s Summer_.)

    _Instrument-shelter._--The American name of the cage or screen in
    which thermometers are exposed at meteorological stations. Called
    _thermometer-screen_ in Great Britain.

    _Inversion._--More fully _temperature inversion_; an increase of
    air temperature with increase of altitude, instead of the normal
    decrease.

    _Ion-counter._--An instrument for determining the number of ions
    present, per unit volume, in a sample of air.

    _Isobar._--A line of equal barometric pressure. (Isobars are
    generally drawn on maps to show the horizontal distribution of
    pressure reduced to sea level, or the pressure at some specified
    altitude; but in a broader sense any line on a chart or diagram
    drawn through places of equal pressure is an isobar.)

    _Isohyet._--A line of equal rainfall.

    _Isotherm._--A line of equal temperature.

    _Isothermal Layer._--(See _stratosphere_.)

    _January Thaw._--A period of mild weather popularly supposed to
    recur each January, especially in New England.

    _Katathermometer._--A device consisting of a dry-bulb and a
    wet-bulb thermometer, designed for measuring the cooling power of
    the atmosphere, with reference to its physiological effects. It was
    invented by Leonard Hill.

    _Kiosk._--The name given by the U. S. Weather Bureau to a small
    street pavilion in which are displayed meteorological instruments,
    maps, tables, etc.

    _Land and Sea Breezes. Land and Lake Breezes._--The breezes that,
    on certain coasts and under certain conditions, blow from the land
    by night and from the water by day.

    _Lenticular Cloud._--A cloud having approximately the form of a
    double-convex lens, marking the position of a standing wave in the
    atmosphere. (See Chapter VI.)

    _Lightning._--A disruptive electrical discharge in the atmosphere,
    or, generally, the luminous phenomena attending such a discharge.
    The various forms of lightning are named and described in Chapter
    IX.

    _Lightning Print._--A collection of marks, often treelike in form,
    sometimes found on the body of a person or animal that has been
    struck by lightning.

    _Lightning Rod._--A metallic rod, connected with a suitable
    “ground,” in earth or water, set up for the purpose of protecting
    some structure from lightning.

    _Light-pillar._--A form of halo, consisting of a column of light,
    vertical or nearly so, extending from or through the sun or moon.
    Called a _sun-pillar_, or a _moon-pillar_, as the case may be.

    _Line-squall._--A more or less continuous line of squalls and
    thunderstorms traveling broadside over the country.

    _Looming._--An apparent elevation of distant objects by mirage.

    _Low._--An area of low barometric pressure, with its attendant
    system of winds. Also called a _barometric depression_ or _cyclone_.

    _Mackerel Sky._--An area of sky covered with cirro-cumulus clouds;
    especially when the clouds resemble the patterns seen on the backs
    of mackerel.

    _Mammato-cumulus._--A form of cloud showing pendulous sacklike
    protuberances. (See Chapter VI.)

    _March._--The variation of a meteorological element in the course
    of a day, year, or other interval of time; e. g., the diurnal
    march of temperature; the annual march of barometric pressure.

    _Mares’-tails._--Cirrus in long slender streaks.

    _Marine Climate._--A type of climate characteristic of the ocean
    and oceanic islands. Its most prominent feature is equability of
    temperature.

    _Meteorograph._--Autographic apparatus for recording simultaneously
    two or more meteorological elements. Certain types of meteorograph
    are connected, electrically or otherwise, with some of the
    instruments at meteorological stations; others are sent aloft
    attached to kites and balloons.

    _Meteorology._--The science of the atmosphere.

    _Millibar._--(See _bar_.)

    _Mirage._--An apparent displacement or distortion of distant
    objects by abnormal atmospheric refraction. Sometimes the images of
    objects are inverted, multiplied, etc.

    _Mist._--Generally, a wet fog or a very fine drizzle of rain; hence
    the expression, “it is misting.” The “Scotch mist” of mountainous
    or hilly regions is a combination of thick fog and heavy drizzle;
    it has been suggested that this occurs when the rain clouds
    actually rest on the earth.

    _Mistpoeffer._--(See _brontide_.)

    _Mistral._--Along the Mediterranean coast, from the mouth of the
    Ebro to the Gulf of Genoa, a stormy, cold northerly wind, blowing
    down from the mountains of the interior. (The name is sometimes
    applied to northerly winds on the Adriatic, in Greece, and in
    Algeria.)

    _Monsoon._--A wind that reverses its direction with the season,
    blowing more or less steadily from the interior of a continent
    toward the sea in winter, and in the opposite direction in the
    summer.

    _Moon Dog._--A paraselene.

    _Mountain and Valley Breezes._--The breezes that, in mountainous
    regions, normally blow up the slopes by day (_valley breeze_) and
    down the slopes by night (_mountain breeze_).

    _Nephoscope._--An instrument for measuring the movements of clouds.

    _Nieve Penitente._--Fields of pinnacled snow found on certain high
    mountains in tropical or subtropical regions.

    _Nimbus._--The rain cloud. (See Chapter VI.)

    _Noctilucent Clouds._--Luminous, cirruslike clouds sometimes
    visible throughout the short nights of summer; supposed to be
    clouds of dust at great altitudes shining with reflected sunlight.
    Such clouds were observed during several summers after the eruption
    of Krakatoa (1883) and are still occasionally reported.

    _Normal._--The average value which, in the course of years, any
    meteorological element is found to have on a specified date or
    during a specified month or other portion of the year, or during
    the year as a whole. Also used as an adjective in such expressions
    as “normal temperature,” etc. Thus, for any station at which
    records have been maintained for many years, we may compute the
    normal temperature of January 1, the normal pressure of February,
    the normal rainfall of the year, etc. The normal serves as a
    standard with which values occurring in a particular year may be
    compared in order to determine the _departure from the normal_.

    _Nucleus._--A particle upon which condensation of water vapor
    occurs in the free atmosphere in the form of a water drop or an ice
    crystal.

    _Oblique Arcs of the Anthelion._--A rare form of halo, consisting
    of intersecting arcs, usually white, passing through the anthelion
    or the place where the anthelion would occur if visible.

    _Ozone._--An allotropic form of oxygen, which occurs transiently
    in small quantities in the lower atmosphere, and is supposed to be
    permanently present and relatively abundant at high atmospheric
    levels.

    _Painter._--A dirty fog frequently experienced on the coast of
    Peru. The brownish deposit from it is sometimes called “Peruvian
    paint.”

    _Paragrêle._--A hail rod.

    _Paranthelion._--A halo phenomenon similar to a parhelion, but
    occurring at a distance of 90 degrees or more in azimuth from the
    sun. The solar distance of the ordinary paranthelia is 120 degrees.
    (Analogous phenomena produced by the moon as source of light are
    called _parantiselenæ_.)

    _Paraselene._--(Plu. _paraselenæ_.) (See _parhelion_.)

    _Paraselenic Circle._--(See _parhelic circle_.)

    _Parasitic Clouds._--The name formerly given to clouds capping the
    summits of mountains.

    _Parhelic Circle._--A halo consisting of a white circle passing
    through the sun and parallel to the horizon. A similar phenomenon
    in connection with the moon is called a _paraselenic circle_.

    _Parhelion._--A mock sun, or sun dog; a form of halo consisting
    of a more or less distinctly colored image of the sun at the
    same altitude as the latter above the horizon, and hence lying
    on the parhelic circle, if present. The ordinary parhelia are 22
    degrees from the sun in azimuth, or a little more, according to
    the altitude of the luminary. Parhelia have occasionally been
    seen about 46 degrees from the sun. Analogous phenomena seen in
    connection with the moon are called _paraselenæ_, _mock moons_, _or
    moon dogs_.

    _Penetrating Radiation._--A form of radiation that has the property
    of passing through a great extent of air without being absorbed and
    of ionizing the air inside hermetically sealed metal vessels. It
    is supposed to consist of a special kind of Gamma rays and to come
    from the higher levels of the atmosphere.

    _Phenology._--The study of the periodic phenomena of animal and
    plant life and their relations to weather and climate.

    _Photochemical Climate._--The chemical activity of sunlight
    characteristic of any place or region. The variations of this
    feature of solar radiation are more or less strikingly different
    from those of solar heat and the brightness of solar light. Certain
    types of “chemical actinometer” are used in its measurement.

    _Pilot Balloon._--A small free balloon the drift of which, as
    observed from the ground, indicates the movements of the air aloft.

    _Pocky Cloud._--Mammato-cumulus.

    _Pogonip._--A fog, composed of fine needles of ice, which occurs in
    mountainous regions of the western United States and is reputed to
    be very injurious to the lungs.

    _Pollution Gauge._--A gauge for measuring soot and other impurities
    found in the atmosphere.

    _Pontias._--A wind that blows by night from a narrow valley at
    Nyons, France.

    _Potential Gradient._--(See _gradient_.)

    _Precipitation._--The collective name for deposits of atmospheric
    moisture in liquid and solid form, including rain, snow, hail, dew,
    hoarfrost, etc.

    _Pressure._--An elliptical expression, current in meteorological
    literature, for _atmospheric pressure_, or _barometric pressure_.

    _Prevailing Westerlies._--The belts of winds lying on the poleward
    sides of the subtropical high-pressure belts.

    _Psuchrainometer._--An instrument devised by J. R. Milne for
    measuring the cooling power of the atmosphere with reference to
    physiological effects.

    _Psychrometer._--An instrument for measuring atmospheric humidity,
    consisting usually of a _dry-bulb thermometer_ and a _wet-bulb
    thermometer_. The former is an ordinary mercurial thermometer.
    The latter has its bulb covered with muslin or other fabric,
    which is either permanently wet or is wetted before use. In some
    psychrometers there is only one thermometer, readings being taken
    both before and after moistening the bulb.

    _Purple Light._--The purple or rosy glow observed over a large
    area of the western sky after sunset and the eastern sky before
    sunrise; it lies above the _bright segment_ that borders the
    horizon.

    _Pyrheliometer._--An instrument that measures solar radiation by
    its heating effects.

    _Qobar._--A dry fog or heat haze of the upper Nile region. (Also
    spelled _kobar_ and _gobar_ and occasionally applied to a hazy
    condition of the atmosphere in other parts of the world.)

    _Rain Balls._--Mammato-cumulus.

    _Rainbow._--A luminous arc formed by the refraction and reflection
    of light in drops of water. (See Chapter X.)

    _Rainfall._--A term sometimes synonymous with _rain_, but most
    frequently used in reference to amounts of precipitation (including
    snow, hail, etc.).

    _Rain Gauge._--An instrument for measuring rainfall.

    _Rain Tree._--A mythical tree which is alleged to exude copious
    showers of water even in the driest weather. (“Rain tree” is also
    the common name of an ornamental tree variously known to botanists
    as _Albizzia saman_, _Pithecolobium saman_, _Enterolobium saman_,
    and _Samanea saman_.)

    _Recoloration._--(See _afterglow_.)

    _Recurrence._--The alleged tendency of any particular type of
    weather to occur at about the same period every year, independently
    of and generally in contrast to the regular march of the seasons.

    _Refraction._--_Astronomical refraction._ Change in the apparent
    position of a heavenly body, due to atmospheric refraction.
    _Terrestrial refraction._ Change in the apparent position of
    distant terrestrial objects, due to the same cause.

    _Relative Wind._--In aeronautics, the motion of the air with
    reference to an aeroplane or airship moving through it.

    _Réseau._--A collection of meteorological stations operating under
    a common direction, or in the same territory. An _international
    réseau_ is a group of stations in different countries cooperating
    for any purpose. The _Réseau mondial_ is a world-wide system of
    selected stations, the observations of which may be utilized in
    studies of the meteorology of the globe.

    _Rime._--1. Hoarfrost. 2. A rough or feathery coating of ice
    deposited on terrestrial objects by fog. (The second meaning is the
    one now used in technical literature.)

    _Roaring Forties._--(See _brave west winds_.)

    _Saddle._--(See _col_.)

    _Saint Elmo’s Fire._--A luminous brush discharge of electricity
    from elevated objects, such as the masts and yardarms of ships,
    lightning rods, steeples, etc., occurring in stormy weather. Also
    called corposant.

    _St. Martin’s Summer._--One of several names given in Europe to a
    mild period in autumn, corresponding approximately to the Indian
    summer of North America.

    _Scarf Cloud._--A thin cirruslike cloud which often drapes the
    summits of tall cumulo-nimbus clouds.

    _Scotch Mist._--(See _mist_.)

    _Scud._--Shreds or small detached masses of cloud moving rapidly
    below a rain cloud or other heavy clouds.

    _Sea Breeze._--(See _land and sea breezes_.)

    _Secondary._--A small cyclone developed, or tending to develop, on
    the border of a large one.

    _Sensible Temperature._--The temperature felt at the surface of
    the human body; formerly identified, by some authorities, with the
    temperature indicated by the wet-bulb thermometer.

    _Silence, Areas, Zones, or Regions of._--Regions within which a
    sound is not heard, though heard in regions more distant from the
    source.

    _Silver Thaw._--A term variously applied to rime, glaze, and a thin
    coating of ice deposited on cold objects by a damp wind.

    _Simoom._--An intensely hot and dry wind of Asian and African
    deserts; often described as a sand storm or dust storm, but certain
    authorities state that the typical simoom is free from sand or dust.

    _Sirocco._--A name applied to various types of warm wind in the
    Mediterranean region. Some of these siroccos are foehns. The term
    is also used as the generic name for winds blowing from a warm
    region toward an area of low pressure in a normally colder region.

    _Sleet._--1. Frozen or partly frozen rain; frozen raindrops in the
    form of particles of clear ice. 2. Snow and rain falling together.
    (Other definitions have been proposed and the proper technical
    application of this term is still a subject of controversy. In
    popular and engineering use the word is often applied to a coating
    of glaze on trees, wires, rails, etc.)

    _Snow Bin._--A large receptacle for collecting the snowfall of an
    entire winter, or other long period, for measurement at one time.

    _Snow Garland._--An elongated mass of snow suspended at the ends
    and sagging in the middle.

    _Snow Mushroom._--An overhanging cap of snow resting on a tree
    stump, post, or the like.

    _Snow Roller._--A mass of snow rolled by the wind; generally
    muff-shaped.

    _Snow Sampler._--A device for collecting a sample of snow, cut
    vertically through a snowfield, for the purpose of determining the
    depth and density of the snow.

    _Snow Survey._--A measurement of the total amount of snow lying
    over a particular area, especially with a view to determining the
    total amount of water it will yield, when melted, for purposes of
    irrigation, etc.

    _Soft Hail._--Graupel.

    _Sounding Balloon._--A free, unmanned balloon carrying a set of
    self-registering meteorological instruments.

    _Specter of the Brocken._--The shadow of an observer and of objects
    in his immediate vicinity cast upon a cloud or fog bank; sometimes
    attended by a series of colored rings, called the _glory_ or
    _Brocken-bow_.

    _Squall._--1. A sudden storm of brief duration; closely akin to a
    thunderstorm but not necessarily attended by thunder and lightning.
    2. A sudden brief blast of wind, of longer duration than a gust.

    _Squaw Winter._--In North America, a brief cold spell popularly
    reputed to precede Indian summer.

    _Static._--(See _stray_.)

    _Storm Card._--A device intended for use on shipboard in
    determining the direction of a storm center from the ship.

    _Storm Water._--The water resulting from a heavy and rapid fall of
    rain; especially the portion that occurs as run-off. (The term has
    been used mainly in connection with the subject of sewers.)

    _Strato-cumulus._--A form of cloud. (See Chapter VI.)

    _Stratosphere._--The upper region of the atmosphere, formerly
    called the _isothermal layer_, in which there is no marked or
    systematic decrease of temperature with altitude. The stratosphere
    is free from clouds (except occasional dust clouds) and from strong
    vertical air currents, and its circulation appears to be more or
    less independent of that of the lower atmosphere. The height of its
    base, which varies with latitude and otherwise, averages between 6
    and 7 miles. (Cf. _troposphere_.)

    _Stray._--A natural electromagnetic wave in the ether. The term
    is used in reference to the effects of such waves in producing
    erratic signals in radiotelegraphic receivers. Strays are known
    collectively as _static_.

    _Summer Day._--A day in which the temperature reaches or exceeds 77
    degrees F.

    _Sun Dog._--A mock sun or parhelion.

    “_Sun Drawing Water._”--The sun popularly said to be “drawing
    water” when crepuscular rays extend down from it toward the horizon.

    _Sunshine Recorder._--An instrument for recording the duration of
    sunshine; certain types also record the intensity of sunshine.

    _Synchronous Chart._--A form of synoptic chart, such as the
    ordinary weather map, which shows the meteorological conditions
    prevailing over any area at a given moment of time.

    _Synoptic Chart._--A chart showing the distribution of
    meteorological conditions over an area at a given moment or the
    average conditions during a given period of time, such as a month
    or a year.

    _Table-cloth._--A sheet of cloud that sometimes spreads over the
    flat top of Table Mountain, near Cape Town.

    _Tangent Arc._--Any halo that occurs as an arc tangent to one of
    the heliocentric halos.

    _Term Hours._--Prescribed hours for taking meteorological
    observations.

    _Thermal Belt._--A well-defined zone, found on some mountain-sides,
    in which vegetation is particularly exempt from frosts in spring
    and autumn. Also called _verdant zone_.

    _Thermograph._--A self-registering thermometer.

    _Thermometer._--An instrument for measuring temperature; in
    meteorology, generally the temperature of the air. _Maximum_
    and _minimum thermometers_ indicate, respectively, the highest
    and lowest temperatures occurring between the times of setting
    the instruments. A _wet-bulb thermometer_ is used in measuring
    humidity. (See _psychrometer_.)

    _Thermometer Screen._--A cage, or sometimes merely a roof, for
    protecting thermometers from direct sunshine and other radiation
    that would cause the readings to indicate a temperature different
    from that of the air. In the United States generally called an
    _instrument shelter_.

    _Thunder._--The sound produced by a lightning discharge.

    _Thunderstorm._--A storm attended by thunder and lightning.
    Thunderstorms are local disturbances, often occurring as episodes
    of cyclones, and, in common with squalls, are marked by abrupt
    variations in pressure, temperature, and wind.

    _Thunderstorm Recorder._--Any device that furnishes a record of
    thunderstorms, either near or distant. Most thunderstorm recorders
    register, by radiotelegraphy, the strays set up by lightning
    discharges.

    _Tornado._--1. A violent vortex in the atmosphere, usually attended
    by a pendulous, more or less funnel-shaped cloud. 2. In West
    Africa: A violent thundersquall.

    _Totalizer._--A form of snow gauge used chiefly in the Alps,
    designed to be read only once a year. (French, _totalisateur_.)

    _Trade Winds._--Two belts of winds, one on either side of the
    equatorial doldrums, in which the winds blow almost constantly from
    easterly quadrants.

    _Troposphere._--The part of the atmosphere lying below the
    stratosphere.

    _Trough._--1. A line drawn at right angles to the path of a
    cyclonic area through all points at which the pressure has reached
    a minimum and is about to rise. 2. An elongated area of low
    barometric pressure.

    _Twilight._--_Astronomical twilight_ is the interval between
    sunrise or sunset and the total darkness of night. _Civil twilight_
    is the period of time before sunrise and after sunset during which
    there is enough daylight for ordinary outdoor occupations.

    “_Twister._”--A tornado; also, a small whirling dust storm.

    _Typhoon._--The name applied in the Far East to a tropical cyclone.

    _Ulloa’s Ring._--1. A glory. 2. A halo (also called _Bouguer’s
    halo_) surrounding a point in the sky diametrically opposite the
    sun; sometimes described as a “white rainbow.”

    _Vane._--A device that shows which way the wind blows; also called
    _weather vane_ or _wind vane_.

    _Variability._--_Interdiurnal variability_ is the mean difference
    between successive daily means of a meteorological element.

    _Verdant Zone._--(See _thermal belt_.)

    _Visibility._--The transparency and illumination of the atmosphere
    as affecting the distance at which objects can be seen. It is often
    expressed on a numerical scale. (This term was formerly applied
    by British meteorologists to a state of unusual clearness of the
    atmosphere, regarded as a weather prognostic.)

    _V-shaped Depression._--A trough of low barometric pressure bounded
    on the weather map by V-shaped isobars.

    _Waterspout._--A tornadolike vortex and cloud occurring over a body
    of water.

    _Weather Well._--(See _blowing well_.)

    _Wedge._--A wedge-shaped area of high barometric pressure.

    _Wet-bulb thermometer._--(See _psychrometer_.)

    _Wind._--Moving air; especially a mass of air having a common
    direction of motion. The term is generally limited to air moving
    horizontally, or nearly so; vertical streams of air are usually
    called “currents.”

    _Wind Aloft._--The wind blowing at high levels as distinguished
    from that near the earth’s surface.

    _Wind Rose._--1. A diagram showing the relative frequency and
    sometimes also the average strength of the winds blowing from
    different directions in a specified region. 2. A diagram showing
    the average relation between winds from different directions and
    the occurrence of other meteorological phenomena.

    _Winter Day._--A day on which the temperature does not at any time
    rise above the freezing point.

    _Wool-pack Cloud._--Cumulus.



Transcribers’ Notes


Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in this book; otherwise they were not
changed.

Simple typographical errors were corrected; occasional unbalanced
quotation marks retained.

Ambiguous hyphens at the ends of lines were retained.

In versions of this book that do not support accented letters, “Canon”
(with a tilde over the middle “n”) is spelled “Canyon”.

Page 103: For consistency with the preceding enumerated definitions,
the Transcriber added extra spacing after the last of them, just before
“We have given”.

Page 348: In some versions of this eBook, the chemical symbols for
methane and phosphine are represented as CH4 and PH3.

Page 367: “The scale runs from 0 = calm to = hurricane” is missing the
value for hurricane, which is “12”.





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