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Title: Practical Exercises in Elementary Meteorology
Author: Ward, Robert DeCourcy
Language: English
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       *       *       *       *       *



                          PRACTICAL EXERCISES

                                   IN

                         ELEMENTARY METEOROLOGY

                                   BY

                          ROBERT DECOURCY WARD

            INSTRUCTOR IN CLIMATOLOGY IN HARVARD UNIVERSITY

                             BOSTON, U.S.A.
                       GINN & COMPANY, PUBLISHERS

                          =The Athenæum Press=

                                  1899


                          COPYRIGHT, 1899, BY
                          ROBERT DECOURCY WARD

                          ALL RIGHTS RESERVED



                                PREFACE.


The advance of meteorology as a school study has been much hampered by the
lack of a published outline of work in this subject which may be
undertaken during the school years. There are several excellent text-books
for more advanced study, but there is no laboratory manual for use in the
elementary portions of the science. In many secondary schools some
instruction in meteorology is given, and the keeping of meteorological
records by the scholars is every year becoming more general. There is yet,
however, but little system in this work, and, in consequence, there is
little definite result. The object of this book is to supply a guide in
the elementary observational and inductive studies in meteorology. This
Manual is not intended to replace the text-books, but is designed to
prepare the way for their more intelligent use. Simple preliminary
exercises in the taking of meteorological observations, and in the study
of the daily weather maps, as herein suggested, will lay a good foundation
on which later studies, in connection with the text-books, may be built
up. Explanations of the various facts discovered through these exercises
are not considered to lie within the scope of this book. They may be found
in any of the newer text-books.

This Manual lays little claim to originality. Its essential features are
based on the recommendations in the Report on Geography of the Committee
of Ten. A scheme of laboratory exercises, substantially the same as that
proposed in this Report, was, for some fifteen years, the basis of the
work in elementary meteorology done in Harvard College under the direction
of Professor William M. Davis. The plan proposed by the Committee of Ten
has been thoroughly tested by the writer during the past five years, not
only in college classes, but also in University Extension work among
school teachers, and the present book embodies such modifications of that
scheme and additions to it as have been suggested by experience. Emphasis
is laid throughout this Manual on the larger lessons to be learned from
the individual exercises, and on the relations of various atmospheric
phenomena to human life and activities. No attempt is made to specify in
exactly what school years this work should be undertaken. At present, and
until meteorology attains a recognized position as a school study,
teachers must obviously be left to decide this matter according to the
opportunities offered in each school. The general outline of the work,
however, as herein set forth, is intended to cover the grammar and the
high school years, and may readily be adapted by the teacher to fit the
circumstances of any particular case.

This book contains specific instructions to the student as to the use of
the instruments; the carrying out of meteorological observations; the
investigation of special simple problems by means of the instruments; and
the practical use of the daily weather maps. The Notes for the Teacher, at
the end of the book, are explanatory, and contain suggestions which may be
useful in directing the laboratory work of the class.

It has been the privilege of the author during the past ten years to study
the science of meteorology, and the methods of teaching that science,
under the constant direction of Professor William Morris Davis, of Harvard
University. To Professor Davis the author is further indebted for many
valuable suggestions in connection with the arrangement and treatment of
the subject-matter of this book. Thanks are due also to Mr. William H.
Snyder, of Worcester (Mass.) Academy, and to Mr. John W. Smith, Local
Forecast Official of the United States Weather Bureau, Boston, Mass., for
valued criticisms.

                                                 ROBERT DEC. WARD.

  HARVARD UNIVERSITY, CAMBRIDGE, MASS.,
              September, 1899.



                               CONTENTS.


                             INTRODUCTION.

                                                                  PAGE

  THE IMPORTANCE OF METEOROLOGY: ITS RELATIONS TO MAN               xi


                PART I.——NON-INSTRUMENTAL OBSERVATIONS.

  CHAPTER I.——OBSERVATIONS OF TEMPERATURE, WIND DIRECTION AND
  VELOCITY, STATE OF SKY, AND RAINFALL                               1


                  PART II.——INSTRUMENTAL OBSERVATIONS.

  CHAPTER II.——ELEMENTARY INSTRUMENTAL OBSERVATIONS                 11

  CHAPTER III.——ADVANCED INSTRUMENTAL OBSERVATIONS                  26


          PART III.——EXERCISES IN THE CONSTRUCTION OF WEATHER
                                 MAPS.

  CHAPTER IV.——THE DAILY WEATHER MAP                                47

  CHAPTER V.——TEMPERATURE                                           51

  CHAPTER VI.——WINDS                                                70

  CHAPTER VII.——PRESSURE                                            76

  CHAPTER VIII.——WEATHER                                            85


           PART IV.——THE CORRELATIONS OF THE WEATHER ELEMENTS
                        AND WEATHER FORECASTING.

  CHAPTER IX.——CORRELATION OF THE DIRECTION OF THE WIND AND THE
  PRESSURE                                                          91

  CHAPTER X.——CORRELATION OF THE VELOCITY OF THE WIND AND THE
  PRESSURE                                                          93

  CHAPTER XI.——FORM AND DIMENSIONS OF CYCLONES AND ANTICYCLONES     96

  CHAPTER XII.——CORRELATION OF CYCLONES AND ANTICYCLONES AND
  THEIR WIND CIRCULATION                                            98

  CHAPTER XIII.——CORRELATION OF THE DIRECTION OF THE WIND AND
  THE TEMPERATURE                                                  101

  CHAPTER XIV.——CORRELATION OF CYCLONES AND ANTICYCLONES AND
  THEIR TEMPERATURES                                               104

  CHAPTER XV.——CORRELATION OF THE DIRECTION OF THE WIND AND
  THE WEATHER                                                      106

  CHAPTER XVI.——CORRELATION OF CYCLONES AND ANTICYCLONES AND
  THE WEATHER                                                      109

  CHAPTER XVII.——PROGRESSION OF CYCLONES AND ANTICYCLONES          111

  CHAPTER XVIII.——SEQUENCE OF LOCAL WEATHER CHANGES                113

  CHAPTER XIX.——WEATHER FORECASTING                                114


            PART V.——PROBLEMS IN OBSERVATIONAL METEOROLOGY.

  CHAPTER XX.——TEMPERATURE                                         125

  CHAPTER XXI.——WINDS                                              130

  CHAPTER XXII.——HUMIDITY, DEW, AND FROST                          132

  CHAPTER XXIII.——CLOUDS AND UPPER AIR CURRENTS                    136

  CHAPTER XXIV.——PRECIPITATION                                     138

  CHAPTER XXV.——PRESSURE                                           139

  CHAPTER XXVI.——METEOROLOGICAL TABLES                             142


                              APPENDIX A.

  SUGGESTIONS TO TEACHERS                                          171


                              APPENDIX B.

  THE EQUIPMENT OF A METEOROLOGICAL LABORATORY                     186


  INDEX                                                            197



                       ACKNOWLEDGMENT OF FIGURES.


  1, 7, 8, 9, 10, 16. Meteorological Instruments. H. J. Green, 1191
      Bedford Avenue, Brooklyn, N. Y.

  2, 4. Instrument Shelter and Rain Gauge. _Instructions for Voluntary
      Observers._ United States Weather Bureau.

  5. Mercurial Barometer. L. E. Knott Apparatus Co., 14 Ashburton
      Place, Boston, Mass.

  12, 15, 53. Thermograph and Barograph Curves, and Cyclonic
      Composite. Davis, _Elementary Meteorology_.

  17. Nephoscope. _Annals Harvard College Observatory_, Vol. XX, Part
      I.

  48. North Atlantic Cyclone. _Pilot Chart of the North Atlantic
      Ocean._ United States Hydrographic Office.

  51. Wind Rose. _Quarterly Journal Royal Meteorological Society_,
      Vol. XXIV, No. 108.



                             INTRODUCTION.

          THE IMPORTANCE OF METEOROLOGY: ITS RELATIONS TO MAN.


We live in the laboratory of the earth’s atmosphere. The changes from hot
to cold, wet to dry, clear to cloudy, or the reverse, profoundly affect
us. We make and unmake our daily plans; we study or we enjoy vacations; we
vary our amusements and our clothing according to these changes. The
weather forecasts for the day in the newspaper are read even before the
telegraphic despatches of important events. Sailors about to put to sea
govern themselves according to the storm warnings of our Weather Bureau.
Farmers and shippers of fruit, meat, and vegetables anxiously watch the
bulletins of cold or warm waves, and guard against damage by frost or
excessive heat. Steam and electric railways prepare their snow-plows when
a severe snowstorm is predicted.

Meteorology, the science of the atmosphere, is thus of very great interest
and importance. There is no subject a knowledge of which does more to make
our daily life interesting. Since we live in the midst of the atmosphere
and cannot escape from the changes that take place in it, we must,
consciously or unconsciously, become observers of these changes. Examples
of the varying processes at work in the atmosphere are always with us.
There is no end to the number and the variety of our illustrations of
these processes. Man is so profoundly affected by weather changes from day
to day that all civilized countries have established weather services.
Observers taking regular weather records are stationed at thousands of
different places in all parts of the world, and the observations which
they make are used by meteorologists in preparing daily weather maps and
forecasts, and in studying the conditions of temperature, winds, and
rainfall. In the United States alone there are about 3000 of these
observers.

These observations are not made on land only. Hundreds of ship captains on
all the oceans of the world are making their regular daily meteorological
records, which at the end of the voyage are sent to some central
office,[1] where they are studied and employed in the preparation of Pilot
Charts for the use of mariners. By means of these ocean meteorological
observations, which were first systematized and carried out on a large
scale under the direction of Lieutenant Matthew Fontaine Maury (born,
1806; died, 1873), of the United States Navy, it has become possible to
lay out the most favorable sailing routes for vessels engaged in commerce
in all parts of the world.

[Footnote 1: In the United States, marine meteorological observations are
forwarded to the United States Hydrographic Office, Navy Department,
Washington.]

So important is a knowledge of the conditions of the winds and the
weather, that scientific expeditions into unexplored or little-known
regions give much of their time to meteorological observations. On the
famous Lady Franklin Bay Expedition (1881-1884) of Lieutenant (now
General) A. W. Greely, of the United States Army, meteorological
observations were kept up by the few feeble survivors, after death by
disease and starvation had almost wiped out the party altogether, and when
those who were left had but a few hours to live unless rescue came at
once. On Nansen’s expedition to the “Farthest North,” on Peary’s trips to
Greenland, and on every recent voyage to the Arctic or the Antarctic,
meteorological instruments have formed an important part of the equipment.

Not content with obtaining records from the air near the earth’s surface,
meteorologists have sent up their instruments by means of small, un-manned
balloons to heights of 10 miles; and the use of kites for carrying up such
instruments has been so successful that, at Blue Hill Observatory, near
Boston, Mass., records have been obtained from a height of over 2 miles.
Observatories have also been established on mountain summits, where
meteorological observations have been made with more or less regularity.
Such observatories are those on Pike’s Peak, Colorado (14,134 feet), Mont
Blanc, Switzerland (15,780 feet), and on El Misti, in southern Peru. The
latter, 19,200 feet above sea level, is the highest meteorological station
in the world.

The study of the meteorological conditions prevailing over the earth has
thus become of world-wide importance. In the following exercises we shall
carry out, in a small way, investigations similar to those which have
occupied and are now occupying the attention of meteorologists all over
the world.



                   PRACTICAL EXERCISES IN ELEMENTARY
                              METEOROLOGY.



                PART I.——NON-INSTRUMENTAL OBSERVATIONS.



                               CHAPTER I.

            OBSERVATIONS OF TEMPERATURE; WIND DIRECTION AND
                 VELOCITY; STATE OF SKY, AND RAINFALL.


Before beginning observations with the ordinary instruments, accustom
yourself to making and recording observations of a general character, such
as may be carried out without the use of any instruments whatever. Such
records include: _Temperature_; _Wind Direction and Velocity_; _State of
the Sky_, _and Rainfall_.

=Temperature.=——In keeping a record[2] of temperature without the use of a
thermometer, excellent practice is given in observations of the
temperature actually felt by the human body. Our bodies are not
thermometers. They do not indicate, by our sensations of heat or cold,
just what is the temperature of the surrounding air, but they try to
adjust themselves to the conditions in which they are. This adjustment
depends on many things beside the temperature of the air; _e.g._, the
moisture or humidity of the air; the movement of the air; the temperature
and the nearness of surrounding objects. In summer, a day on which the
temperature reaches 80° or 85° often seems much hotter than another day on
which the temperature rises to 95°. In winter, temperatures registered by
the thermometer as 10° or 15° above zero often feel a great deal colder
than temperatures of -5° or -10°. In recording your observations on
temperature, the record book may be divided into columns as follows:——

[Footnote 2: Each scholar will need a blank book in which to preserve the
observations.]

                     SAMPLE RECORD OF TEMPERATURE.

  +---------+---------+--------------+------------------------------------+
  |  DATE.  |  HOUR.  | TEMPERATURE. |         REMARKS.                   |
  +---------+---------+--------------+----------------------------------- +
  |Jan. 16  |  9 A.M. |   Chilly     |                                    |
  | “    “  | 12 M.   |   Warmer     |                                    |
  | “    “  |  4 P.M. |     “        | Growing slowly warmer all day.     |
  | “   17  |  8 A.M. |    Warm      | About the same as Jan. 16, 4 P.M.  |
  | “    “  | 11 A.M. |   Cooler     | Began to grow cooler about 10 A.M. |
  | “    “  |  3 P.M. |   Colder     | Steadily becoming colder.          |
  +---------+---------+--------------+------------------------------------+

The following are some of the questions you should ask yourself in
carrying out this work. It is not expected that you will be able to answer
all these questions at once, but that you will keep them in mind during
your studies, and try to discover the answers, as a result of your own
observations.

How does it feel to you out of doors to-day? Is it hot, warm, cool, or
cold? What is the difference between your feelings yesterday and to-day?
Between day before yesterday and to-day? Have you noticed any _regular_
change in your feelings as to warmth and cold during three or four
successive days? During the past week or two? During the past month? Is
there any difference between the temperature of morning, noon, afternoon,
and evening? Is there any _regular_ variation in temperature during the
day? Have there been any _sudden_ changes in temperature during the last
few days? Have these sudden changes brought warmer or cooler weather? Has
the warmer or cooler weather continued for a day or so, or has another
change quickly followed the first? Have the sudden changes, if you have
noted any, come at any regular times (as morning, afternoon, evening) or
at irregular intervals? Does there seem to you to be any definite system,
of any kind, in our changes of temperature? In what ways are people in
general affected by hot weather? By cold weather? What difference does a
very hot or a very cold day make in your own case?

=Wind Direction and Velocity.=——Wind is an important meteorological
element because it has many close relations to human life. It affects very
markedly our bodily sensations of heat or cold. A cold, calm day is
pleasanter than a cold, windy day. On the other hand, a hot, calm day is
usually much more uncomfortable than a hot, windy day. High winds cause
wrecks along seacoasts and damage houses, crops, and fruit trees. Sea
breezes bring in fresh, cool, pure air from the ocean on hot summer days.
In the tropics the sea breeze is so important in preserving the health of
Europeans in many places that it is known as “the doctor.” The movement of
wind through large cities carries off the foul air which has collected in
the narrow streets and alleys, and is thus a great purifying agent.

Record the _direction of the wind_ according to the four cardinal points
of the compass (N., E., S., and W.) and the four intermediate points (NE.,
SE., SW., and NW.). The direction of the wind is the point _from_ which
the wind blows. You can determine the points of the compass roughly by
noting where the sun rises and where it sets.

Note the _velocity of the wind_ according to the following scale, proposed
by Professor H. A. Hazen of the United States Weather Bureau.

  0 CALM.
  1 LIGHT; just moving the leaves of trees.
  2 MODERATE; moving branches.
  3 BRISK; swaying branches; blowing up dust.
  4 HIGH; blowing up twigs from the ground, swaying whole trees.
  5 GALE; breaking small branches, loosening bricks on chimneys.
  6 HURRICANE or TORNADO; destroying everything in its path.

The record book will need two additional columns when wind observations
are begun, as follows:——

                 SAMPLE RECORD OF TEMPERATURE AND WIND.

  +--------+-----------+--------------+------------+----------+--------------------------+
  |        |           |              |    WIND    |   WIND   |                          |
  |  DATE. |   HOUR.   | TEMPERATURE. |  DIRECTION.| VELOCITY.|         REMARKS.         |
  +--------+-----------+--------------+------------+----------+--------------------------+
  | Oct. 3 | 7.30 A.M. |     Cool     |     NE.    | Moderate | Temperature falling      |
  |        |           |              |            |          | since last evening.      |
  |        |           |              |            |          | Wind velocity increasing.|
  |        |           |              |            |          |                          |
  |  “ “   |   11 A.M. |     “ “      |    “ “     |   Brisk  | Temperature the          |
  |        |           |              |            |          |  same. Wind velocity     |
  |        |           |              |            |          |  still increasing.       |
  |        |           |              |            |          |                          |
  |  “ “   |    3 P.M. |      “ “     |    “ “     |   High   | Wind velocity still      |
  |        |           |              |            |          |  increasing.             |
  +--------+-----------+--------------+------------+----------+--------------------------+

What is the direction of the wind to-day? What is its velocity? Has its
direction or velocity changed since yesterday? If so, was the change
sudden or gradual? Have you noticed any calms? What was the direction of
the wind before the calm? What after the calm? Does there seem to be more
wind from one compass point than from another? Is there any relation
between the direction of the wind and its velocity? _i.e._, is the NW.
wind, for instance, usually a brisk or a high wind, or, is the SE. or S.
wind usually moderate? Does the wind usually change its direction
gradually, as from SE. to S., then to SW., then to W., etc., or does it
jump all at once, as from SE. to W.? Is there any relation between the
velocity of the wind and the hour of the day, _i.e._, does the wind seem
stronger or weaker at noon than in the morning or at night? Is it a common
occurrence to have a wind from the same direction for several successive
days, or are we apt to have different winds almost every day? Do you
notice any _systematic_ changes in wind direction which are often
repeated? What are these changes? Can you make a simple rule for them? In
what ways does the wind affect us?

=State of the Sky.=——By the _state of the sky_ is meant the condition of
the sky as to its cloudiness. Clouds add much to the beauty and variety of
nature. They are often gorgeously colored at sunset. By their changes in
form, color, and amount from day to day they relieve what might otherwise
be a wearisome succession of the same weather types. Prevailingly overcast
skies have a depressing effect. Prevailingly clear skies become
monotonous. A proper amount of bright sunshine is essential for the
ripening of crops, but too much sunshine may parch soil and vegetation,
and become injurious. Clouds bring rain; hence a sufficient amount of
cloudiness is just as necessary as a sufficient amount of sunshine. The
drift of clouds shows us the direction of movement of the air above us,
and is of considerable help in forecasting the weather. Fog, which is a
very low cloud, is in some cases so common as to be a meteorological
element of great importance. In the city of London, where fogs are very
prevalent, especially in winter, the average number of hours of bright
sunshine in December and January is only fifteen in each month. The London
fogs are, in great part, due to the presence in the air of vast numbers of
particles of soot and smoke from millions of fires. These particles
increase the density of the fog and prolong its duration.

The amount of cloudiness is recorded on a scale of _tenths_. A _clear_ sky
is one that is less than 3/10 cloudy; a _fair_ sky is from 3/10 to 7/10
cloudy; and a _cloudy_ sky is over 7/10 cloudy. In observing the state of
the sky, note such points as the times of clouding and of clearing; the
arrangement of the clouds, _i.e._, whether they are few and scattered, or
cover the sky with a uniform layer; the common forms of clouds; the
changes in the amounts of cloudiness, etc.

Another new column must be added in the record book for the cloudiness.
The table will now appear thus:——

       SAMPLE RECORD OF TEMPERATURE, WIND, AND STATE OF THE SKY.

  +-------+----------+--------------+----------------------+---------+----------------------+
  |       |          |              |         WIND.        |  STATE  |                      |
  | DATE. |  HOUR.   | TEMPERATURE. +-----------+----------+ OF SKY. |      REMARKS.        |
  |       |          |              | DIRECTION.| VELOCITY.|         |                      |
  +-------+----------+--------------+-----------+----------+---------+----------------------+
  |Dec. 18| 9 A.M.   | Very cold    |    NW.    | Brisk    |  Clear  |Very cold all night.  |
  |       |          |              |           |          |         |Everything frozen     |
  |       |          |              |           |          |         |up.                   |
  |       |          |              |           |          |         |                      |
  | “   “ | 5 P.M.   |  “     “     |     “     |    “     |    “    |Same conditions.      |
  |       |          |              |           |          |         |                      |
  | “   19| 8.30 A.M.| A little     |     “     | Moderate |   Fair  |Wind less violent.    |
  |       |          |   warmer     |           |          |         |Small clouds scattered|
  |       |          |              |           |          |         |over the sky.         |
  +-------+----------+--------------+-----------+----------+---------+----------------------+

Is the sky _clear_, _fair_, or _cloudy_ to-day? Is there more or less
cloud than there was yesterday? Than the day before yesterday? Is to-day a
day of increasing or of decreasing cloudiness? Is the sky usually
perfectly clear, or is it oftenest somewhat clouded over? How long does it
take for the sky to become completely covered with clouds from the time
when it first begins to become cloudy? When there are a few clouds in the
sky, are these usually scattered all over the sky, or are they in groups?
Have you noticed any particular form of clouds which seemed familiar to
you? Do clouds seem to have certain definite shapes and appearances which
are to be seen often? Do you discover any variation of cloudiness during
the day, _i.e._, is it apt to be more cloudy in the afternoon than in the
morning or at night? Can you make a list describing some of the clouds
that you see most often? Can you give these common kinds of clouds some
names of your own that shall describe them briefly? In what ways does a
clear sky, with bright sunshine, affect us?

=Rainfall.=——Under the general term _rainfall_, meteorologists include,
besides _rain_ itself, _snow_, _hail_, _sleet_, etc. The term
_precipitation_ is also often used. Rainfall stands in close relation to
human life and occupations. It feeds lakes and rivers, thus furnishing
means of transportation, power for running mills and factories, and water
supplies for cities. Regions of abundant rainfall are usually heavily
forested, like the Amazon valley in South America, and parts of Equatorial
Africa. In civilized countries lumbering is apt to be an important
occupation in districts of heavy rainfall, as in Oregon and Washington in
our own country, and in Southern Chile in South America. Where there is a
moderate rainfall, and other conditions are favorable, there agriculture
is possible, and farming becomes one of the chief occupations, as in the
Mississippi and Missouri valleys in the United States, and in Western
Canada. Districts which have a rainfall too small for successful
agriculture, but are not by any means deserts, are often excellent grazing
lands, as in the case of parts of Texas, Nebraska, and Kansas in the
United States, and the Argentine Republic in South America. Where there is
very little rainfall deserts are found. Cities are not built in deserts,
because there are no occupations to attract large numbers of men. The
inhabitants of the desert are wandering tribes, which move from place to
place in search of water and food for themselves and their animals. Rain
and snow cleanse the air, washing out impurities such as dust and smoke.
Hence they are important agents in preserving health.

Note the _kind_ of precipitation (rain, snow, hail, sleet); the _amount_
(heavy, moderate, light, trace); and the _time of the beginning and
ending_ of the storm or shower.

The record book must now be further subdivided into columns, to make room
for the rainfall observations, in this manner:——

  SAMPLE RECORD OF TEMPERATURE, WIND, STATE OF SKY, AND PRECIPITATION.

  +-------+---------+------------+--------------------+--------+-------------------+------------------+
  |       |         |            |        WIND.       |        |   PRECIPITATION.  |                  |
  |       |         |            +----------+---------+ STATE  +-------+-----+-----+                  |
  | DATE. |  HOUR.  |TEMPERATURE.|          |         |   OF   | TIME  |     |     |     REMARKS.     |
  |       |         |            |DIRECTION.|VELOCITY.|  SKY.  |  OF   |KIND.|AM’T.|                  |
  |       |         |            |          |         |        |BEGINN.|     |     |                  |
  +-------+---------+------------+----------+---------+--------+-------+-----+-----+------------------+
  |Mar. 21|8.30 A.M.|   Mild     |    S.    |  Light  |Overcast|8 A.M. |Rain |Light|  Raining.        |
  |       |         |            |          |         |        |       |     |     |                  |
  | “   “ |  12 M.  |     “      |    “     |    “    |Overcast|       |  “  |  “  |     “            |
  |       |         |            |          |         |        |       |     |     |                  |
  | “   “ |  4 P.M. |     “      |    “     |Moderate |Overcast|       |     |     |Stopped raining   |
  |       |         |            |          |         |        |       |     |     | about 3 P.M.     |
  |       |         |            |          |         |        |       |     |     |                  |
  | “   22|  8 A.M. |   Cool     |   NW.    |  Brisk  | Clear  |       |     |     |Cleared off during|
  |       |         |            |          |         |        |       |     |     | the night.       |
  +-------+---------+------------+----------+---------+--------+-------+-----+-----+------------------+

Does most of our rain come in brief showers, or in storms lasting a day or
two? Do we have about the same amount of rain or snow every week and every
month, or does the amount vary a good deal from week to week and from
month to month? Do you notice much difference in the characteristics of
successive storms, or do they all seem pretty much alike? Are
thunderstorms limited to any particular season of the year? If so, to what
season? Have you discovered any rule as to the time of day when rainstorms
or snowstorms begin? When thunderstorms begin and end? Is it common or
uncommon for us to have a storm lasting three or four days? How long does
a thunderstorm usually last? Do we have most hail in winter or in summer?
In what ways does a rainy day affect people? How are you yourself
affected? How does a heavy snowstorm affect travel and transportation? In
what ways does a snowstorm differ from a rainstorm as to the character of
the precipitation and its effects?

After studying the _temperature_, _wind_, _state of sky_, and _rainfall_
separately, take two elements together and see what relation one has to
the other. Try to answer such questions as these:——

=Temperature and Wind.=——What relations can you discover between the
direction of the wind and the temperature? Which winds are the coolest?
Which the warmest? Does a hot, calm day seem warmer or cooler than a hot,
windy day? Does a cold, calm day seem colder or warmer than a cold, windy
day? Does the _velocity_ of the wind have any effect on your feeling of
cold or of warmth? If so, what effect?

=Wind and State of Sky.=——Has the direction of the wind anything to do
with the cloudiness? Is there more apt to be considerable cloudiness with
wind from one direction than from another? What winds are usually
accompanied by the largest amount of cloud? What winds usually blow when
the sky is clear? Is the relation of cloudiness to certain wind directions
so close that, if you know the wind direction, you can make a prediction
as to the probable cloudiness? Are the winds with clouds more common in
one month than another? In one season than another? If so, which month?
which season?

=Temperature and State of Sky.=——Do you notice any relation between the
temperature and the state of the sky? In winter are our coldest days
usually cloudy or clear? In summer are our hottest days cloudy or clear?
Are the winds that give us the most cloudiness warm or cold winds in
winter and in summer? Is a cloudy night colder or warmer than a clear
night? Is a cloudy day colder or warmer than a clear day?

=State of Sky and Precipitation.=——How is rainfall or snowfall related to
the cloudiness? Do we ever have rain or snow when the sky is not
completely covered with clouds? Does the sky usually become quickly
covered with clouds before a rain? Does a sky wholly covered with clouds
always give us rain or snow? Does the sky clear rapidly or slowly after a
rain? Are any particular kinds of clouds associated with rain or with
snowstorms? With brief showers? With thunderstorms?

=Wind and Precipitation.=——Are any particular wind directions more likely
than others to give us rain or snow? Are these the same winds as those
which give us the most cloudiness? What winds are they? Has the velocity
of the wind any relation to the rain or snowstorm? Does the wind blow
harder before, during, or after the rain or snow? What changes of wind
direction have you noted before, during, and after any storm? Have you
noticed these same changes in other storms? Are they so common in our
storms that you can make a rule as to these changes?

=Temperature and Precipitation.=——Does a shower or a rainstorm in the
hotter months affect the temperature of the air in any way? How? In the
winter does the temperature show any changes before a snowstorm? Is it
usually warmer or colder then than a day or two before the storm and the
day after? Is it usually uncomfortably cold during a snowstorm? Are rainy
spells in the spring and the autumn months cooler or warmer than clear dry
weather?



                  PART II.——INSTRUMENTAL OBSERVATIONS.



                              CHAPTER II.

                 ELEMENTARY INSTRUMENTAL OBSERVATIONS.


The non-instrumental observations, suggested in the preceding chapter,
prepare the way for the more exact records of the weather elements which
are obtainable only by the use of instruments. The non-instrumental
records are not to be entirely given up, even after the instrumental work
and the weather-map exercises have begun, but should be continued
throughout the course. Notes on the forms and changes of clouds, on the
times of beginning and ending, and on the character of the precipitation,
as outlined in the last chapter, and other observations made without the
use of instruments, are an essential part of even the most advanced
meteorological records.

The simpler instruments are the _ordinary thermometer_, the _wind vane_,
the _rain gauge_, and the _mercurial barometer_ (in a modified form).
Observations with these instruments, although of a simple character, can
be made very useful. The advance over the non-instrumental observations,
which latter may be termed observations of _sensation_, is a decided one.
In place of the vague and untrustworthy statements concerning hot and
cold, warm and cool days, we now have actual degrees of temperature to
serve as a basis for comparison of day with day or month with month. The
measurements of rain and snowfall enable us to study the amounts brought
in different storms, the average precipitation of the various months, etc.
The important facts of change of pressure now become known, and also the
relation of these changes to the weather. Just as we have, in the earlier
work, become familiar with our typical weather changes and types, so we
shall now have our eyes opened to the actual values of the temperatures
and precipitation connected with these changes.

[Illustration: FIG. 1.]

The =ordinary thermometer= (Greek: _heat measure_), the most commonly used
and most widely known of all meteorological instruments, was in an
elementary form known to Galileo, and was used by him in his lectures at
the University of Padua during the years 1592 to 1609. Thermometers enable
us to measure the temperatures of different bodies by comparison with
certain universally accepted standards of temperature. These standards are
the freezing and boiling points of distilled water. In its common form the
thermometer consists of a glass tube, closed at the top, and expanding at
its lower end into a hollow spherical or cylindrical glass bulb. This bulb
and part of the tube are filled with mercury or alcohol. As the
temperature rises, the liquid expands, flows out of the bulb, and rises in
the tube. As the temperature falls, the mercury or alcohol contracts, and
therefore stands at a lower level in the tube. In order that the amount of
this rise or fall may be accurately known, some definite scale for
measurement must be adopted. The scale commonly used in this country owes
its name to Fahrenheit (born in Danzig in 1686; died in 1736), who was the
first to settle upon the use of mercury as the liquid in thermometers, and
also the first definitely to adopt two fixed points in graduating the
scale. The division of this scale into 180° between the freezing point
(32°) and the boiling point (212°) seems to have been taken from the
graduation of a semicircle. Fahrenheit was a manufacturer of all sorts of
physical apparatus, and it has been thought probable that he had some
special facilities for dividing his thermometer tubes into 180 parts.
Mercury is most commonly used as the liquid in thermometers, because it
readily indicates changes of temperature, and because over most of the
world the winter cold is not sufficient to freeze it. The freezing point
of mercury is about 40° below zero (-40° F.). Alcohol, which has a much
lower freezing point, is therefore used in thermometers which are to be
employed in very cold regions. Alcohol thermometers must, for instance, be
used in Northern Siberia, where the mean January temperature is 60° below
zero.

The temperature which meteorologists desire to obtain by the ordinary
thermometer is the _temperature of the free air in the shade_. In order
that thermometers may indicate this temperature, they must, if possible,
be placed in an open space where there is an unobstructed circulation of
the air, and they must be protected from the direct rays of the sun. They
are, therefore, usually _exposed_ inside of a cubical enclosure of wooden
lattice work, in an open space away from buildings, and at a height of 4
to 10 feet above the ground, preferably a grass-covered surface. This
enclosure is called the _shelter_, and its object is to screen the
instrument from the direct and reflected sunshine, to allow free
circulation of air around the bulb, and to keep the thermometer dry.
Sometimes the shelter, instead of being in an open space on the ground, is
built on the roof or against the north wall of a building, or outside of
one of the windows. Fig. 2 shows an ordinary shelter.

A still simpler method of exposure is described in the “Instructions for
Voluntary Observers” (United States Weather Bureau, 1892) as follows:
“Select a north window, preferably of an unoccupied room, especially in
winter. Fasten the blinds open at right angles to the wall of the house.
Fasten a narrow strip 3 inches wide across the window outside, and from 8
to 12 inches from the window-pane. To this fasten the thermometers.” If
none of these methods of sheltering the instrument is feasible, the
thermometer may be fastened to the window frame, about a foot from the
window, and so arranged that it can be read from the inside of the room
without opening the window.

[Illustration: FIG. 2.]

Readings of the thermometer, to the nearest degree of temperature
indicated on the scale by the top of the mercury column, are to be made at
the regular observation hours, and are to be entered in your record book.
Temperatures below zero are preceded by a minus sign (-). A table similar
to that suggested towards the close of the last chapter (p. 8) may be used
in keeping these instrumental records, except that actual thermometer
readings can now be entered in the column headed “Temperature,” instead of
using only the general terms _warm_, _cold_, _chilly_, etc. This is a
great gain. You will now be able to give fairly definite answers to many
of the questions asked in Chapter I. Answer these questions with the help
of your thermometer readings, as fully as you can.

The greater part of the Temperate Zone, in which we live, is peculiar in
having frequent and rapid changes of temperature, not only from season to
season, but from day to day, and during a single day. In winter, we are
apt to have a warm wind immediately after a spell of crisp cold weather.
In summer, cloudy, cool days come as a sudden relief when we have been
suffering from intense heat, with brilliant sunshine.

These changes give a variety to our climate which is, on the whole, very
beneficial to man. The North Temperate Zone, with strong seasonal changes
in temperature and weather, is the zone of the highest civilization and of
the greatest energy of man. In the Torrid Zone the changes of temperature
are, as a whole, small. There is no harsh winter. The climate is
monotonous and deadening, rather than enlivening. Man finds it easy to
live without much work, and the inhabitants of the Torrid Zone have not,
as a rule, advanced far in the scale of civilization.

The =wind vane= used by the Weather Bureau is about 6 feet long, and has a
divided tail made of pine boards, the two pieces making an angle of
22-1/2°. The purpose of this divided tail is to steady the vane and to
make it more sensitive to light currents.

[Illustration: FIG. 3.]

A common wind vane on a neighboring church steeple or flagstaff will
usually serve sufficiently well for ordinary use. Observations of wind
direction (to eight compass points) are to be made as a part of the
ordinary weather record, and to be entered in the proper column of the
record book.

The =rain gauge= consists of three separate parts, the receiver _A_, the
overflow attachment _B_, and the measuring tube _C_.

[Illustration: FIG. 4.]

The inside diameter of the top of the receiver in the standard Weather
Bureau gauge is 8 inches (at _a_ in Fig. 4). This receiver has a
funnel-shaped bottom, so that all the precipitation which falls into it
is carried at once into the measuring tube _C_, whose inside height is 20
inches. The diameter of the measuring tube is 2.53 inches. The rain
falling into the receiver _A_ fills this tube _C_ to a depth greater than
the actual rainfall, in proportion as the area of the receiver is greater
than the area of the measuring tube. In the standard Weather Bureau gauges
the ratio of the area of the receiver to the area of the measuring tube is
such that the depth of rainfall is magnified exactly ten times. The object
of magnifying the amount in this way is to measure a very small quantity
more easily. The narrow portion of the receiver [_d_] fits over the top of
the measuring tube, holding the latter firmly in place and preventing any
loss of rainfall. An opening, _e_, in the lower portion of the receiver
[_d_], just on a level with the top of the measuring tube, serves as an
escape for the water into the overflow attachment _B_, in case the
rainfall is so heavy as to more than fill the tube. The inside diameter of
the overflow attachment is the same as that of the receiver (8 inches), as
will be seen from the figure.

The rain gauge should be firmly set in a wooden frame, so arranged that
the overflow attachment can readily be removed from the frame. The box in
which the gauge is sent out by the manufacturer is usually designed to
serve as a permanent support when the gauge is set up. The best exposure
for the gauge is an open space unobstructed by large trees, buildings, or
fences. Fences, walls, or trees should be at a distance from the gauge not
less than their own height. If an exposure upon the ground is out of the
question, the gauge may be placed upon a roof, in which case the middle of
a flat unobstructed roof is the best position.

=Records of Rainfall.=——Every rain gauge is provided with a measuring
stick, which is graduated into inches and hundredths. It must be
remembered that the amount of rain in the measuring tube is, by the
construction of the ordinary gauge, ten times greater than the actual
rainfall. This fact need not, however, be taken into account by the
observer, for the numbers used in graduating the measuring sticks have all
been divided by 10, and therefore they represent the actual rainfall. The
graduations on the stick indicate hundredths of an inch, and should appear
in the record as decimals (.10, .20, etc.). Ten inches of water in the
measuring tube will reach the mark 1.00 on the stick; thus 1.00 denotes 1
inch and zero hundredths of rain. One inch of water in the tube will reach
the .10 mark, indicating 10/100 of an inch. The shortest lines on the
measuring stick denote successive hundredths of an inch. Thus, if water
collected comes to a point halfway between the .10 and .20 lines, the
amount is .15 inch, and so on. In measuring rainfall, the stick is lowered
through the bottom of the receiver into the measuring tube, and on being
withdrawn the wet portion of the stick at once shows the depth of water in
the tube. Care must be exercised to put the end of the stick where the
numbering begins first into the gauge, and to pass the stick through the
middle of the tube. After each observation the gauge should be emptied
and drained, and immediately put back into place. When the total rainfall
more than fills the measuring tube, _i.e._, exceeds 2 inches, the receiver
should first be lifted off and the tube removed with great care so as not
to spill any water. After emptying the tube, the surplus water in the
overflow attachment must be poured into the measuring tube and measured.
The amount of rainfall thus found is to be added to the 2 inches contained
in the measuring tube in order to give the total rainfall. If any water
happens to be spilled during its removal from the overflow attachment,
then the amount in the tube will be less than 2 inches, and it must be
carefully measured before the latter is emptied.

During the winter season, in all regions where snow forms the chief part
of the precipitation, the only portion of the rain gauge that need be
exposed is the overflow attachment. The snow which falls into the gauge
may be measured by first melting the snow and then measuring the water as
rainfall. About 10 inches of snow give, on the average, 1 inch of water,
but the ratio varies very greatly according to the density of the snow.
Besides the measurement of the melted snow collected in the gauge, it is
customary to keep a record of the depth of snowfall in inches, as measured
by means of an ordinary foot rule or a yardstick, on some level place
where there has been little or no drifting.

Measurements of rain and snowfall are usually made once a day, at 8 P.M.,
and also at the end of every storm. Enter the amounts of precipitation in
the column of the table headed “Amount” and state always whether it is
_rain_ or _melted snow_ that you have measured. When there has been no
precipitation since the last observation, an entry of 0.00 should be made
in the column of the record book devoted to “Amount of Precipitation.”
When the amount is too small to measure, the entry T (for _Trace_) should
be made.

Continue your non-instrumental record of the time of beginning and ending
of the precipitation as before. Whenever it is possible, keep a record of
the total amount of precipitation in each storm, noting this under
“Remarks.” Try to answer such questions as are asked in Chapter I with the
help of your instrumental record of the rain and snowfall. Note what
depths of snow in different snowstorms are necessary, when melted, to make
1 inch of water.

=The Mercurial Barometer.=——Air has weight. At sea level this weight
amounts to nearly 15 pounds on every square inch of surface. Imagine a
layer of water, 34 feet deep, covering the earth. The weight of this water
on every square inch of surface would be the same as the weight of the
air. Under ordinary circumstances the weight of the air is not noticeable,
because air presses equally in all directions, and the pressure within a
body is the same as that outside of it. On account of this equal pressure
in all directions, we speak of the _pressure_ of the air instead of its
_weight_. The effects of the air pressure may become apparent when we
remove the air from a surface. By working the piston of a pump in a well
we may remove the pressure on the surface of the water in the tube of the
pump. When this is done, a column of water rises in the tube until the top
of this column is about 34 feet above the level of the rest of the water
in the well. The pressure of the atmosphere on the water _outside_ of the
tube holds up this column of water _inside_ the tube.

Galileo (1564-1642) first taught that the air has weight. His pupil
Torricelli went a step further. Torricelli saw that the column of water,
held up by the pressure of the air in the tube of the pump, must exactly
balance a similar column of air, reaching from the surface of the water in
the well to the top of the atmosphere. The column of water, in other
words, exactly replaces this column of air. While working on this subject,
Torricelli, in 1643, performed the following experiment. He filled a glass
tube, about 3 feet long and closed at one end, with mercury. After
filling the tube, he put his finger over the open end and inverted the
tube over a vessel containing mercury. When the lower end of the tube was
below the surface of the mercury in the dish, he removed his finger. At
once the column of mercury fell in the tube until it stood at a height of
about 30 inches, leaving a vacant space of 6 inches in the upper part of
the tube. This space has since been known as the _Torricellian vacuum_.
Torricelli had proved what he had expected, viz., that the height of the
column of liquid which replaces and balances an air column of the same
size varies with the weight of that liquid. It takes a column of water 34
feet long to balance a similar column of air. It takes a column of mercury
only 30 inches long to balance a similar column of air. This, as
Torricelli correctly explained, is due to the fact that mercury is so much
(13-1/2 times) heavier than water. The column of water weighs just the
same as the column of mercury. Each column exactly balances an air column
of similar cross-section. The height of the water or of the mercury is a
measure of the weight or pressure of the air. The greater the pressure on
the surface of the water in the well, the higher will be the top of the
water in the pump. The greater the pressure on the surface of the mercury
in the basin, in the experiment of Torricelli, the higher will the mercury
column stand in the glass tube. Either water or mercury may be used as the
liquid in the barometer. Otto von Guericke (1602-1686), of Magdeburg,
constructed a water barometer about 36 feet long, which he attached to the
outside wall of his house. This barometer he used for some months, and
made predictions of coming weather changes by means of it. A water
barometer is, however, a very unwieldy thing to manage, on account of the
great length of its tube. Furthermore, water barometers cannot be used in
any countries where the temperatures fall to freezing. Mercury is the
liquid universally employed in barometers. It is so heavy that only a
small column of it is necessary to balance the atmospheric pressure.
Therefore a mercurial barometer is portable. Further, mercury does not
freeze until the temperature falls to 40° below zero.

Another name which should be mentioned in connection with the barometer is
that of Blaise Pascal, who in 1648 fully confirmed Torricelli’s results.
Pascal saw that if the mercury column is really supported by the weight of
the air, the height of that column must be less on the summit of a
mountain than at the base, because there is less air over the top of the
mountain than at the bottom, and therefore the weight of the air must be
less at the summit. To prove this, he asked his brother-in-law Perrier,
who lived at Clermont, in France, to carry the Torricellian tube up the
Puy-de-Dôme, a mountain somewhat over 3500 feet high in Central France.
This Perrier did on Sept. 19, 1648, and he found, as predicted by Pascal,
that the mercury fell steadily in the tube as he went up the mountain, and
that at the top of the mountain the column of mercury was over 3 inches
shorter than at the base.

The pressure of the atmosphere is a weather element which, unlike the
other elements already considered, cannot be observed without an
instrument. We cannot, under ordinary conditions at sea level, determine
by any of our senses whether the pressure is rising or falling, or is
stationary. The pressure on the upper floors of one of our high buildings
is shown by a barometer to be considerably lower than it is at the level
of the street below, and yet we notice no difference in our feelings at
the two levels.

It is only when we ascend far into the air, as in climbing a high mountain
or in a balloon, that the much-diminished pressure at these great heights
perceptibly influences the human body. Mountain climbers and aëronauts who
reach altitudes of 15,000 to 20,000 feet or more, usually suffer from
headache, nausea, and faintness, which have their cause in the reduced
pressure encountered at these heights.

[Illustration: FIG. 5.]

The ordinary mercurial barometer in use to-day is, essentially, nothing
more than the glass tube and vessel of Torricelli’s famous experiment. A
simple form of the mercurial barometer is shown in Fig. 5. It consists of
a glass tube about one-quarter of an inch in inside diameter and about 36
inches long. This tube, closed at the top and open at the bottom, is
filled with mercury, the lower, open end dipping into a cup of mercury
known as the _cistern_. The space above the mercury is a vacuum. The
mercury extends inside the tube to a height corresponding to the weight or
pressure of the air, the vertical height of the top of the mercury column
above the level of the mercury in the cistern, in inches and hundredths of
an inch, being the barometer reading. At sea level the normal barometer
reading is about 30 inches. There is an opening near the top of the
cistern, at the back of the instrument, through which the air gains access
to the mercury and holds up the mercury column. It will readily be seen
that, as the mercury in the tube rises, the level of the mercury in the
cistern falls, and _vice versa_, so that there is a varying relation
between the two levels. In order to have the reading accurate, it is
necessary that the surface of the mercury in the cistern should be just at
the zero of the barometer scale when a reading is made. To accomplish
this, the bottom of the cistern consists of a buckskin bag which may be
raised or lowered by means of a thumb-screw, seen at the lower end of the
instrument. The level of the mercury may thus be changed and adjusted to
the top of a black line, marked on the outside of the cistern, and which
indicates the zero of the scale. Before making a reading, the surface of
the mercury in the cistern must be raised or lowered until it _just_
reaches this black line. Then the top of the mercury column will give the
pressure of the air. The reading is made on an aluminum scale at the top
of the wooden back on which the tube is mounted, this scale being
graduated both on the English and on the metric system. This barometer may
be hung against the wall of a room.

The =aneroid barometer= (Greek: _without fluid_), although less desirable
in many ways than the mercurial, is nevertheless a useful instrument for
rough observations. The aneroid is not good for careful scientific work,
because its readings are apt to be rather inaccurate. To be of much value
in indicating exact pressures, it should frequently be compared with and
adjusted to a mercurial barometer. An ordinary aneroid barometer is shown
in Fig. 6.

[Illustration: FIG. 6.]

In this instrument the changes in atmospheric pressure are measured by
their effects in altering the shape of a small metallic box, known as the
_vacuum chamber_. The upper and lower surfaces of this box are made of
thin circular sheets of corrugated German silver, soldered together around
their outer edges, thus forming a short cylinder. From this the air is
exhausted, and it is then hermetically sealed. A strong steel spring,
inside or outside of the vacuum chamber, holds apart the corrugated
surfaces, which tend to collapse, owing to the pressure of the external
air upon them. An increase or decrease in the air pressure is accompanied
by an approach, or a drawing apart, of the surfaces of the chamber. These
slight movements are magnified by means of levers, a chain, and a spindle,
and are made to turn an index hand or pointer on the face of the
instrument. The outer margin of the face, underneath the glass, is
graduated into inches and hundredths, and the pressure may thus be read at
once.

As the tension of the steel spring varies with the temperature, aneroids
are usually _compensated_ for temperature by having one of the levers made
of two different metals, _e.g._, brass and iron, soldered together, or
else by leaving a small quantity of air in the vacuum chamber. This air,
when heated, expands, and thus tends to compensate for the weaker action
of the spring, due to the higher temperature. At best, however, this
compensation is but imperfect, and this fact, together with the friction
of the different parts, the changes in the spring with age, and the need
of frequent adjustments, makes aneroids rather inaccurate. They may be
adjusted to mercurial barometers by means of a small screw, whose head may
be found on the lower surface of the instrument. The words _fair_,
_stormy_, etc., which frequently appear on the face of aneroid barometers,
are of little use in foretelling weather changes, as no definite pressures
always occur with the same weather conditions. The instrument should be
tapped lightly a few times with the finger before a reading is made. The
second pointer, which is often found in aneroids, is set by the observer
on the position marked by the index hand when he makes his reading. The
difference between the pressure marked by this set pointer and that shown
by the index hand at the next observation is the measure of the change of
pressure in the interval.

Another column must now be added to the record book (preferably between
the columns devoted to _temperature_ and _wind_) to receive the “Pressure
in Inches and Hundredths.”

Is the pressure constant (_i.e._, are the readings always the same) or
does it vary? If it varies, is there any apparent system in the
variations? Is there a tendency to a daily maximum? To a daily minimum? If
so, about what time do these occur, respectively? What is the average
variation (in inches and hundredths) in the course of a day? What is the
greatest difference in pressure which you have observed in a day? What is
the least? Does the pressure seem to vary more or less in the colder
months than in the warmer? Has the height of the mercury column any
relation to the weather? Are we likely to have rainy weather with rising
barometer? Is the velocity of the wind related to the pressure in any way?
How? Can you make any general rules for weather prediction based on the
action of the barometer? What rules?

=Tabulation of Observations.=——The tables suggested in the preceding
chapter can be used unchanged with the simple instruments just described.

=Summary of Observations.=——At the end of each month summarize your
instrumental observations in the following way:——

_Temperature._——Add together all your temperature readings; divide their
sum by the total number of observations of temperature, and the quotient
will give you a sufficiently accurate _mean_ or _average_ temperature for
the month in question. It is to be noted that the mean monthly
temperatures obtained from these observations will be much more accurate
if the thermometer readings are made at 7 A.M. and 7 P.M., at 8 A.M. and 8
P.M., etc., and the mean of these is taken; or if the mean is derived from
the maximum and the minimum temperatures, discussed in Chapter III. This
_mean_ temperature should be written at the bottom of the temperature
column, and marked “Mean.” The mean monthly temperature is one of the
important meteorological data in considering the climatic conditions of
any place.

_Wind._——Determine the frequency of the different wind directions by
counting the total number of times the wind has blown from N., NE., E.,
etc., during the month. The wind which you have observed the greatest
number of times is the _prevailing_ wind. It may, of course, happen that
two or three directions have been observed an equal number of times. The
number of calms should also be recorded.

_Rainfall._——The total monthly precipitation is obtained by adding
together all the separate amounts of rainfall noted in your record book,
and expressing the total, in inches and hundredths, at the bottom of the
rainfall column. You now have the means for comparing one month’s rainfall
with that of another month, and of seeing how these amounts vary.

Examine carefully also your _non-instrumental observations_. See whether
you can draw any general conclusions as to the greater prevalence of
cloud, or of rain or snow, in one month than in another. Did the last
month have more high winds than the one before? Or than the average? Were
the temperature changes more sudden and marked? Was there more or less
precipitation than in previous months?



                              CHAPTER III.

                  ADVANCED INSTRUMENTAL OBSERVATIONS.


The instruments for more advanced study are the following: _maximum and
minimum thermometers, wet and dry-bulb thermometers, sling psychrometer,
standard barometer, thermograph, barograph, and anemometer_.

[Illustration: FIG. 7.]

=Maximum and minimum thermometers= are usually mounted together on a
board, as shown in Fig. 7, the lower one of the two being the maximum, and
the upper the minimum. In the view of the instrument shelter (Fig. 2),
these thermometers are seen on the left. The minimum thermometer, when
attached to its support, is either exactly horizontal or else slopes
downward somewhat towards the bulb end, as shown in Fig. 7. These
instruments, as their names imply, register the highest and the lowest
temperatures, respectively, which occur during each day of 24 hours. The
maximum thermometer is filled with mercury. Its tube is narrowed just
above the bulb, in such a way that the mercury passes through the
constriction with some difficulty. As the temperature rises, the mercury,
in expanding, is forced out from the bulb through this narrow passage.
When the temperature falls, however, the mercury above this point cannot
get back into the bulb, there being nothing to force it back. The length
of the mercury column, therefore, remains the same as it was when the
temperature was highest, and the instrument is read by observing the
number of degrees indicated by the top, or right-hand end, of the mercury
column upon the scale. After reading, the thermometer is set by removing
the brass pin upon which the bulb end rests, and whirling the instrument
rapidly around the pin to which its upper end is fastened. By this process
the mercury is driven back into the bulb, past the constriction. Care must
be taken to stop the thermometer safely while it is whirling. After
setting, the reading of the maximum thermometer should agree closely with
that of the ordinary or dry-bulb thermometer.

The _minimum thermometer_ is filled with alcohol, and contains within its
tube a small black object, called the _index_, which resembles a
double-headed black pin. The instrument is so constructed that this index,
when placed with its upper, or right-hand end, at the surface of the
alcohol, is left behind, within the alcohol, when the temperature rises.
On the other hand, when the temperature falls, the index is drawn towards
the bulb by the surface cohesion of the alcohol, the top or right end of
the index thus marking the lowest temperature reached. The upper end of
the thermometer is firmly fastened, by means of a screw, to a brass
support, while the lower end rests upon a notched arm. In setting this
instrument, the bulb end is raised until the index slides along the tube
to the end of the alcohol column. The thermometer is then carefully
lowered back into the notch just referred to. Maximum and minimum
thermometers need to be read only once a day, in the evening. The
temperatures then recorded are the highest and lowest reached during the
preceding 24 hours. The observation hour is preferably 8 P.M., but if this
is inconvenient, or impracticable, the reading may be made earlier in the
afternoon. The hour, however, should be as late as possible, and should
not be varied from day to day. The maximum temperature sometimes occurs in
the night. The maximum and the minimum temperatures should be entered
every day, in a column headed “Maximum and Minimum Temperatures,” in your
record book.

The =wet- and dry-bulb thermometers=, together commonly known as the
_psychrometer_ (Greek: _cold measure_), are simply two ordinary mercurial
thermometers, the bulb of one of which is wrapped in muslin, and kept
moist by means of a wick leading from the muslin cover to a small vessel
of water attached to the frame (see Fig. 8). The wick carries water to the
bulb just as a lamp wick carries oil to the flame. The psychrometer is
seen inside the shelter on the right in Fig. 2.

[Illustration: FIG. 8.]

The air always has more or less moisture in it. Even the hot, dry air of
deserts contains some moisture. This moisture is either invisible or
visible. When invisible it is known as _water vapor_, and is a gas. When
visible, it appears as _clouds_ and _fog_, or in the liquid or solid form
of _rain_, _snow_, and _hail_. The amount of moisture in the air, or the
_humidity_ of the air, varies according to the temperature and other
conditions. When the air contains as much water vapor as it can hold, it
is said to be _saturated_. Its humidity is then high. When the air is not
saturated, evaporation goes on into it from moist surfaces and from
plants. Water which changes to vapor is said to _evaporate_.

This process of evaporation needs energy to carry it on, and this energy
often comes from the heat of some neighboring body. When you fan yourself
on a very hot day in summer, the evaporation of the moisture on your face
takes away some of the heat from the skin, and you feel cooler. The drier
the air on a hot day, the greater is the evaporation from all moist
bodies, and hence the greater the amount of cooling of the surfaces of
those bodies. For this reason a hot day in summer, when the air is
comparatively dry, that is, not saturated with moisture, is cooler, other
things being equal, than a hot day when the air is very moist. Over
deserts the air is often so hot and dry that evaporation from the face and
hands is very great, and the skin is burned and blistered. Over the
oceans, near the equator, the air is hot and excessively damp, so that
there is hardly any cooling of the body by evaporation, and the conditions
are very uncomfortable. This region is known as the “Doldrums.”

The temperatures that are felt at the surface of the skin, especially
where the skin is exposed, as on the face and hands, have been named
_sensible temperatures_. Our sense of comfort in hot weather depends on
the _sensible_ temperatures. These sensible temperatures are not the same
as the readings of the ordinary (dry-bulb) thermometer, because our
sensation of heat or cold depends very largely on the amount of
evaporation from the surface of the body, and the temperature of
evaporation is obtained by means of the wet-bulb thermometer. Wet-bulb
readings at the various stations of the Weather Bureau are entered on
all our daily weather maps. In summer (July) the sensible (wet-bulb)
temperatures are 20° below the ordinary air temperature in the dry
southwestern portion of the United States (Nevada, Arizona, Utah). The
mean July sensible temperatures there are from 50° to 65°; while on the
Atlantic coast, from Boston to South Carolina, they are between 65° and
75°. Hence over the latter district the temperatures actually experienced
in July average higher than in the former.

Unless the air is saturated with water vapor, the evaporation from the
surface of the wet-bulb thermometer will lower the temperature indicated
by that instrument below that shown by the dry-bulb thermometer next to
it, from which there is no evaporation. The drier the air, the greater the
evaporation, and therefore the greater the difference between the readings
of the two thermometers. By means of tables, constructed on the basis of
laboratory experiments, we may, knowing the readings of the wet and
dry-bulb thermometers, easily determine the _dew-point_ and the _relative
humidity_ of the air——important factors in meteorological observations
(see Chapter XXVI). In winter, when the temperature is below freezing, the
muslin of the wet-bulb thermometer should be moistened with water a little
while before a reading is to be made. The amount of water vapor which air
can contain depends on the temperature of the air. The higher the
temperature, the greater is the capacity of the air for water vapor. Hence
it follows that, if the temperature is lowered when air is saturated, the
capacity of the air is diminished. This means that the air can no longer
contain the same amount of moisture (invisible water vapor) as before.
Part of this moisture is therefore changed, _condensed_, as it is said,
from the condition of water vapor into that of cloud, fog, rain, or snow.
The temperature at which this change begins is called the _dew-point_ of
the air.

The _relative humidity_ of the air is the ratio between the amount of
water vapor which the air contains at any particular time and the total
amount which it could contain at the temperature it then has. Relative
humidity is expressed in percentages. Thus, air with a relative humidity
of 50% has just half as much water vapor in it as it _could_ hold.

It is found that the readings of the wet-bulb thermometer are considerably
affected by the amount of air movement past the bulb, and that in a light
breeze, or in a calm, the reading does not give accurate results as to the
humidity of the general body of air outside the shelter.

To overcome this difficulty another form of psychrometer has been devised.

The =sling psychrometer= (Fig. 9) consists simply of a pair of wet and
dry-bulb thermometers, fastened together on a board or a strip of metal,
to the upper part of which a cord with a loop at the end is attached. In
this form of psychrometer there is no vessel of water and no wick, but the
muslin cover of the wet-bulb thermometer must be thoroughly wet, by
immersion in water, just before each observation. The instrument is then
whirled around the hand at the rate of about 12 feet a second. After
whirling about 50 times, note the readings, and then whirl the instrument
again, and so on, until the wet bulb reaches its lowest reading. The
lowest reading of the wet bulb, and the reading of the dry bulb at the
same time, are the two observations that should be recorded. Take care to
have the muslin wet throughout each observation, and in windy weather
stand to leeward of the instrument, so that it may not be affected by the
heat of your body. The true reading may be obtained within two or three
minutes.

[Illustration: FIG. 9.]

Make observations with the wet-bulb thermometer or the sling psychrometer
as a part of your regular daily weather record. Note the temperatures
indicated by the wet and dry bulbs, and, by means of the table in Chapter
XXVI, obtain the _dew-point_ and the _relative humidity_ of the air at
each observation. Enter these data in your record book, in a column headed
“Humidity,” and subdivided into two columns, one for the dew-point and one
for the relative humidity.

[Illustration: FIG. 10.]

By means of observations with the psychrometer you will be able to answer
such questions as the following:——

Does the relative humidity vary from day to day? Has it any relation to
the direction of the wind? To the state of the sky? To precipitation? Does
it show any _regular_ variations during the course of a day? How does a
high degree of relative humidity affect you in cold weather? In hot
weather? Between what limits of percentages does the relative humidity
vary? Do the changes come gradually or suddenly? Are these changes related
in any way to the changes in the other weather elements? How do the
sensible temperatures vary? In what weather conditions do the sensible
temperatures differ most from the air temperatures? In what seasons?
Compare the sensible temperatures obtained by your own observations with
the sensible temperatures at various stations of the Weather Bureau, as
given on the daily weather map. Are there any fairly regular differences
between the sensible temperatures observed at your own station and the
Weather Bureau stations?

=Standard Mercurial Barometer.=——A simple form of barometer has been
described in Chapter II. The ordinary standard mercurial barometer used by
the Weather Bureau (Fig. 10) has the glass tube containing the mercury
surrounded by a thin brass covering, through which openings are cut, near
the top, on the front and back, exposing to view the glass tube and the
top of the mercury column. On one side of this opening there is a strip of
metal, graduated to inches and tenths or twentieths, by means of which the
height of the barometer is determined. This strip, for barometers used at
or near sea level, is about 4 inches long, the variations in pressure
under normal conditions not exceeding that amount. In addition to this
fixed scale there is a small scale, also graduated, which may be moved up
and down the opening in the enclosing brass case by means of a milled head
outside and a small rack and pinion inside the brass case. This movable
scale, known as the _vernier_ from the name of its inventor, Vernier, is
an ingenious device, by means of which more accurate readings of the
barometer can be made than is possible with the ordinary fixed scale. A
_vernier_ graduated into twenty-five parts enables the observer to make
readings accurately to the one-thousandth part of an inch. On the front of
the barometer there is a small thermometer, known as the _attached
thermometer_. The bulb of this thermometer, concealed within the metal
casing of the barometer, is nearly in contact with the glass tube
containing the mercury. The air, upon whose weight the height of the
mercury column depends, gains access to the top of the cistern through
leather joints, by which the cistern is joined to the glass tube.

Mercurial barometers of the Weather Bureau pattern are best hung in a
barometer box, fastened securely against the wall of a room, where there
is a good light on the instrument and where the temperature is as constant
as possible.

In all accurate work certain corrections have to be applied to barometer
readings to make them strictly comparable. These are: (1) _correction for
altitude_; (2) _correction for temperature_; and (3) _correction for
latitude_. The first is necessary because of the fact that the weight of
the air decreases upwards, and a barometer reading on a hill or a
mountain is not comparable with one at sea level unless the former has
been corrected by the addition of the weight of the column of air between
the hill or mountain and sea level. The correction for temperature is
rendered necessary by the fact that with increasing temperature the
mercury in the barometer tube expands more than the metallic scale,
because mercury is more sensitive to heat, and unless some allowance is
made for this fact, barometer readings made at high temperatures will show
somewhat too high a pressure. The readings of the attached thermometer
give the temperature of the mercury and are used in making the corrections
for temperature. As gravity varies from a maximum value at the poles to a
minimum value at the equator, barometer readings made at different
latitudes are _corrected for latitude_, which means that they are reduced
to latitude 45°, midway between 0° and 90°. The correction is +0.08″ at
the poles and -0.08″ at the equator. Tables for use in correcting
barometer readings for altitude and for temperature are given in Chapter
XXVI.

[Illustration: FIG. 11.]

=Thermograph and Barograph.=——Two instruments of much interest are the
self-recording thermometer, or _thermograph_, and the self-recording
barometer, or _barograph_, manufactured by Richard Brothers of Paris. In
the thermograph (Fig. 11) there is a brass cylinder around which a sheet
of paper is wound, this paper being divided into two-hour intervals of
time and into spaces representing differences of 5° or 10° of temperature.
The cylinder revolves once in a week, being driven by clock-work contained
within it. The thermometer consists of a flat, bent, hollow brass tube
containing alcohol, one end of the tube being fastened to the metallic
frame seen at the right of the figure, and the other end being free to
move. With rising temperature, the liquid in the tube expanding more than
the metallic casing, by reason of its greater sensitiveness to heat, tends
to straighten the tube, while with falling temperature the elasticity of
the tube turns it into a sharper curve. These movements of the free end of
the tube are carried through a train of levers and thus magnified. At the
end of the last lever is a metallic pen filled with ink, which rests
lightly against the paper on the revolving drum. A rise or fall in
temperature is thus recorded by a rise or fall of the pen on the record
sheet, and a continuous curve of temperature is secured. The pen of the
thermograph should be frequently adjusted to make the reading of the
instrument accord with that of a standard mercurial thermometer, and care
should be taken to have the clock keep good time. These adjustments can
readily be made by means of a screw and a regulator, respectively. The
thermograph should be exposed in the instrument shelter with the other
thermometers. The sheets should be changed, the clock wound, and the pen
filled once a week, preferably on Monday, at 8 A.M., or at noon.

The continuous records written by a thermograph are a valuable addition to
the fragmentary observations which are the result of eye readings of the
ordinary thermometer. From the former any omitted thermometer readings may
be supplied. The interest of thermograph records may be seen in the
following figure (Fig. 12), in which curves traced under different
conditions are reproduced. Curve _a_ illustrates a period of clear
warming weather at Nashua, N. H., April 27-30, 1889. Curve _b_ was traced
during a spell of cloudy weather at Nashua, accompanying the passage of a
West India hurricane, Sept. 13-16, 1889. Curve _c_ illustrates the change
from a time of moderate winter weather to a cold spell (Nashua, Feb.
22-25, 1889). Curve _d_ exhibits a steady fall of temperature from the
night of one day over the next noon to the following night, during the
approach of a winter cold spell (Nashua, Jan. 19-21, 1889). Curve _e_
shows a reverse condition, viz., a continuous rise of temperature through
a night from noon to noon (Nashua, Dec. 16-17, 1888). Curve _f_ shows the
occurrence of a high temperature at night, caused by warm southerly winds,
followed by cold westerly winds (Cambridge, Mass., Nov. 30-Dec. 1, 1890).
Curve _g_ illustrates the sudden rise of temperature due to the coming of
a hot, dry wind (chinook) at Fort Assiniboine, Mont. (Jan. 19, 1892). A
study of such records leads to the discovery of many important facts,
which would be completely lost sight of without a continuous record.

[Illustration: FIG. 12.]

The =barograph= (Fig. 13) is very similar to the thermograph in general
appearance. The essential portion of this instrument consists of a series
of six or eight hollow shells of corrugated metal screwed one over the
other in a vertical column. These shells are exhausted of air, and form,
in reality, an aneroid barometer which is six or eight times as sensitive
as the ordinary single-chamber aneroid. The springs for distending the
shells are inside. The base of the column being fixed, the upper end rises
and falls with the variations in pressure. The movements of the shells are
magnified by being carried through a series of levers, and, as in the
thermograph, the motion is finally given to a pen at the end of the long
lever. The compensation for temperature is the same as in the ordinary
aneroid. A small quantity of air is left in one of the shells to
counteract, by its own expansion at increased temperature, the tendency of
the barometer to register too low on account of the weakening of the
springs. The barograph may be placed upon a shelf in the schoolroom, where
it can remain free from disturbance, and yet where the record may be
clearly seen. The general care of the barograph is the same as that of the
thermograph. Brief instructions concerning the care and adjustments of
these instruments are sent out by the makers with each instrument.
Frequent comparison with a mercurial barometer is necessary, the
adjustment of the barograph being made by turning a screw, underneath the
column of shells, on the lower side of the wooden case.

[Illustration: FIG. 13.]

Barograph records are fully as interesting as those made by the
thermograph. The week’s record traced on the writer’s barograph during a
winter voyage from Punta Arenas, Strait of Magellan, to Corral, Chile,
Aug. 2-9, 1897, gives a striking picture of the rapid and marked changes
of pressure during seven days in the South Pacific Ocean (Fig. 14).

[Illustration: FIG. 14.]

The following figure (Fig. 15) presents samples of barograph curves traced
at Harvard College Observatory, Cambridge, Mass., during Feb. 22-28, 1887,
and May 17-23, 1887. The February curve illustrates well the large and
irregular fluctuations in pressure, characteristic of our winter months;
while the May curve shows clearly the more even quality of the pressure
changes in our summer.

The =anemometer= shown in Fig. 16 is the most generally used of
instruments designed to measure wind velocity. It is known as the Robinson
cup anemometer, and consists of four hollow hemispherical cups upon arms
crossed at right angles, and all facing the same way around the circle.
The cross-arms are fixed upon a vertical axis having an endless screw at
its lower end. When the cups move around, the endless screw turns two
dials which register the number of miles traveled by the wind. The Weather
Bureau pattern of anemometer has the dials mounted concentrically, the
outer dial having 100, and the inner, 99 divisions. The revolutions of
the outer dial are recorded on the inner one, and in making an observation
of the number of miles traveled by the wind, the hundreds and tens of
miles are taken from the inner dial, and the miles and tenths from the
outer one. Take from the inner scale the hundreds and tens of miles
contained between the zero of that scale and the zero of the outer one.
Take on the outer scale the miles and tenths of miles contained between
the zero of that scale and the index point of the instrument. The sum of
these readings is the reading of the instrument at the time of the
observation.

[Illustration: FIG. 15.]

[Illustration: FIG. 16.]

Wind velocities are recorded in miles per hour. The velocity of the wind
at any particular moment is found by noting the number of miles and tenths
of miles recorded by the index before and after an interval of one minute,
or of five minutes, and multiplying this rate by 60 or by 12 as the case
may be. This gives the number of miles an hour that the wind is blowing at
the time of observation.

Records of wind velocity (in miles per hour) are to be made at each
regular observation hour, and are to be entered in the proper column of
the table in your record book. The total wind movement in each 24 hours is
to be observed once a day, always at the same hour, and is to be entered
in its proper column in the record book.

The total wind movement for 24 hours is obtained as follows: Subtract the
reading of the anemometer at 12 noon (or 8 A.M., or any other hour) of the
preceding day from the reading taken at 12 noon or the corresponding hour
of the current day, and the difference will be the total movement of the
wind. When the reading of the anemometer is less than the reading of the
preceding day, 990 miles should be added to it; and the remainder, after
subtracting the reading of the preceding day, will be the total wind
movement for the 24 hours. Thus: To-day’s reading = 91 miles; yesterday’s
reading = 950 miles. Hence 91 + 990 = 1081 miles, 1081 - 950 = 131 miles
= total wind movement for the current day.

By means of an electrical attachment the anemometer may be arranged so as
to record continuously on a cylinder rotating by clock-work, a pen making
a mark on the paper for every mile traveled by the wind. The anemometer
should be exposed on top of a building where there is as little
obstruction as possible by tall chimneys, higher buildings, and the like.

The =nephoscope= (Greek: _cloud observer_) is an instrument used in
determining the directions of movement of clouds. These directions, if
determined by ordinary eye observation of the clouds as they drift across
the sky, are apt to be quite inaccurate. The best method of observing
directions of cloud movement is to note the path of the reflection of the
cloud in a horizontal mirror, the observer looking at this reflection
through an eyepiece which remains fixed during the operation. Such a
horizontal mirror, adapted to measure the direction of motion of clouds,
is known as a _nephoscope_. A form of nephoscope devised by Mr. H. H.
Clayton, of Blue Hill Observatory, Hyde Park, Mass., is shown in Fig. 17.

[Illustration: FIG. 17.]

This instrument consists of a circular mirror, 13 inches in diameter,
sunk in a narrow circular wooden frame, on top of which is fastened a
brass circle, S.W.N.E., divided to 5° of arc. Inside of this fixed circle
is a movable brass one, to which is attached a brass arc, _BD_, rising
above the mirror and bearing a movable eyepiece, _C_. This arc forms the
quadrant of a circle whose center is the center of the mirror, and is
divided to 5° of arc. Its top is held vertically over the center of the
mirror by two rods fastened to the movable circle. The center of the
mirror _A_ is marked by cross lines on the reflecting surface, the glass
of which is thin. In order to determine the motion of a cloud, the movable
circle and tripod are revolved until the arc _BD_ is in the vertical plane
formed by the cloud, the center of the mirror, and the eye. The eyepiece
_C_ is then shifted until some point of the cloud image, as seen through
the eyepiece, is projected on the intersection of the cross lines on the
glass. The cloud image soon changes its position, and while the eye is
still held at the eyepiece, a small index is placed on the part of the
cloud image which previously appeared on the center of the mirror. If now
a ruler be placed on the index and the center of the mirror and extended
backward, its intersection with the divided scale will give the direction
from which the cloud came to the nearest degree, if all the measurements
have been accurately made. The height of the cloud above the horizon is
found by reading the position of the eyepiece on the divided quadrant.

The nephoscope may be placed on a table, out of doors in fine weather, or
close to a window from which the clouds to be observed can be seen. The
instrument must be properly oriented, so that the four points marked N.,
E., S., and W. on the frame shall correspond to the four chief compass
directions. The zero (0°) of the movable brass scale is usually put at the
S. Hence, if a cloud is found moving from exactly SW., the angular
measurement of its direction of motion will be 45°. If a cloud is moving
from due E., the angular measurement of its direction of motion will be
270°.

When the sky is completely overcast with a uniform layer of cloud, it is
usually impossible to determine any direction of movement, because of the
difficulty of selecting and keeping in view, on the mirror, some
particular point of cloud.

Observations with the nephoscope may be made as often as is desired, and
should be entered in an appropriate column in the record book.

=Tabulation of Observations.=——A convenient form of table which may be
used in the complete instrumental observations is given on the next page.
The number of columns and their arrangement may, of course, be varied to
suit the number and the nature of the records.

=Summary of Observations.=——In the preceding chapter we have seen how to
obtain the mean monthly temperature from the daily observations, the
frequency of the different wind directions for each month, and the total
monthly precipitation. The addition of the new instruments, the maximum
and minimum thermometers, the psychrometer, the anemometer, and the
nephoscope, enables us to obtain the following additional data in our
monthly summaries.

_Temperature._——The _mean monthly temperature_ may be obtained from the
maximum and minimum temperatures as follows: Add together all the daily
maximum and minimum temperatures for a month. Divide this sum by the total
number of readings you have made of each thermometer (_i.e._, one reading
of the maximum and one of the minimum each day, making two readings a
day), and the result will be the _mean monthly temperature_ derived from
the maximum and minimum temperature. This is a more accurate mean
temperature than the one noted in the summary of the preceding chapter.

Add together all the maximum temperatures noted during one month. Divide
this sum by the number of observations, and the result gives the _mean
maximum temperature_ for the month.

                    TABLE FOR METEOROLOGICAL RECORD.

  ||                                      | DATE.                          ||
  |+--------------------------------------+--------------------------------+|
  ||                                      | HOUR.                          ||
  |+--------------------------------------+--------------------------------+|
  ||                                      | PRESSURE (in                   ||
  ||                                      |  inches).                      ||
  |+--------------------------------------+----------------+---------------+|
  ||                                      | DRY BULB.      |               ||
  |+--------------------------------------+----------------+               ||
  ||                                      | WET BULB.      | TEMPERATURE.  ||
  |+--------------------------------------+----------------+               ||
  ||                                      | MAX.           |               ||
  |+--------------------------------------+----------------+               ||
  ||                                      | MIN.           |               ||
  |+--------------------------------------+----------------+---------------+|
  ||                                      | DEW-POINT.     |               ||
  |+--------------------------------------+----------------+               ||
  ||                                      | RELATIVE       | HUMIDITY.     ||
  ||                                      |  HUMIDITY.     |               ||
  |+--------------------------------------+----------------+---------------+|
  ||                                      | DIRECTION.     |               ||
  |+--------------------------------------+----------------+               ||
  ||                                      | VELOCITY       | WIND.         ||
  ||                                      |  (miles per    |               ||
  ||                                      |  hr.).         |               ||
  |+--------------------------------------+----------------+               ||
  ||                                      | TOTAL MILES    |               ||
  ||                                      |   PER DAY.     |               ||
  |+--------------------------------------+----------------+---------------+|
  ||                                      | KIND.          |               ||
  |+--------------------------------------+----------------+               ||
  ||                                      | AMOUNT (in     |               ||
  ||                                      |  tenths).      |               ||
  |+--------------------------------------+----------------+               ||
  ||                                      | DIRECTION OF   | CLOUDS.       ||
  ||                                      |  MOVEMENT.     |               ||
  |+--------------------------------------+----------------+               ||
  ||                                      | ANGULAR        |               ||
  ||                                      |  ALTITUDE      |               ||
  |+--------------------------------------+----------------+---------------+|
  ||                                      | TIME OF        |               ||
  ||                                      |  BEGINNING     |               ||
  ||                                      |  AND ENDING.   |               ||
  |+--------------------------------------+----------------+               ||
  ||                                      | KIND.          | PRECIPITATION.||
  |+--------------------------------------+----------------+               ||
  ||                                      | AMOUNT.        |               ||
  |+--------------------------------------+----------------+---------------+|
  ||                                      | REMARKS.                       ||
  ||                                      |                                ||
  ||                                      |                                ||
  ||                                      |                                ||
  ||                                      |                                ||

A similar operation applied to the minimum temperatures gives the _mean
minimum temperature_ for the month.

In meteorological summaries it is customary also to include the _absolute
maximum_ and the _absolute minimum_ temperatures, _i.e._, the highest and
lowest single readings of the thermometer made during each month. These
can easily be determined by simple inspection of your record book. Note
also the dates on which the absolute maximum and the absolute minimum
occurred.

The _absolute monthly range_ of temperature is the difference, in degrees,
between the absolute maximum and the absolute minimum.

_Humidity._——The _mean relative_ humidity is obtained by adding together
all the different percentages of relative humidity obtained during the
month, and dividing this sum by the whole number of observations of this
weather element.

_Wind._——The _mean velocity_ of the wind corresponding to the different
wind directions is readily obtained by adding together all the different
velocities (in miles per hour) observed in winds from the different
directions, and dividing these sums by the number of cases. The wind
summaries will thus give the frequency of the different directions during
each month, and the corresponding mean velocities.

The _maximum hourly wind velocity_ is obtained by inspection of the
velocity column.

The _total monthly wind movement_ is readily deduced from the daily
records in the twelfth column of the table on p. 44.

_State of the Sky._——In connection with the more advanced records
described in this chapter, the observations of cloudiness should record
the number of _tenths_ of the sky cloudy, as closely as the amount can be
estimated by eye, instead of indicating the state of the sky as _cloudy_,
_fair_, etc. A detailed record of cloudiness in tenths gives opportunity
to determine the _mean cloudiness_ for each month, by averaging, as in the
case of the other means already described.

If nephoscope observations are made, the monthly summary may include the
_mean direction of cloud movement_ for each month. This is obtained by
adding together all the different angular measurements of directions of
cloud movement, and dividing by the whole number of such observations.

By means of your monthly summaries compare one month with another. Notice
how the means and the extremes of the different weather elements are
related; how they vary from month to month. Are there any _progressive_
changes in temperature, cloudiness, precipitation, etc., from month to
month? What are the changes? Summarize, in a short written statement, the
meteorological characteristics of each month as shown by your tables.



       PART III.——EXERCISES IN THE CONSTRUCTION OF WEATHER MAPS.



                              CHAPTER IV.

                         THE DAILY WEATHER MAP.


The first daily weather maps were issued in connection with the Great
Exhibition of 1851 in London. The data were collected by the Electric
Telegraph Company and transmitted to London over its wires. These maps
were published and sold daily (excepting Sundays) from Aug. 8 to Oct. 11,
1851. The first official weather map of the United States Weather Service
was prepared in manuscript on Nov. 1, 1870, and on Jan. 14, 1871, the work
of manifolding the maps for distribution was begun at Washington. Previous
to the publication of this government map, Professor Cleveland Abbe had
issued in Cincinnati, with the support of the Chamber of Commerce of that
city, the first current weather maps published in the United States (Feb.
24 to Dec. 10, 1870). In France, daily weather maps have been published
continuously since Sept. 16, 1863.

Two things are essential for the publication of a daily synoptic weather
map; _first_, simultaneous meteorological observations over an extended
area; and, _second_, the immediate collection of these observations by
telegraph. The weather map of the United States is based on simultaneous
observations made at about 150 stations in different parts of this
country, besides several coöperating stations in Canada, Central America,
Mexico, and the West Indies. At each of our stations, whose location may
be seen on any weather map, the Weather Bureau employs one or more
observers, who, twice a day, at 8 A.M. and 8 P.M., “Eastern Standard
Time,” make regular observations of the ordinary weather elements, _i.e._,
temperature, pressure, humidity, wind direction and velocity,
precipitation, cloudiness, etc. The instruments at these stations are all
standard, but the completeness of the equipment varies according to the
importance of the station. The 8 A.M. observations are the only ones now
generally used in the preparation of weather maps. When the Weather
Service was first established, tri-daily charts were for some time issued
from the central office in Washington. On April 1, 1888, the number was
reduced to two a day, and on Sept. 30, 1895, a further change was made,
and now there is but one map a day.

The 8 A.M. observations, as soon as made, are corrected for certain
instrumental errors, and the barometer readings are reduced to sea level.
The data are then put into cipher, not for secrecy, but to facilitate
transmission and to lessen the chances of error, and are telegraphed from
all parts of the country to the central office of the Weather Bureau in
Washington. Besides sending their own messages to Washington, all the
important stations of the Weather Bureau receive, by a carefully devised
system of telegraphic circuits, a sufficient number of the reports from
other stations to enable their observers to draw and issue local weather
maps.

The observations are received at the central office of the Weather Bureau
in Washington by special wires, and are usually all there within an hour
after the readings were made. As the messages are received in the forecast
room, they are translated from the cipher back again into the original
form, and the data are entered upon blank maps. The official charged with
making the forecasts then draws upon the maps lines of equal temperature,
lines of equal pressure, lines of equal pressure-change and
temperature-change during the past 24 hours. These several sets of lines,
together with those showing the regions of precipitation during the past
24 hours, furnish the necessary data on which the forecasts can be based.
In other words, the forecast official has before him, on the several maps,
a bird’s-eye view of the weather conditions over the United States as they
were an hour before, and also of the changes that have taken place in
these conditions during the preceding 24 hours. Thus, by knowing the
general laws which govern the movements of areas of high and low
temperature, of fair and stormy weather, across the country, he can make a
prediction as to the probable conditions which any state or section of the
country will experience in 12, 24, or 36 hours.

In a later chapter some suggestions will be given for studies of
forecasting.

The forecasts made in Washington, and printed on the Washington daily
weather map, relate to all sections of the United States, and include
predictions of cold waves, killing frosts, storm winds, river floods, and
the like, besides the ordinary changes in weather conditions. These
forecasts, as soon as made, are at once given to the local newspapers and
to the press associations. They are also sent by telegraph to all regular
stations of the Weather Bureau, and to all stations at which cautionary or
storm signals are to be displayed, along the Atlantic or Gulf coasts, and
on the Great Lakes.

The Washington weather map is about 24 by 16 inches in size, and is newly
lithographed each day. The total number of maps issued from the central
office during the fiscal year ending June 30, 1898, was 310,250. In
addition to these, there are now 84 stations of the Weather Bureau in
different parts of the country, at which daily weather maps are issued and
local forecasts made. These latter forecasts are made by a corps of local
forecast officials, each of whom has to make the weather prediction for
his own district. At first, and until within a few years, one predicting
officer in Washington made all the forecasts for the country, but it was
found better to have the country divided into geographical sections, over
each one of which the meteorological conditions are fairly similar, and to
have a local forecast official in charge of each section. These local
forecast officials have the double advantage of being able to study the
weather conditions over the whole country, as sent them by telegraph each
morning, and also of knowing the special peculiarities of their own
regions. This enables them to make more accurate predictions than can be
made by an official who may be one or two thousand miles distant, in
Washington.

The greater portion of the maps issued at the map stations outside of
Washington are prepared by what is known as the chalk-plate process,
suggested by Mr. J. W. Smith, local forecast official at Boston. This
process is as follows: A thin covering of specially prepared chalk, 1/8 of
an inch in thickness, is spread upon a steel plate of the size of the
prospective weather map. On this chalk are engraved, by means of suitable
instruments, the various weather symbols, the lines of equal pressure and
of equal temperature, and the wind arrows. The plate is then stereotyped
in the ordinary way, and printed on a sheet prepared for the purpose,
which has a blank outline map of the United States at the top, and space
in the lower half for the forecasts, summary, and tables.

The size of the chalk-plate map itself is 10 by 6-1/2 inches; the size of
the whole sheet, which includes also the text and tables, 16 by 11 inches.
Weather maps prepared by the chalk-plate process are now issued from 28 of
the 84 stations which publish daily maps. At the remaining stations the
maps are prepared by a stencil process, the size of the map being 13-1/2
by 22 inches. The total number of weather maps issued at the various
stations during the fiscal year 1897-1898 was 5,239,300.

Besides recording the usual meteorological data, and publishing weather
maps and forecasts, the various stations of the Weather Bureau serve as
distributing centers for cold wave, frost, flood, and storm warnings.
These warnings are promptly sent out by telegraph, telephone, and mail.
Besides these usual methods of distributing forecasts, other means have
also been adopted. In some places factory whistles are employed to inform
those within hearing as to the coming weather; railway trains are provided
with flags, whose various colors announce to those who are near the train
fair or stormy weather, rising or falling temperature; and at numerous
so-called “display stations,” scattered all over the country, the
forecasts are widely disseminated by means of flags.



                               CHAPTER V.

                              TEMPERATURE.


_A._ =Lines of Equal Temperature.=——Temperature is the most important of
all the weather elements. It is therefore with a study of the distribution
of temperature over the United States, and of the manner of representing
that distribution, that we begin our exercises in map drawing. In carrying
out the work we shall proceed in a way similar to that adopted by the
officials of the Weather Bureau in Washington and at the other
map-publishing stations over the country.

Enter on a blank weather map the temperature readings found in the first
column of the table in Chapter VIII. These readings are given in degrees
of the ordinary Fahrenheit scale [those which are preceded by the minus
sign (-) being below zero], and were made at the same time (7 A.M.,
“Eastern Standard Time”) all over the United States. Make your figures
small but distinct, and place them close to the different stations to
which they belong. This is done every morning at the Weather Bureau in
Washington, when the telegraphic reports of weather conditions come in
from all over the country. When all the temperature readings have been
entered on the outline map, you have before you a view of the actual
temperature distribution over the United States at 7 A.M., on the first
day of the series. Describe the distribution of temperature in general
terms, comparing and contrasting the different sections of the country in
respect to their temperature conditions. Where are the lowest
temperatures? Where are the highest? What was the lowest thermometer
reading recorded anywhere on the morning of this day? At what station was
this reading made? What was the highest temperature recorded? And at what
station was this reading made?

Notice that the warmest districts on the map are in Florida, along the
Gulf Coast, and along the coast of California. The marked contrasts in
temperature between the Northwest and the Pacific and Gulf Coasts at once
suggest a reason why Florida and Southern California are favorite winter
resorts. To these favored districts great numbers of people who wish to
escape the severe cold of winter in the Northern States travel every
year, and here they enjoy mild temperature and prevailingly sunny
weather. To the cold Northwest, on the other hand, far from the warm
waters of the Pacific, where the days are short and the sun stands low in
the sky, no seekers after health travel. This annual winter migration
from the cities of the North to Florida and Southern California has led
to the building of great hotels in favored locations in these States, and
during the winter and spring fast express trains, splendidly equipped,
are run from north to south and from south to north along the Atlantic
Coast to accommodate the great numbers of travelers between New York,
Philadelphia, Boston, Chicago, and other large northern cities, and the
Florida winter resorts. Southern California also is rapidly developing as
a winter resort, and rivals the far-famed Riviera of Southern Europe as a
mild and sunny retreat from the severe climates of the more northern
latitudes. The control which meteorological conditions exercise over
travel and over habitability is thus clearly shown. Florida and Southern
California are also regions in which, owing to the mildness of their
winter climates, certain fruits, such as oranges and lemons, which are
not found elsewhere in the country, can be grown out of doors, and these
are shipped to all parts of the United States.

Let us take another step in order to emphasize more clearly the
distribution of temperature over the United States on the first day of our
series. Draw a line which shall separate all places having a temperature
_above_ 30° from those having temperatures _below_ 30°, 30° being nearly
the freezing point and, therefore, a critical temperature. Evidently this
will help us to make our description of the temperature distribution more
detailed. If this line is to separate places having temperatures _above_
30° from those having temperatures _below_ 30°, it must evidently pass
through all places whose temperature is exactly 30°. Examine the
thermometer readings entered on your map to see whether there are any
which indicate exactly 30°. You will find this reading at Norfolk, Va.,
Wilmington, S. C., Atlanta, Ga., Chattanooga, Tenn., Ft. Smith, Ark., and
Portland, Ore. Through all these stations the line of 30° must be drawn.
Begin the line on the Atlantic Coast at Norfolk, Va., and draw it wherever
you find a thermometer reading of 30°. It is best to trace the line
faintly with pencil at first, so that any mistakes can be easily
rectified, and it should be drawn in smooth curves, not in angles. From
Norfolk the line must run southwest through Wilmington, and then westward
through Atlanta, passing just north of Augusta, which has 31°. From
Atlanta the line goes northwest through Chattanooga, and thence westward,
curving south of Memphis (28°) and Little Rock (26°), and then
northwestward again through Ft. Smith.

In fixing the _exact_ position of the 30° line south of Memphis and Little
Rock, the following considerations must be our guide: Memphis has 28°;
Vicksburg has 35°. Neither of these stations has 30°. Suppose, however,
that you had started from Memphis, with a thermometer, and had traveled
very rapidly to Vicksburg. The thermometer reading at starting in Memphis
would have been 28°, and at the end of your journey in Vicksburg it would
have been 35°, presuming that no change in temperature at either station
took place during the journey. Evidently the mercury rose during the
journey, and in rising from 28° to 35° it must, somewhere on the way, have
stood at exactly 30°. Now this place, where the temperature was exactly
30°, is the point through which our 30° line ought to pass. How are we to
determine its location? Assume, as is always done in such cases, that the
temperature increased at a uniform rate between Memphis and Vicksburg. The
total rise was from 28° to 35° = 7°. In order to find a temperature 7°
higher than at Memphis, you had to travel the whole distance from Memphis
to Vicksburg. Suppose you had only wished to find a temperature 5° higher.
Then, assuming a uniform rate of increase between the two stations, you
would have had to travel only 5/7 of the distance, and your thermometer at
that place would have read 28° + 5° = 33°. But assume you had wanted to
find the place where the thermometer stood at 30°. In this case you would
have been obliged to go but 2/7 of the total distance from Memphis to
Vicksburg, and at that point your thermometer reading would have been 28°
+ 2° = 30°, which is the point we wish to find. In this way, then, when we
do not find the _exact_ temperature we are looking for on the map, we can
calculate where that temperature prevails by noting places which have
temperatures somewhat higher and somewhat lower, and proceeding as in the
case just described. Take another example. Little Rock, Ark., has 26°;
Shreveport, La., has 40°. 40° - 26° = 14°, which is the total difference.
From 26° to 30° is 4°. Therefore a point 4/14 or 2/7 of the distance from
Little Rock to Shreveport should have a temperature of 26° + 4° = 30°,
which is the point we wish to find, and through which our 30° line must
pass.

From Ft. Smith the line cannot go north or northwest or west, because the
temperatures there are all below 30°. To the south the temperatures are
all above 30°. Evidently there is only one direction in which you can
prolong the line, and that is to the southwest. Temperatures of 30° cannot
be found north of El Paso (28°), because there the temperature distinctly
falls, Santa Fé having 4°, Denver, -14°, and Cheyenne, -23°. Therefore
temperatures _above_ 28° must be found south of El Paso. From Ft. Smith
you may, therefore, continue the 30° line southwest and west, passing
close to El Paso, but to the south of it. In determining the further
course of the 30° line, note that Yuma and all the California stations
have temperatures above 30°, while Winnemucca, Nev., has 13°, and
Portland, Ore., has exactly 30°. From El Paso you may, therefore, continue
the line to the northwest, passing up through Central California parallel
with the coast line, and to the east of all the California stations and of
Roseburg, Ore., and thence running through Portland, Ore., ending just
west of Seattle, Wash. Notice that the 30° line should be nearer to
Sacramento, Cal., with 36°, than to Red Bluff with 44°.

Thus you have drawn the line which passes through all places that have a
temperature of 30° on the map under discussion. This may be called _a line
of equal temperature_. _Isotherm_, a compound of two Greek words meaning
_equal temperature_, is the name given in meteorology to such lines as
this. You have drawn the isotherm of 30°. All parts of the United States
north and east of this line are below 30°, while all districts south and
west of it are above 30°. You see, therefore, how much easier the drawing
of this one line has made the description of the temperature distribution
over the United States.

Carry this process a step further by drawing the line which shall pass
through all places with a temperature of 40°. This line begins at
Jacksonville, Fla. (40°), and runs west, passing between Montgomery, Ala.
(33°), and Pensacola, Fla. (46°). Thence it turns to the northwest,
passing between Vicksburg, Miss. (35°), and New Orleans, La. (48°), and
through Shreveport, La. (40°). From Shreveport it turns to the southwest,
passing to the north and west of Palestine, Tex. (46°), and down through
San Antonio, Tex. (40°). Its further exact location cannot be determined
in Mexico, because there are no observations from Mexican stations, but
the readings at Yuma, Ariz. (41°), and at San Diego (42°), Los Angeles
(44°), San Francisco (45°), Red Bluff (44°), and Cape Mendocino (43°), all
in California, show that the 40° isotherm may be started again just north
of Yuma, and may be carried up through California, nearly parallel with
the Pacific Coast, ending between Cape Mendocino, Cal. (43°), and
Roseburg, Ore. (37°). You have now drawn the isotherms of 30° and of 40°,
and in order to avoid confusion, mark the ends of the first line 30° and
the ends of the second line 40°.

Isotherms on weather maps are drawn for every even 10° of temperature.
They are drawn in smooth curves and not in angular sections. Two isotherms
cannot cross one another, for if they did you would have two temperatures,
differing by 10°, at the point of crossing, which is obviously impossible.
Complete the chart for this day by drawing the remaining isotherms,
_i.e._, those for 50°, 20°, 10°, 0°, -10°, -20°, and -30°, bearing in mind
what has been said in regard to the determination of the positions of
isotherms when the _exact_ temperature you are seeking is not given on the
map.

The dotted lines in Fig. 18 show the positions of the isotherms when
drawn. Notice how clearly the temperature distribution now stands out, and
how simple the description of that distribution has become. Observe that
the isotherms, although more or less irregular, show a good deal of
uniformity in their general courses, and this uniformity is a great
assistance in drawing them. Study the distribution of temperature on this
map, and the positions of the isotherms, very carefully.

Construct isothermal charts for the remaining days of the series. Use a
new blank map for each day, and take the temperature observations from
the table in Chapter VIII. Proceed as in the case of the first day. Draw
the isotherms for every even 10° of temperature, taking care to study the
course of each line before you begin to draw the line. The charts when
completed form a series in which the temperature distribution over the
United States is shown at successive intervals of 24 hours.

[Illustration: FIG. 18.——Isotherms. First day.]

In order to bring out the temperature distribution on the maps more
clearly, color (with colored pencils or water colors) all that portion of
each map which lies within the -20° isotherm a dark blue; that portion
which is between the 0° isotherm and the -20° isotherm a somewhat lighter
shade of blue, and those districts which are between 0° and +30° a still
lighter blue. The portion of the map above 30° and below 40° may be left
uncolored, while the districts having temperatures over 40° may be colored
red. In the map for the third day the district which has temperatures
below -50° should be colored darker blue than any shade used on the other
maps, or black, in order to emphasize the extremely low temperatures
there found. Figs. 19-24, on which the isotherms are shown, also
illustrate the appearance of these maps when the different temperature
areas are colored, as has been suggested.

[Illustration: FIG. 19.——Temperature. First Day.]

[Illustration: FIG. 20.——Temperature. Second Day.]

[Illustration: FIG. 21.——Temperature. Third Day.]

[Illustration: FIG. 22.——Temperature. Fourth Day.]

[Illustration: FIG. 23.——Temperature. Fifth Day.]

[Illustration: FIG. 24.——Temperature. Sixth Day.]

Study the maps individually at first. Describe the temperature
distribution on each map. Ask yourself the following questions in each
case: Where is it coldest? Where warmest? What is the lowest temperature
on the map? What is the highest? At what stations were these readings
made?

Then compare the successive maps and answer these questions: What changes
have taken place in the intervening 24 hours? In what districts has the
temperature risen? What is the greatest rise that has occurred? Where? In
what districts has the temperature fallen? What was the greatest fall in
temperature and where did it occur? Has the temperature remained nearly
stationary in any districts? In which? You will find it a help in
answering such questions to make out a table of all the stations, and to
indicate in columns, after the names of the stations, the number of
degrees of rise or fall in temperature at each place during the 24-hour
interval between the successive maps. When the temperature is higher at
any station than it was on the preceding day, note this by writing a plus
sign (+) before the number of degrees of rise in temperature. When the
temperature has fallen, put a minus sign (-) before the number of degrees
of fall. Thus, New Orleans, La., had a temperature of 48° on the first
day. On the second it had 33°. Therefore the change at New Orleans was
-15° in the 24 hours. At Key West, Fla., the change was +11° in the same
time.

Write a brief account of the temperature distribution on each day of the
series, and of the changes which took place between that day and the one
preceding, naming the districts and States over which the most marked
falls and rises in temperature occurred, with some indication of the
amount of these changes. Note especially the changes in position, and the
extent, of the districts with temperatures below -20°; between 0° and
-20°, and between 30° and 0°. Write out a clear, concise statement of the
temperature distribution and changes shown on the whole set of six maps.

=Cold Waves.=——The series of charts for these six days furnishes an
excellent illustration of a severe cold wave.

A _cold wave_, as the term is now used by the Weather Bureau, means,
during December, January, and February, a fall in temperature of from 20°
to 16° in 24 hours, with a resulting reduction of temperature to between
0° and 32°, and, during the months from March to November inclusive, a
fall of from 20° to 16° in 24 hours, with a reduction of temperature from
16° to 36°. During December, January, and February a _cold wave_ means
the following falls and reductions of temperature. Over the Northwestern
States, from western Wisconsin to Montana, including Wyoming, Nebraska,
and western Iowa, and over northeastern New York and northern New
Hampshire, northern Vermont and northern Maine, a fall of 20° or more to
zero or below; over southern New England and adjoining districts, the
Lake region, the central valleys and west to Colorado, including northern
New Mexico and northwestern Texas, a fall of 20° or more to 10° or below;
over southern New Jersey, Delaware, eastern Maryland, Virginia, western
North Carolina, northwestern South Carolina, northern Georgia, northern
Alabama, northern Mississippi, Tennessee, southern Kentucky, Arkansas,
Oklahoma, and southern New Mexico, a fall of 20° or more to 20° or below;
over eastern North Carolina, central South Carolina, central Georgia,
central Alabama, central Mississippi, central and northern Louisiana and
central and interior Texas, a fall of 18° or more to 25° or below; along
the Gulf coasts of Texas, Louisiana, Mississippi, and Alabama, over all
of Florida, and over the coasts of Georgia and South Carolina, a fall of
16° or more to 32° or below. From March to November inclusive a _cold
wave_ means falls of temperature of the same amounts over the same
districts, with resulting temperatures of 16°, 24°, 28°, 32°, and 36°
respectively.

Notice that the region from which the greatest cold came in this cold wave
is Canada. In that northern country, with its short days and little
sunshine, and its long, cold nights, everything is favorable to the
production of very low temperatures.

Cold waves occur only in winter. In the summer cool spells, with similar
characteristics, may be called _cool waves_.

=Cold-Wave Forecasts.=——A severe cold wave in winter does much damage to
fruit and crops growing out of doors in our Southern States, and to
perishable food products in cars, on the way from the South to supply the
great cities of the North. Therefore it is important that warnings should
be issued giving early information of the coming cold, so that farmers and
fruit growers and shippers may take every precaution to protect their
crops and produce. Our Weather Bureau takes special pains to study the
movements of cold waves and to make forecasts of them, and so well are the
warnings distributed over the country that the fruit growers and the
transportation companies, and the dealers in farm produce, are able every
winter to save thousands of dollars’ worth of fruit and vegetables which
would otherwise be lost. Cold-wave warnings are heeded by many persons
besides those who are directly interested in fruits and farm products. The
ranchmen in the West, with thousands of cattle under their charge; the
trainmen in charge of cattle trains; the engineers of large buildings,
such as hotels, stores, and office buildings, who must have their fires
hotter in cold weather,——these and many more watch, and are governed by,
the cold-wave forecasts of our Weather Bureau.

=Mean Annual and Mean Monthly Isothermal Charts.=——We have thus far
considered isothermal charts for the United States only, based on the
temperature observations made at a single moment of time. It is, of
course, possible to draw isothermal charts, the data for which are not
the temperatures at a given moment, but are the mean or average
temperatures for a month or a year. Such charts have been constructed for
other countries besides our own, as well as for the whole world. An
isothermal chart based on the mean annual temperatures is known as a
_mean annual isothermal chart_. These charts show at once the average
distribution of temperature for the month or for the year, just as the
ones we have drawn show the distribution of temperature over the United
States at a single moment.

_B._ =Direction and Rate of Temperature Decrease. Temperature
Gradient.=——Take your isothermal map for the first day and imagine
yourself at Kansas City, Mo. In what direction must you go from Kansas
City in order to enter most rapidly into colder weather? In what direction
must you go from Kansas City in order to enter most rapidly into warmer
weather? Take the case of Salt Lake City. In what direction must you go
from that station in order to enter most rapidly into colder weather? Into
warmer weather? What are the corresponding directions in the case of
Spokane, Wash.? Of Bismarck, N. Dak.? Of Buffalo, N. Y.? Of Montreal,
Que.? Of Portland, Me.? Of Sacramento, Cal.?

Draw a line from Kansas City to the nearest point at which there is a
temperature 10° lower than at Kansas City. Evidently this point is on the
isotherm of 0°, and will be found if a line be drawn from Kansas City
towards, and at right angles to, the isotherm of 0°. Continue the line
beyond the 0° isotherm in the direction of still lower temperatures,
_i.e._, to the isotherms of -10°, -20°, and -30°. Beyond the isotherm of
-30° the line must stop. Draw similar lines from Seattle, Wash.; Salt Lake
City, Utah; Denver, Col.; St. Paul, Minn.; Cleveland, O.; and New York, N.
Y. Prolong these lines all across the map, so that they will extend from
the regions of highest temperature to those of the lowest. A number of
intermediate lines may also be added. Note that the various directions
followed by these lines are square to, or at right angles to, the
successive isotherms, and that although the lines all run from higher to
lower temperatures, they do not all trend in the same direction. These
lines may be called _lines of decrease of temperature_. Fig. 25 shows a
few of these lines of decrease of temperature drawn for the first day.

Draw similar lines on the other isothermal charts, for the same stations.
Are the directions of temperature decrease the same on these charts as on
the chart for the first day, for Kansas City, Seattle, Salt Lake City,
Denver, St. Paul, Cleveland, New York? Draw lines of decrease of
temperature from the following additional stations: Key West, Fla.; New
Orleans, La.; Charleston, S. C.; El Paso, Tex.; San Diego, Cal.; Hatteras,
N. C.

Compare the directions of these lines on the different days. How do they
change from one day to the next?

[Illustration: FIG. 25.——Temperature Gradients. First Day.]

Next select some line of decrease of temperature on the map for the first
day which begins in Texas, and follow it northward. Where, along this
line, is the decrease of temperature most rapid? Evidently this must be
where the isotherms are closest together, because every isotherm that is
crossed means a change of temperature of 10°, and the more isotherms there
are in a given distance, the more rapidly the temperature is changing.
Where the isotherms are closest together, a given decrease of temperature
is passed over in the least distance, or, conversely, a greater decrease
of temperature is experienced in a given distance. Study this question of
rapidity or slowness of temperature decrease on the whole series of
charts. On which of the charts, and where, do you find the most rapid
decrease? The slowest decrease? Is there any regularity in these _rates_
of temperature decrease either on one map or in the whole series of maps?

The term _temperature gradient_ is used by meteorologists to describe the
_direction_ and _rate of temperature decrease_ which we have been
studying.

If we are to compare these rates of temperature change, we must have some
definite scale of measurement. Thus, for example, in speaking of the wind
velocity we say the velocity of the wind is so many miles per hour; in
describing the grade of a railroad we say it is so many feet in a mile. In
dealing with these temperature changes, we adopt a similar scheme. We say:
The rate of temperature decrease is so many degrees Fahrenheit in a
distance of one latitude degree (about 70 miles). In order to make our
measurements, we use a scale of _latitude degrees_, just as, in
calculating railroad grades, we must have a way to measure the miles of
track in which the ascent or descent of the roadbed is so many feet. Take
a strip of paper 6 inches long, with a straight edge, and lay this edge
north and south at the middle of the weather map, along a longitudinal or
meridian line. Mark off on the strip of paper the points where any two
latitude lines cross the meridian line. These latitude lines are five
(latitude) degrees apart. Therefore divide the space between them on your
paper into five divisions, and each of these will measure just one
latitude degree. Continue making divisions of the same size until you have
ten altogether on the strip of paper. Select, on any weather map, some
station lying between two isotherms at which you wish to measure the rate
of temperature decrease. Take, for instance, Buffalo, N. Y., on the first
day. What you want to find is this: What is the _rate of temperature
decrease_, or the _temperature gradient_, at Buffalo? Lay your paper scale
of latitude degrees through Buffalo, from the isotherm of 10° to the
isotherm of 0°, and as nearly as possible at right angles to the
isotherms.[3] Count the number of latitude degrees on your scale between
the isotherms of 10° and 0°, on a line running through Buffalo. There are,
roughly, about two degrees of latitude in this distance. That is, in the
district in which Buffalo lies, the temperature is changing _at the rate_
of 10° Fahrenheit (between isotherms 10° and 0°) in two latitude degrees.
As our standard of measurement is the amount of change of temperature in
one latitude degree, we divide the 10 (the number of degrees of
temperature) by the 2 (the number of degrees of latitude), which gives us
5 as the rate of decrease of temperature per latitude degree at Buffalo,
N. Y., at 7 A.M., on the first day of the series. The temperature gradient
at Buffalo is therefore 5. The rule may be stated as follows: Select the
station for which you wish to know the rate of temperature decrease or
temperature gradient. Lay a scale of latitude degrees through the station,
and as nearly as possible at right angles to the adjacent isotherms. If
the station is exactly on an isotherm then measure the distance _from_ the
station to the nearest isotherm indicating a temperature 10° lower. The
scale must, however, be laid perpendicularly to the isotherm, as before.
Divide the number of degrees of difference of temperature between the
isotherms (always 10°) by the distance (in latitude degrees) between the
isotherms, and the quotient is the _rate of temperature decrease per
latitude degree_. Or, to formulate the operation:

                              _R = T / D_,

in which _R_ = rate; _T_ = temperature difference between isotherms
(always 10°), and _D_ = distance between isotherms in latitude degrees.
Thus, a distance of 10 latitude degrees gives a rate of 1; a distance of 5
gives a rate of 2; a distance of 2 gives a rate of 5; a distance of 4
gives a rate of 2.5, etc.

[Footnote 3: Unless the isotherms are exactly parallel, the scale cannot
be at right angles to both of them. It should, however, be placed as
nearly as possible in that position.]

Determine the rates of temperature decrease in the following cases:——

_A._ For a considerable number of stations in different parts of the same
map, as for each of the six days of the series.

And, using the school file of weather maps,

_B._ For one station during a winter month and during a summer month,
measuring the rate on each map throughout the month and obtaining an
average rate for the month.

_C._ For a station on the Pacific Coast, and one on the Atlantic Coast
during the same months.

_D._ For a station on the Gulf of Mexico, or in Florida, and one in the
Northwest during a winter month.

_E._ For a station in the central United States, and one on the Pacific
Coast, the Gulf Coast, and the Atlantic Coast, respectively, during
different months of the winter and summer.

The determination of the rates of temperature decrease under these
different conditions over the United States prepares us for an
appreciation of the larger facts, of a similar kind, to be found on the
mean annual and mean monthly isothermal charts of various countries, and
also of the whole world.

=Temperature Gradients on Isothermal Charts of the Globe.=——The mean
annual isothermal charts of the globe (see page 63) bring out some very
marked contrasts in rates of temperature decrease. Thus, along the eastern
side of the North American continent the isotherms are crowded close
together, while on the western coast of Europe they are spread far apart.
Between southern Florida and Maine there is the same change in mean annual
temperature as is found between the Atlantic coast of the Sahara and
central England. The latter is a considerably longer distance, and this
means that the decrease of temperature is much slower on the European
Atlantic coast than on the North American Atlantic coast. In fact, the
rate of temperature decrease with latitude in the latter case is the most
rapid anywhere in the world, in the same distance. These great
contrasts in temperature which occur within short distances along the
eastern coast of North America have had great influence upon the
development of this region, as has been pointed out by Woeikof, an eminent
Russian meteorologist. The products of the tropics and of the Arctic are
here brought very near together; and at the same time intercommunication
between these two regions of widely differing climates is very easy.
Labrador is climatically an Arctic land, and man is there forced to seek
his food chiefly in the sea, for nature supplies him with little on shore,
while southern Florida is quite tropical in its temperature conditions and
in the abundance of its vegetation. Between the Pacific coasts of Asia and
of North America there is a similar but less pronounced contrast, the
isotherms being crowded together on the eastern coast of China and
Siberia, and being spread apart as they cross the Pacific Ocean and reach
our Pacific Coast.

In general, we naturally expect to find that the temperature decreases as
one goes poleward from the equator; from lower latitudes, where the sun is
always high in the heavens, to higher latitudes, where it is near the
horizon, and its warming effect is less. But there are some curious
exceptions to this general rule. The lowest temperatures on the January
isothermal chart (-60°) are found in northeastern Siberia, and not, so far
as our observations go, near the North Pole. If you find yourself at this
“cold pole,” as it is called, in Siberia in January, you can reach higher
temperatures by traveling north, south, east, or west. In other words,
here is a case of _increase_ of temperature in a _northerly_ direction, as
well as east, south, and west. Again, there is a district of high
temperature (90°) over southern Asia in July, from which you can travel
south towards the equator and yet reach lower temperatures.

In our winter months the contrasts of temperature in the United States
are, as a rule, violent, there being great differences between the cold of
the Northwest and the mild air of Florida and the Gulf States. In the
summer, on the other hand, the distribution of temperature is relatively
equable, the isotherms being, as a rule, far apart. In summer, therefore,
we approach the conditions characteristic of the Torrid Zone. These are
uniformly high temperatures over large areas. The same thing, on a larger
scale, is seen over the whole Northern Hemisphere. During our winter
months the isotherms are a good deal closer together than they are during
the summer, or, in more technical language, the temperature
gradient between the equator and the North Pole is steeper in winter than
in summer.



                              CHAPTER VI.

                                 WINDS.


The observational work already done, whether non-instrumental or
instrumental, has shown that there is a close relation between the
_direction of the wind_ at any station and the _temperature_ at that
station. Our second step in weather-map drawing is concerned with the
winds on the same series of maps which we have thus far been studying from
the point of view of temperature alone.

In the second column of the table in Chapter VIII are given the wind
directions and the wind velocities (in miles per hour) recorded at the
Weather Bureau stations at 7 A.M., on the first day of the series. Enter
on a blank weather map, at each station for which a wind observation is
given in the table, a small arrow flying _with_ the wind, _i.e._, pointing
in the direction _towards_ which the wind is blowing. Make the lengths of
the wind arrows roughly proportionate to the velocity of the wind, the
winds of higher velocities being distinguished by longer arrows, and those
of lower velocities by shorter arrows. The letters _Lt._ (= light) in the
table denote wind velocities of 5 miles, or less, per hour.

When you have finished drawing these arrows, you will have before you a
picture of the wind directions and velocities all over the United States
at the time of the morning observation on this day. (See solid arrows in
Fig. 26.)

The wind arrows on your map show the wind directions at only a few
scattered points as compared with the vast extent of the United States. We
must remember that the whole lower portion of the atmosphere is moving,
and not merely the winds at these scattered stations. It will help you to
get a clearer picture of this actual movement of the atmosphere as a
whole, if you draw some additional wind arrows between the stations of
observation, but in sympathy with the observed wind directions given in
the table and already entered on your map. These new arrows may be drawn
in broken lines, and may be curved to accord in direction with the
surrounding wind arrows. Heavier or longer lines may be used to indicate
faster winds. (See broken arrows, Fig. 26.)

[Illustration: FIG. 26.——Winds. First Day.]

It is clear that the general winds must move in broad sweeping paths,
changing their directions gradually, rather than in narrow belts, with
sudden changes in direction. Therefore long curving arrows give a better
picture of the actual drift of the atmospheric currents than do short,
straight, disconnected arrows.

Study the winds on this chart with care. Describe the conditions of wind
distribution in a general way. Can you discover any apparent relation
between the different wind directions in any part of the map? Is there
any system whatever in the winds? Write out a brief and concise
description of the results of the study of this map.

Enter on five other blank maps the wind directions given in the table in
Chapter VIII for the other five days of the series, making, as before, the
lengths of the arrows roughly proportionate to the velocity of the wind,
and adding extra broken arrows as suggested. (See Figs. 27-31.)

_A._ Study the whole series of six maps. Describe the wind conditions on
each map by itself, noting carefully any system in the wind circulation
that you may discover. Examine the wind velocities also. Are there any
districts in which the velocities are especially high? Have these
velocities any relation to whatever wind systems you may have discovered?
If so, include in your description of these systems some consideration of
the wind _velocities_ as well as of the wind _directions_.

[Illustration: FIG. 27.——Winds. Second Day.]

[Illustration: FIG. 28.——Winds. Third Day.]

[Illustration: FIG. 29.——Winds. Fourth Day.]

[Illustration: FIG. 30.——Winds. Fifth Day.]

[Illustration: FIG. 31.——Winds. Sixth Day.]

_B._ Compare each map of the series with the map preceding it. Note what
changes in direction and velocity have taken place at individual
stations. Group these changes as far as possible by the districts over
which similar changes have occurred. Compare the wind systems on each map
with those on the map for the preceding day. Has there been any
alteration in the position or relation of these systems? Write for each
day an account of the conditions on that map, and of the changes that have
taken place in the preceding 24-hour interval.

_C._ Write out a short connected account of the wind conditions and
changes illustrated on the whole set of six maps.

In the last chapter we studied the progression of the cold wave of low
temperatures in an easterly direction across the United States. Notice now
the relation of the winds on the successive maps of our series to the
movement of the cold wave. Place your wind charts and isothermal charts
for the six days side by side, and study them together. The temperature
distribution on the second day differs from that on the first. What are
the chief differences? Examine the wind charts for these two days. Do you
detect any differences in the wind directions or systems on these days? Do
these differences help to explain some of the changes in temperature?

Compare the temperature distribution on the second day with that on the
third. What are the most marked changes in the distribution? What changes
in the winds on the corresponding wind maps seem to offer an explanation
of these variations?

Proceed similarly with each map of the series. Formulate, in writing, the
general relation between winds and cold waves, discovered through your
study of these charts.

=Cold Waves in Other Countries.=——Cold waves in the United States come, as
has been seen, from the northwest, that being the region of greatest
winter cold. In Europe, cold waves come from the northeast. This is
because northwest of Europe there is a large body of warm water supplied
by the Gulf Stream drift, and therefore this is a source of warmth and not
of cold. The cold region of Europe is to the northeast, over Russia and
Siberia.

Cold waves have different names in different countries. In southern France
the cold wind from the north and northeast is known as the _mistral_,
derived from the Latin word _magister_, meaning _master_, on account of
its strength and violence. In Russia the name _buran_ or _purga_ is given
to the cold wave when it blows along with it the fine dry snow
from the surface of the ground. This _buran_ is apt to cause the loss of
many lives, both of men and cattle. In the Argentine Republic the coolest
wind is from the southwest. It is known as a _pampero_, from the Spanish
_pampa_, a _plain_.

=Cyclones and Anticyclones.=——A system of winds blowing towards a common
center (such as is well shown over the Gulf States on the weather map for
the second day, and over the middle Atlantic coast on the third day) is
called by meteorologists a _cyclone_. The name was first suggested by
Piddington early in this century. It is derived from the Greek word for
_circle_, and hence it embodies the idea of a circular or spiral movement
of the winds. A system of _outflowing_ winds, such as that over the
northwestern United States shown on the maps for the first five days, and
over the western Gulf States on the sixth day is called an _anticyclone_.
This name was proposed by Galton in 1863, and means the opposite of
_cyclone_.



                              CHAPTER VII.

                               PRESSURE.


_A._ =Lines of Equal Pressure: Isobars.=——One of the most important
weather elements is the _pressure_ of the atmosphere. This has already
been briefly discussed in the sections on the mercurial barometer (Chapter
II). It was there learned that atmospheric pressure is measured by the
number of inches of mercury which the weight of the air will hold up in
the glass tube of the barometer. Our sensation of heat or cold gives us
some general idea as to the air temperature. We can tell the wind
direction when we know the points of the compass, and can roughly estimate
its velocity. No instrumental aid is necessary to enable us to decide
whether a day is clear, fair or cloudy, or whether it is raining or
snowing. Unlike the temperature, the wind, or the weather, the pressure
cannot be determined by our own senses without instrumental aid. The next
weather element that we shall study is pressure.

Proceed as in the case of the thermometer readings. Enter upon a blank
map the barometer readings for the different stations given in the third
column of the table in Chapter VIII. When this has been done you have
before you the actual pressure distribution over the United States at 7
A.M., on the first day of the series. Describe the distribution of
pressure in general terms. Where is the pressure highest? Where lowest?
What are the highest and the lowest readings of the barometer noted on the
map? What is the difference (in inches and hundredths) between these
readings?

[Illustration: FIG. 32.——Isobars. First Day.]

Draw lines of equal pressure, following the same principles as were
adopted in the case of the isotherms. The latter were drawn for every even
10° of temperature. The former are to be drawn for every even .10 inch of
pressure. Every station which has a barometer reading of an even .10 inch
will be passed through by some line of equal pressure. Philadelphia, Pa.,
with 29.90 must be passed through by the 29.90 line; Wilmington, N. C.,
with 30.00, must have the 30.00 line passing through it, etc. Chicago,
with 30.17 inches, must lie between the lines of 30.10 and 30.20 inches,
and nearer the latter than the former. Denver, Col., with 30.35 inches,
must lie midway between the 30.30 and 30.40 lines (Fig. 32).

Lines of equal pressure are called _isobars_, a word derived from two
Greek words meaning _equal pressure_.

Describe the distribution of pressure as shown by the arrangement of the
isobars. Note the differences in form between the isotherms and the
isobars. The words _high_ and _low_ are printed on weather maps to mark
the regions where the pressure is highest and lowest.

Draw isobars for the other days, using the barometer readings given in the
table in Chapter VIII. Figs. 33-38 show the arrangement of the isobars on
these days.

The pressure charts may be colored, as indicated by the shading in these
figures, in order to bring out more clearly the distribution of pressure,
according to the same general scheme as that adopted in the temperature
charts. Color _brown_ all parts of your six isobaric charts over which the
pressures are below 29.50 inches; color _red_ all parts with pressure
above 30.00 inches. Use a _faint shade of brown_ for pressures between
29.50 inches and 29.00 inches, and a _darker shade_ for pressures below
29.00 inches. In the case of pressures over 30.00 inches, use a _pale red_
for pressures between 30.00 and 30.50 inches, and a _darker shade of red_
for pressures above 30.50 inches. By means of these colors the pressure
distribution will stand out very clearly. The scheme of color and shading
may, of course, be varied to suit the individual fancy.

Study the isobaric chart of each day of the series by itself at first.
Describe the pressure distribution on each chart.

Then compare the successive charts. Note what changes have taken place in
the interval between each chart and the one preceding; where the pressures
have risen; where they have fallen, and where they have remained
stationary. Write a brief account of the facts of pressure change
illustrated on the whole series of six charts.

[Illustration: FIG. 33.——Pressure. First Day.]

[Illustration: FIG. 34.——Pressure. Second Day.]

[Illustration: FIG. 35.——Pressure. Third Day.]

[Illustration: FIG. 36.——Pressure. Fourth Day.]

[Illustration: FIG. 37.——Pressure. Fifth Day.]

[Illustration: FIG. 38.——Pressure. Sixth Day.]

Compare the charts of temperature and of pressure, first individually,
then collectively. What relations do you discover between temperature
distribution and pressure distribution on the isothermal and the isobaric
charts for the same day? What relations can you make out between the
changes in temperature and pressure distribution on successive days? On
the whole series of maps? Write out the results of your study concisely
and clearly.

Compare the wind charts and the pressure charts for the six days. Is there
any relation between the direction and velocity of the winds and the
pressure? Observe carefully the changes in the winds from day to day on
these charts, and the changes in pressure distribution. Formulate and
write out a brief general statement of all the relations that you have
discovered.

=Mean Annual and Mean Monthly Isobaric Charts.=——We have thus far been
studying isobaric charts based on barometer readings made at a single
moment of time. Just as there are mean annual and mean monthly isothermal
charts, based on the mean annual and mean monthly temperatures, so there
are mean annual and mean monthly isobaric charts for the different
countries and for the whole world, based on the mean annual and mean
monthly pressures. The mean annual and mean monthly isobaric charts of
the world show the presence of great oval areas of low and high pressure
covering a whole continent, or a whole ocean, and keeping about the same
position for months at a time. Thus, on the isobaric chart showing the
mean pressure over the world in January, there are seen immense areas of
high pressure (anticyclones) over the two great continental masses of the
Northern Hemisphere. These anticyclonic areas, although vastly greater in
extent than the small ones seen on the weather maps of the United States,
have the same system of spirally _outflowing_ winds. Over the
northeastern portion of the North Pacific and the North Atlantic, in
January, are seen immense areas of low pressure (cyclones) with spirally
_inflowing_ winds. In July the northern continents are covered by
cyclonic areas, and the central portion of the northern oceans by
anticyclonic areas.

_B._ =Direction and Rate of Pressure Decrease: Pressure Gradient.=——In
Chapter V we studied the direction and rate of temperature decrease, or
temperature gradient. We saw that the direction of this decrease varies in
different parts of the map, and that the rate, which depends upon the
closeness of the isotherms, also varies. An understanding of temperature
gradients makes it easy to study the directions and rates of pressure
decrease, or _pressure gradients_, as they are commonly called. Examine
the series of isobaric charts to see how the lines of pressure decrease
run. Draw lines of pressure decrease for the six isobaric charts, as you
have already done on the isothermal charts. When the isobars are near
together, the lines of pressure decrease may be drawn heavier, to indicate
a more rapid rate of decrease of pressure. Fig. 39 shows lines of pressure
decrease for the first day. Note how the arrangement and direction of
these lines change from one map to the next. Compare these lines with the
lines of temperature decrease.

[Illustration: FIG. 39.——Pressure Gradients. First Day.]

Next study the _rate_ of pressure decrease. This rate depends upon the
closeness of the isobars, just as the rate of temperature decrease depends
upon the closeness of the isotherms. Examine the rates of pressure
decrease upon the series of isobaric charts. On which charts do you find
the most rapid rate? Where? On which the slowest? Where? Do you discover
any relation between rate of pressure decrease and the pressure itself?
What relation?

When expressed numerically, the barometric gradient is understood to mean
the number of hundredths of an inch of change of pressure in one latitude
degree. Prepare a scale of latitude degrees, and measure rates of pressure
decrease, just as you have already done in the case of temperature. In
this case, instead of dividing the difference in temperature between the
isotherms (10° = _T_) by the distance between the isobars (_D_), we
substitute for 10° of temperature .10 inch of pressure (_P_). Otherwise
the operation is precisely the same as described in Chapter V. The rule
may be stated as follows: Select the station for which you wish to know
the rate of pressure decrease or the barometric gradient. Lay your scale
through the station, and as nearly as possible at right angles to the
adjacent isobars. If the station is exactly on an isobar, then measure the
distance _from_ the station to the nearest isobar indicating a lower
pressure. The scale must, however, be laid perpendicularly to the isobars,
as before. Divide the number of hundredths of an inch of pressure
difference between the isobars (always .10 inch) by the number expressing
the distance (in latitude degrees) between the isobars; the quotient is
the rate of pressure decrease per latitude degree. Or, to formulate the
operation,

                            _R_ = _P_ / _D_,

in which _R_ = rate; _P_ = pressure difference between isobars (always .10
inch), and _D_ = distance between the isobars in latitude degrees.

Determine the rates of pressure decrease in the following cases:——

_A._ For a number of stations in different parts of the same map, as,
_e.g._, Boston, New York, Washington, Charleston, New Orleans, St. Louis,
St. Paul, Denver, and on the same day.

_B._ For one station during a winter month and during a summer month,
measuring the rate on each map throughout the month, and obtaining an
average rate for the month.

Have these gradients at the different stations any relation to the
proximity of low or high pressure? To the velocity of the wind?

=Pressure Gradients on Isobaric Charts of the Globe.=——The change from
low pressure to high pressure or _vice versa_ with the seasons, already
noted as being clearly shown on the isobaric charts of the globe,
evidently means that the directions of pressure decrease must also change
from season to season. The rates of pressure decrease likewise do not
remain the same all over the world throughout the year. If we examine
isobaric charts for January and July, we shall find that these gradients
are stronger or steeper over the Northern Hemisphere in the former month
than in the latter.



                             CHAPTER VIII.

                                WEATHER.


Hitherto nothing has been said about the _weather_ itself, as shown on the
series of maps we have been studying. By weather, in this connection, we
mean the state of the sky, whether it is clear, fair, or cloudy, or
whether it is raining or snowing at the time of the observation. While it
makes not the slightest difference to our feelings whether the _pressure_
is high or low, the _character of the weather_ is of great importance.

The character of the weather on each of the days whose temperature, wind,
and pressure conditions we have been studying is noted in the table in
this chapter. The symbols used by the Weather Bureau to indicate the
different kinds of weather on the daily weather maps are as follows: [full
moon] clear; [quarter moon] fair, or partly cloudy; [new moon] cloudy;
[circle-R] rain; [circle-S] snow.

Enter on a blank map, at each station, the sign which indicates the
weather conditions at that station at 7 A.M., on the first day, as given
in the table. When you have completed this, you have before you on the map
a bird’s-eye view of the weather which prevailed over the United States at
the moment of time at which the observations were taken. Describe in
general terms the distribution of weather here shown, naming the districts
or States over which similar conditions prevail. Following out the general
scheme adopted in the case of the temperature and the pressure
distribution, separate, by means of a line drawn on your map, the
districts over which the weather is prevailingly cloudy from those over
which the weather is partly cloudy or clear. In drawing this line,
scattering observations which do not harmonize with the prevailing
conditions around them may be disregarded, as the object is simply to
emphasize the _general_ characteristics. Enclose also, by means of another
line, the general area over which it was snowing at the time of
observation, and shade or color the latter region differently from the
cloudy one. Study the weather distribution shown on your chart. What
general relation do you discover between the kinds of weather and the
temperature, winds, and pressure?

Proceed similarly with the weather on the five remaining days, as noted in
the table. Enter the weather symbols for each day on a separate blank map,
enclosing and shading or coloring the areas of cloud and of snow as above
suggested. In Figs. 40-45 the cloudy areas are indicated by single-line
shading, and the snowy areas by double-line shading.

Now study carefully each weather chart with its corresponding temperature,
wind, and pressure charts. Note whatever relations you can discover among
the various meteorological elements on each day. Then compare the weather
conditions on the successive maps. What changes do you note? How are these
changes related to the changes of temperature; of wind; of pressure?
Write a summary of the results derived from your study of these four sets
of charts.

[Illustration: FIG. 40.——Weather. First Day.]

[Illustration: FIG. 41.——Weather. Second Day.]

[Illustration: FIG. 42.——Weather. Third Day.]

[Illustration: FIG. 43.——Weather. Fourth Day.]

[Illustration: FIG. 44.——Weather. Fifth Day.]

[Illustration: FIG. 45.——Weather. Sixth Day.]

=The Weather of Temperate and Torrid Zones.=——The facts of the presence of
clear weather in one region while snow is falling in another, and of the
variability of our weather from day to day in different parts of the
United States, are emphasized by these charts of weather conditions. This
changeableness of weather is a marked characteristic of the greater
portion of the Temperate Zones, especially in winter. The weather maps for
successive days do not, as a rule, show a repetition of the same
conditions over extended regions. In the Torrid Zone it is different. Over
the greater part of that zone the regularity of the weather conditions is
such that, day after day, for weeks and months, the same features are
repeated. There monotony, here variety, is the dominant characteristic of
the weather.



               PART IV.——THE CORRELATIONS OF THE WEATHER
                   ELEMENTS AND WEATHER FORECASTING.



                              CHAPTER IX.

            CORRELATION OF THE DIRECTION OF THE WIND AND THE
                               PRESSURE.


The study of the series of weather maps in Chapters V-VIII has made it
clear that some fairly definite relation exists between the general flow
of the winds and the distribution of pressure. We now wish to obtain some
more definite result as to the relation of the direction of the wind and
the pressure. In doing this it is convenient to refer the wind direction
to the _barometric_ or _pressure gradient_ at the station at which the
observation is made. The barometric gradient, it will be remembered, is
the line along which there is the most rapid change of pressure, and lies
at right angles to the isobars (Chapter VII).

[Illustration: FIG. 46.]

Take a small piece of tracing paper, about 3 inches square, and draw upon
it a diagram similar to the one here shown. Select the station (between
two isobars on any weather map) at which you intend to make your
observation. Place the center of the tracing paper diagram over the
station, with the dotted line along the barometric gradient, the minus end
of the line being towards the area of low pressure. Observe into which of
the four sectors (marked _right_, _left_, _with_, _against_) the wind
arrow at the station points. Keep a record of the observation. Repeat the
observation at least 100 times, using different stations, on the same map
or on different maps. Tabulate your results according to the following
scheme, noting in the first column the date of the map, in the second,
third, fourth, and fifth columns the number of winds found blowing _with_,
to the _right_ or _left_ of, and _against_, the gradient.

           TABLE I.——CORRELATION OF THE DIRECTION OF THE WIND
                           AND THE PRESSURE.

  +------------+--------+---------+--------+---------+
  |   DATES    |  WITH  |  RIGHT  |  LEFT  | AGAINST |
  +------------+--------+---------+--------+---------+
  |            |        |         |        |         |
  |            |        |         |        |         |
  |            |        |         |        |         |
  |            |        |         |        |         |
  |            |        |         |        |         |
  +------------+--------+---------+--------+---------+
  |   Sums     |        |         |        |         |
  +------------+--------+---------+--------+---------+
  |Percentages |        |         |        |         |
  +------------+--------+---------+--------+---------+

At the bottom of each column write down the number of cases in that
column, and then determine the percentages which these cases are of the
total number of observations. This is done by dividing the number of cases
in each column by the sum-total of all the observations. When you have
obtained the percentage of each kind of wind direction, you have a
numerical result.

A graphical presentation of the results may be made by laying off radii
corresponding in position to those which divide the sectors in Fig. 46,
and whose lengths are proportionate to the percentages of the different
wind directions in the table. Thus, for a percentage of 20, the radii may
be made 1 inch long, for 40%, 2 inches, etc. When completed, the relative
sizes of the sectors will show the relative frequencies of winds blowing
in the four different directions with reference to the gradient, as is
indicated in Fig. 47.

=The Deflection of the Wind from the Gradient: Ferrel’s Law.=——The law of
the deflection of the wind prevailingly to the right of the gradient is
known as _Ferrel’s Law_, after William Ferrel, a noted American
meteorologist, who died in 1891. The operation of this law has already
been seen in the spiral circulation of the winds around the cyclone and
the anticyclone, as shown on the maps of our series. In the case of the
cyclone the gradient is directed inward towards the center; in the case
of the anticyclone the gradient is directed outward from the center. In
both cases the right-handed deflection results in a spiral whirl, inward
in the cyclone, outward in the anticyclone. The operation of this law is
further seen in the case of the _Northeast Trade Winds_. These winds blow
from about Lat. 30° N. towards the equator, with wonderful regularity,
especially over the oceans. Instead of following the gradient and blowing
as north winds, these trades turn to the right of the gradient and become
_northeast_ winds, whence their name. From about Lat. 30° N. towards the
North Pole there is another great flow of winds over the earth’s surface.
These winds do not flow due north, as south winds. They turn to the
right, as do the trades, and become southwest or west-southwest winds,
being known as the _Prevailing Westerlies_. Ferrel’s Law thus operates in
the larger case of the general circulation of the earth’s atmosphere, as
well as in the smaller case of the local winds on our weather maps.

[Illustration: FIG. 47.]



                               CHAPTER X.

              CORRELATION OF THE VELOCITY OF THE WIND AND
                             THE PRESSURE.


Prepare a scale of latitude degrees, as explained in Chapter V. Select
some station on the weather map at which there is a wind arrow, and at
which you wish to study the relation of wind velocity and pressure. Find
the rate of pressure change per degree as explained in Chapter VII. Note
also the velocity, in miles per hour, of the wind at the station. Repeat
the operation 100 or more times, selecting stations in different parts of
the United States. It is well, however, to include in one investigation
either interior stations alone (_i.e._, more than 100 miles from the
coast) or coast stations alone, as the wind velocities are often
considerably affected by proximity to the ocean. And, if coast stations
are selected, either onshore or offshore winds should alone be included in
one exercise. The investigation may, therefore, be carried out so as to
embrace the following different sets of operations:——

  _A._ Interior stations.
  _B._ Coast stations with onshore winds.
  _C._ Coast stations with offshore winds.

Enter your results in a table similar to the one here given:——

         TABLE II.——CORRELATION OF WIND VELOCITY AND BAROMETRIC
                               GRADIENT.

For interior (or coast) stations, with onshore (or offshore) winds, in the
United States during the month (or months) of

  +--------------------+-------+-------+-------+--------+--------+---------+------+
  |Rates of Pressure   |       |       |       |        |        |         |      |
  |Change per Latitude | ∞-20  | 20-10 | 10-5  | 5-     | 3-1/2- | 2-1/2   | etc. |
  |Degree              |       |       |       |  3-1/2 |  2-1/2 |  -2     |      |
  +--------------------+-------+-------+-------+--------+--------+---------+------+
  |Distances between   |       |       |       |        |        |         |      |
  |Isobars in Latitude | 0-1/2 | 1/2-1 |  1-2  |  2-3   |  3-4   |   4-5   | etc. |
  |Degrees             |       |       |       |        |        |         |      |
  +--------------------+-------+-------+-------+--------+--------+---------+------+
  +--------------------+-------+-------+-------+--------+--------+---------+------+
  |                    |       |       |       |        |        |         |      |
  |                    |       |       |       |        |        |         |      |
  |Wind Velocities     |       |       |       |        |        |         |      |
  | (miles per hour)   |       |       |       |        |        |         |      |
  |                    |       |       |       |        |        |         |      |
  |                    |       |       |       |        |        |         |      |
  +--------------------+-------+-------+-------+--------+--------+---------+------+
  |      Sums          |       |       |       |        |        |         |      |
  +--------------------+-------+-------+-------+--------+--------+---------+------+
  |      Cases         |       |       |       |        |        |         |      |
  +--------------------+-------+-------+-------+--------+--------+---------+------+
  |      Means         |       |       |       |        |        |         |      |
  +--------------------+-------+-------+-------+--------+--------+---------+------+

The wind velocity for each station is to be entered in the column at whose
top is the rate of pressure change found for that station. Thus, if for
any station the rate of pressure change is 3-1/2 (_i.e._, .03 inch in one
latitude degree), and the wind velocity at that station is 17 miles an
hour, enter the 17 in the fourth and fifth columns of the table. When you
find that the rate of pressure change for any station falls into two
columns of the table, as, _e.g._, 10, or 5, or 3-1/3, then enter the
corresponding wind velocity in both those columns.

In the space marked _Sums_ write the sum-total of all the wind velocities
in each column. The _Cases_ are the number of separate observations you
have in each column. The _Means_ denote the average or mean wind
velocities found in each column, and are obtained by dividing the sums by
the cases.

Study the results of your table carefully. Deduce from your own results a
general rule for wind velocities as related to barometric gradients.

=The dependence of wind velocities on the pressure gradient= is a fact of
great importance in meteorology. The ship captain at sea knows that a
rapid fall of his barometer means a rapid rate of pressure change, and
foretells high winds. He therefore makes his preparations accordingly, by
shortening sail and by making everything fast. The isobaric charts of the
globe for January and July show that the pressure gradients are stronger
(_i.e_., the rate of pressure change is more rapid) over the Northern
Hemisphere in January than in July. This fact would lead us to expect
that the velocities of the general winds over the Northern Hemisphere
should be higher in winter than in summer, and so they are. Observations
of the movements of clouds made at Blue Hill Observatory, Hyde Park,
Mass., show that the whole atmosphere, up to the highest cloud level,
moves almost twice as fast in winter as in summer. In the higher
latitudes of the Southern Hemisphere, where the barometric gradients are
prevailingly much stronger than in the Northern, the wind velocities are
also prevailingly higher than they are north of the equator. The
prevailing westerly winds of the Southern Hemisphere, south of latitude
of 30° S., blow with high velocities nearly all the time, especially
during the winter months (June, July, August). These winds are so strong
from the westward that vessels trying to round Cape Horn from the east
often occupy weeks beating against head gales, which continually blow
them back on their course.



                              CHAPTER XI.

           FORM AND DIMENSIONS OF CYCLONES AND ANTICYCLONES.


_A._ =Cyclones.=——Provide yourself with a sheet of tracing paper about
half as large as the daily weather map. Draw a straight line across the
middle of it; mark a dot at the center of the line, the letter _N_ at one
end, and the letter _S_ at the other. Place the tracing paper over a
weather map on which there is a fairly well enclosed center of low
pressure (_low_), having the dot at the center of the _low_, and the line
parallel to the nearest meridian, the end marked _N_ being towards the top
of the map. When thus placed, the paper is said to be _oriented_. Trace
off the isobars which are nearest the center. In most cases the 29.80-inch
isobar furnishes a good limit, out to which the isobars may be traced.
Continue this process, using different weather maps, until the lines on
the tracing paper begin to become too confused for fairly easy seeing.
Probably 15 or 20 separate areas of low pressure may be traced on to the
paper. It is important to have all parts of the cyclonic areas represented
on your tracing. If most of the isobars you have traced are on the
southern side of cyclones central over the Lakes or lower St. Lawrence, so
that the isobars on the northern sides are incomplete, select for your
further tracings weather maps on which the cyclonic centers are in the
central or southern portions of the United States, and therefore have
their northern isobars fully drawn.

When your tracing is finished you have a _composite portrait_ of the
isobars around several areas of low pressure. Now study the results
carefully. Draw a heavy pencil or an ink line on the tracing paper, in
such a way as to enclose the average area outlined by the isobars. This
average area will naturally be of smaller dimensions than the outer
isobars on the tracing paper, and of larger dimensions than the inner
isobars, and its form will follow the general trend indicated by the
majority of the isobars, without reproducing any exceptional shapes.

Write out a careful description of the average _form_, _dimensions_
[measured by a scale of miles or of latitude degrees (70 miles = 1 degree
about)] and _gradients_ of these areas of low pressure, noting any
tendency to elongate in a particular direction; any portions of the
composite where the gradients are especially strong, weak, etc.

_B._ =Anticyclones.=——This investigation is carried out in precisely the
same manner as the preceding one, except that anticyclones (_highs_) are
now studied instead of cyclones. The isobars may be traced off as far away
from the center as the 30.20-inch line in most cases. When, however, the
pressure at the center is exceptionally high, it will not be necessary to
trace off lower isobars than those for 30.30, or 30.40, or sometimes 30.50
inches.

=Loomis’s Results as to Form and Dimensions of Cyclones and
Anticyclones.=——One of the leading American meteorologists, Loomis, who
was for many years a professor in Yale University, made an extended study
of the form and dimensions of areas of low and high pressure as they
appear on our daily weather maps. In the cases of areas of low pressure
which he examined, the average form of the areas was elliptical, the
longer diameter being nearly twice as long as the shorter (to be exact
the ratio was 1.94 : 1). The average direction of the longer diameter he
found to be about NE. (N. 36° E.), and the length of the longer diameter
often 1600 miles. In the case of areas of high pressure, Loomis also
found an elliptical form predominating; the longer diameter being about
twice as long as the shorter (ratio 1.91 : 1), and the direction of trend
about NE. (N. 44° E.). These characteristics hold, in general, for the
cyclonic and anticyclonic areas of Europe also. The cyclones of the
tropics differ considerably from those of temperate latitudes in being
nearly circular in form.



                              CHAPTER XII.

          CORRELATION OF CYCLONES AND ANTICYCLONES WITH THEIR
                           WIND CIRCULATION.


_A._ =Cyclones.=——Something as to the control of pressure over the
circulation of the wind has been seen in the preliminary exercises on the
daily weather maps. We now proceed to investigate this correlation further
by means of the composite portrait method. This method is a device to
bring out more clearly the general systems of the winds by throwing
together on to one sheet a large number of wind arrows in their proper
position with reference to the controlling center of low pressure. In this
way we have many more observations to help us in our investigation than if
we used only those which are given on one weather map, and the circulation
can be much more clearly made out.

Provide yourself with a sheet of tracing paper, prepared as described in
Chapter XI. Place the paper over an area of low pressure on some weather
map, with the dot at the center of the _low_, and having the paper
properly oriented, as already explained. Trace off all the wind arrows
around the center of low pressure, making the lengths of these arrows
roughly proportionate (by eye) to the velocity of the wind, according to
some scale previously determined upon. Include on your tracing all the
wind arrows reported at stations whose lines of pressure-decrease converge
towards the low pressure center. Repeat this operation, using other
centers of low pressure on other maps, until the number of arrows on the
tracing paper is so great that the composite begins to become confused. Be
careful always to center and orient your tracing paper properly. Select
the weather maps from which you take your wind arrows so that the
composite shall properly represent winds in all parts of the cyclonic
area.

Deduce a general rule for the circulation and velocity of the wind in a
cyclonic area, as shown on your tracing, and write it out.

_B._ =Anticyclones.=——This exercise is done in precisely the same way as
the preceding one, except that anticyclones and their winds are studied
instead of cyclones.

Deduce a general rule for the circulation and velocity of the wind in an
anticyclonic area, as shown on your tracing, and write it out.

=The control of the wind circulation by areas of low and high pressure= is
one of the most important laws in meteorology. Buys-Ballot, a Dutch
meteorologist, first called attention to the importance of this law in
Europe, and it has ever since been known by his name. Buys-Ballot’s Law is
generally stated as follows: _Stand with your back to the wind, and the
barometer will be lower on your left hand than on your right._[4] This
statement, as will be seen, covers both cyclonic and anticyclonic systems.
The circulations shown on your tracings hold everywhere in the Northern
Hemisphere, not only around the areas of low and high pressure seen on the
United States weather maps, but around those which are found in Europe and
Asia, and over the oceans as well. Mention has already been made, in the
chapter on isobars (VII), of the occurrence of immense cyclonic and
anticyclonic areas, covering the greater portion of a continent or an
ocean, and lasting for months at a time. These great cyclones and
anticyclones have the same systems of winds around them that the smaller
areas, with similar characteristics, have on our weather maps. A further
extension of what has just been learned will show that if in any region
there comes a change from low pressure to high pressure, or _vice versa_,
the system of winds in that region will also change. Many such changes of
pressures and winds actually occur in different parts of the world, and
are of great importance in controlling the climate. The best-known and the
most-marked of all these changes occurs in the case of India.
During the winter, an anticyclonic area of high pressure is central over
the continent of Asia. The winds blow out from it on all sides, thus
causing general northeasterly winds over the greater portion of India.
These winds are prevailingly dry and clear, and the weather during the
time they blow is fine. India then has its dry season. As the summer comes
on, the pressure over Asia changes, becoming low; a cyclonic area replaces
the winter anticyclone, and inflowing winds take the place of the
outflowing ones of the winter. The summer winds cross India from a general
southwesterly direction, come from over the ocean, and are moist and
rainy. India then has its rainy season. These seasonal winds are known as
_Monsoons_, a name derived from the Arabic and meaning _seasonal_.

[Footnote 4: In the Northern Hemisphere.]

[Illustration: FIG. 48.]

The accompanying figure (Fig. 48) is taken from the _Pilot Chart of the
North Atlantic Ocean_, published by the Hydrographic Office of the United
States Navy for the use of seamen. It shows the wind circulation around
the center of a cyclone which is moving northward along the Atlantic Coast
of the United States. The long arrow indicates the path of movement; the
shorter arrows indicate the directions of the winds. By means of such a
diagram as this a captain is able to calculate, with a considerable degree
of accuracy, the position of the center of the cyclone, and can often
avoid the violent winds near that center by sailing away from it, or by
“lying to,” as it is called, and waiting until the center passes by him at
a safe distance. These cyclones which come up the eastern coast of the
United States at certain seasons are usually violent, and often do
considerable damage to shipping. The Weather Bureau gives all the warning
possible of the coming of these _hurricanes_, as they are called, by
displaying _hurricane signals_ along the coast, and by issuing telegraphic
warnings to newspapers. In this way ship captains, knowing of the approach
of gales dangerous to navigation, may keep their vessels in port until all
danger is past. Millions of dollars’ worth of property and hundreds of
lives have thus been saved.



                             CHAPTER XIII.

            CORRELATION OF THE DIRECTION OF THE WIND AND THE
                              TEMPERATURE.


It is evident, from even the most general observation of the weather
elements, that the temperature experienced at any place is very largely
dependent upon the direction of the wind. Thus, for instance, in the
United States, a wind from some northerly point is likely to bring a lower
temperature than a southerly wind. To investigate this matter more
closely, and to discover how the winds at any station during any month are
related to the temperatures noted at that station, we proceed as
follows:——

Select the Weather Bureau station at which you wish to study these
conditions. Note the direction of the wind and the temperature at that
station on the first day of any month. Prepare a table similar to the
following one.

          TABLE III.——CORRELATION OF THE DIRECTION OF THE WIND
                          AND THE TEMPERATURE.


  At ..................... during the Month of ........

  +--------------+----+-----+----+-----+----+-----+----+-----+-------+-------+
  | WIND         |    |     |    |     |    |     |    |     |               |
  | DIRECTIONS   | N. | NE. | E. | SE. | S. | SW. | W. | NW. |               |
  +--------------+----+-----+----+-----+----+-----+----+-----+               +
  |              |    |     |    |     |    |     |    |     |               |
  +              +----+-----+----+-----+----+-----+----+-----+               +
  |              |    |     |    |     |    |     |    |     |               |
  +              +----+-----+----+-----+----+-----+----+-----+               +
  | TEMPERATURES |    |     |    |     |    |     |    |     |               |
  +              +----+-----+----+-----+----+-----+----+-----+               +
  |              |    |     |    |     |    |     |    |     |               |
  +              +----+-----+----+-----+----+-----+----+-----+               |
  |              |    |     |    |     |    |     |    |     |               |
  +--------------+----+-----+----+-----+----+-----+----+-----+-------+-------+
  | Sums         |    |     |    |     |    |     |    |     |       | Total |
  +--------------+----+-----+----+-----+----+-----+----+-----+-------+-------+
  | Cases        |    |     |    |     |    |     |    |     |       | Total |
  +--------------+----+-----+----+-----+----+-----+----+-----+-------+-------+
  | Means        |    |     |    |     |    |     |    |     |       | Mean  |
  +--------------+----+-----+----+-----+----+-----+----+-----+-------+-------+

Enter the temperature at 8 A.M. on the first day of the month in a column
of the table under the proper wind direction. Thus, if the wind is NE.,
and the temperature 42°, enter 42 in the second column of the table.
Repeat the observation for the same station, and for all the other days of
the month, recording the temperatures in each case in their appropriate
columns in the table. Omit all cases in which the wind is _light_, because
winds of low velocities are apt to be considerably affected by local
influences. When the observations for the whole month have been entered in
the table, add up all the temperatures in each column (_sums_). Find the
mean temperature (_means_) observed with each wind direction by dividing
the sums by the number of observations in each column (_cases_). Add all
the sums together; divide by the total number of cases, and the result
will be the mean temperature[5] for the month at the station. The general
effect of the different wind directions upon the temperature is shown by a
comparison of the means derived from each column with the mean for the
month.

[Footnote 5: Derived from the 8 A.M. observations. This does not give the
true mean temperature.]

[Illustration: FIG. 49.]

A graphic representation of the results of this investigation will help to
emphasize the lesson. Draw, as in the accompanying figure (Fig. 49), eight
lines from a central point, each line to represent one of the eight wind
directions. About the central point describe a circle, the length of whose
radius shall correspond to the mean temperature of the month, measured on
some convenient scale. Thus, if the mean temperature of the month is 55°
and a scale of half an inch is taken to correspond to 10° of temperature,
the radius of the circle must be five and a half times half an inch, or
2-3/4 inches. Next lay off on the eight wind lines the mean temperatures
corresponding to the eight different wind directions, using the same scale
(1/2 in. = 10°) as in the previous case. Join the points thus laid off by a
heavy line, as shown in Fig. 49. The figure, when completed, gives at a
glance a general idea of the control exercised by the winds over the
temperatures at the station selected. Where the heavy line crosses a wind
line _inside_ the circle it shows that the average temperature
accompanying the corresponding wind direction is below the mean. When the
heavy line crosses any wind line _outside_ the circle, it shows that the
average temperature accompanying the corresponding wind direction is above
the mean. Such a figure is known as a _wind rose_.

=The cold wave and the sirocco= are two winds which exercise marked
controls over the temperature at stations in the central and eastern
United States. The _cold wave_ has already been described in Chapter V.
It is a characteristic feature of our winter weather. It blows down from
our Northwestern States or from the Canadian Northwest, on the western
side of a cyclone. It usually causes sudden and marked falls in
temperature, sometimes amounting to as much as 50° in 24 hours. The
_sirocco_ is a southerly or southwesterly wind. It also blows into a
cyclone, but on its southern or southeastern side. Coming from warmer
latitudes, and from over warm ocean waters, the sirocco is usually a warm
wind, in marked contrast to the cold wave. In winter, in the Mississippi
Valley and on the Atlantic Coast, the sirocco is usually accompanied by
warm, damp, cloudy, and snowy or rainy weather. The high temperatures
accompanying it (they may be as high as 50° or 60° even in midwinter) are
very disagreeable. Our warm houses and our winter clothing become
oppressive and we long for the bright, crisp, cold weather brought by the
_cold wave_. In summer when a sirocco blows we have our hottest spells.
Then sunstrokes and prostrations by the heat are most common, and our
highest temperatures are recorded. The word _sirocco_ (from
_Syriacus_=Syrian) was first used as the name of a warm southerly wind in
Italy. The cause and the characteristics of the Italian sirocco and of
the American sirocco are similar, and the name may therefore be applied
to our wind as well as to the Italian one. In the Southern Hemisphere, at
Buenos Ayres, Argentine Republic, there is a similar contrast between two
different winds. The _pampero_ is similar in many respects to our cold
wave. It is a dry, cool, and refreshing wind, blowing over the vast level
stretches of the Argentine pampas from the southwest. The _norte_ is a
warm, damp, depressing northerly wind corresponding to our sirocco.



                              CHAPTER XIV.

           CORRELATION OF CYCLONES AND ANTICYCLONES AND THEIR
                             TEMPERATURES.


_A._ =Cyclones.=——It follows from the two preceding exercises that some
fairly definite distribution of temperature, depending upon the wind
direction, should exist around areas of low and high pressure. Try to
predict, on the basis of the results obtained in Chapters XII and XIII,
what this relation of temperatures and cyclones and anticyclones is. Then
work out the relation independently of your prediction, by studying actual
cases obtained from the weather maps, as follows:——

[Illustration: FIG. 50.]

Prepare a sheet of tracing paper as shown in Fig. 50. The diameter of the
circle should be sufficiently large to include within the circle the
average area covered by a cyclone on the weather maps. Place the tracing
paper, properly divided in accordance with the figure, over a well-defined
area of low pressure on a weather map, centering and orienting it
carefully. Take the temperature at the station which lies nearest the
center of the figure as the standard. Notice the temperatures at all the
other stations which fall within the limits of the circle, and mark down
at the proper places on the tracing paper, the + or - departures of these
local temperatures from the standard temperature. Thus, if the standard is
37°, and a station has a temperature of 46°, enter +9° at the proper place
on your tracing paper. Again, if a certain station has 24°, enter -13° at
the proper place on the paper. Continue this process until your paper has
all of its divisions well filled. It is best to select all the maps used
in this investigation from the same month, for in that case the data are
more comparable than if different months are taken. When a sufficient
number of examples has been obtained, find the average departure (+ or -)
of the temperatures in each division of the tracing from the central
standard temperature. Express these averages graphically by means of a
_wind rose_, as in the last exercise.

=Another Method.=——The above correlation may be investigated by means of
another method, as follows:——

Prepare a piece of tracing paper by drawing an N. and S. line upon it, and
placing a dot at the center of the line. Lay the paper over an area of low
pressure on any weather map, centering and orienting it properly, as in
the previous exercises. Trace off the isotherms which are near the center
of low pressure. Repeat this process with several maps, selecting
different ones from those used in the first part of this exercise.
Formulate a rule for the observed distribution of temperature, and
determine the reasons for this distribution. Note carefully any effects of
the cyclone upon the temperature gradient.

_B._ =Anticyclones.=——The correlation of anticyclones with their
temperatures is studied in precisely the same way as the preceding
correlation. Both methods suggested in the case of cyclones should be used
in the case of anticyclones. When your results have been obtained,
formulate a general rule for the observed distribution of temperature in
anticyclones, and determine the reasons for this distribution.

Find from your composites the average temperature of cyclones and of
anticyclones, and compare these averages.

=The unsymmetrical distribution of temperature around cyclones=, which is
made clear by the foregoing exercises, is very characteristic of these
storms in our latitudes, and especially in the eastern United States.
That this has an important effect upon weather changes is evident, and
will be further noted in the chapter on _Weather Forecasting_. The
cyclones which begin over the oceans near the equator at certain seasons,
and thence travel to higher latitudes,——_tropical cyclones_, so
called,——differ markedly from our cyclones in respect to the distribution
of temperature around them. The temperatures on all sides of tropical
cyclones are usually remarkably uniform, the isotherms coinciding fairly
closely with the isobars. The reason for this is to be found in the
remarkable uniformity of the temperature and humidity conditions over the
surrounding ocean surface, from which the inflowing winds come. In the
case of our own cyclones, in the eastern United States, the warm southerly
wind, or sirocco, in front of the center has very different
characteristics from those of the cold northwesterly wind, or _cold wave_,
in the rear, as has become evident through the preceding exercise. These
winds, therefore, naturally show their effects in the distribution of the
temperatures in different parts of the cyclonic area.



                              CHAPTER XV.

            CORRELATION OF THE DIRECTION OF THE WIND AND THE
                                WEATHER.


Select a file of daily weather maps for some month. Commencing with the
first map in the set, observe the weather and the direction of the wind at
a considerable number of stations in the same general region (as, _e.g._,
the Lake region, the lower Mississippi Valley, the Pacific Coast, etc.).
Enter each case in a table, similar to Table IV below, by making a check
in the column under the appropriate wind direction and on a line with the
appropriate type of weather.

          TABLE IV.——CORRELATION OF THE DIRECTION OF THE WIND
                            AND THE WEATHER.

At ________ during the Month of ________.

  +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+
  |             | N. | NE. | E. | SE. | S. | SW. | W. | NW. |TOTALS|PERCENTAGES|
  +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+
  |Clear        |    |     |    |     |    |     |    |     |  D   |     J     |
  +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+
  |Fair         |    |     |    |     |    |     |    |     |  F   |     K     |
  +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+
  |Cloudy       |    |     |    |     |    |     |    |     |  G   |     L     |
  +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+
  |Rain and Snow|    |     |    |     |    |     |    |     |  H   |     M     |
  +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+
  |Totals       |    |     |    |     |    |     |    |     |  A   |           |
  +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+
  |Percentages  | B  |  C  |etc.|     |    |     |    |     |      |           |
  +-------------+----+-----+----+-----+----+-----+----+-----+------+-----------+

Count every observation of _rain_ or _snow also as cloudy_, for it usually
rains or snows only when the sky is cloudy. Continue your observations on
all the maps for the month you have chosen. Then count up the whole number
of cases of _clear_ weather you have found with north winds, and write
down this sum in the first space, in the column reserved for N. winds. Do
the same with _fair_ and _cloudy_ weather. Add up and enter at the bottom
of the column in the space marked _Totals_ the whole number of
observations of _clear_, _fair_, and _cloudy_ weather you have observed
with N. winds. Then find what percentage of the weather with N. winds was
_clear_, and enter this percentage next to the sum of _clear_ weather
observations, in the first division in the column headed N. Do the same
for _fair_, _cloudy_, and _rainy_ or _snowy_ weather, deriving the
percentages of rain or snow from the total of _clear_ and _fair_ and
_cloudy_. Repeat this process of summarizing in every column. Your
results will then show the percentages of the different kinds of weather
noted with the different wind directions.

The lower division of the table and the last two columns on the right are
to be used for a general summary of the whole investigation. By adding
together all the totals of _clear_, _fair_, and _cloudy_ weather observed
with all the eight wind directions you obtain the whole number of
observations you have made. Enter this in the space marked A, at the right
of the table. From this grand total and the total number of observations
in each column you may find (in percentages) the relative frequencies of
the different wind directions. These should be entered under the totals at
the bottom of each column, in the spaces marked _Percentages_ (spaces B,
C, etc.). The total number of observations of _clear_, _fair_, _cloud_,
and _rain_ or _snow_, noted with all the wind directions, are to be
entered in spaces D, F, G, and H, at the right of the table. From these
totals, and from the grand total in space A, we can determine the relative
frequency (in percentages) of each kind of weather during the month. These
results should be entered in spaces J, K, L, and M.

Study these results carefully. Formulate them in a brief written
statement. Express graphically the following:——

_A._ The percentages of frequency of the different wind directions during
the month.

_B._ The percentages of the different kinds of weather noted during the
whole month for all wind directions.

[Illustration: FIG. 51.]

A wind rose, indicating the percentages of frequency of different winds
during a month, or a year, or several years, may be constructed as shown
in Fig. 51.

A certain convenient scale is adopted as representing a frequency of 10%,
and a circle is drawn with this unit as a radius. A second circle, with a
radius twice as long, represents a frequency of 20%, and a third circle,
with a radius three times as long, represents a frequency of 30%.
Additional circles may be added if necessary. Distances corresponding to
the different percentages of frequency of the eight wind directions are
then laid off along the eight radii of the circles, and the points thus
fixed are joined by a line.

The results asked for under question _B_ may be plotted as a weather rose
on a diagram similar to that above figured. In this case the percentages
of frequency of the different varieties of weather (_clear_, _fair_,
_cloudy_, _stormy_) may be indicated on the same figure by using different
kinds of lines. Thus, a _solid_ line may be employed to represent _clear_
weather; a _broken_ line for _fair_; a _broken and dotted_ line for
_cloudy_; and a _dotted_ line for _stormy_ weather.



                              CHAPTER XVI.

            CORRELATION OF CYCLONES AND ANTICYCLONES AND THE
                                WEATHER.


_A._ =Cyclones.=——Prepare a piece of tracing paper as shown in Fig. 52,
making the diameter of the outer circle about 1000 miles[6] and of the
inner circle 500 miles. Place this diagram over a cyclone on any weather
map, centering and orienting it carefully. Trace off the weather signs
(indicating _clear_, _fair_, _cloudy_, _rain_ or _snow_) around the
cyclonic center from the map on to the tracing paper, taking only
observations which are not more than halfway from the cyclonic center to
the neighboring anticyclonic center. Repeat this process with successive
weather maps until the diagram is well filled in all its different
divisions.

[Footnote 6: Use the scale of miles given on the weather map.]

_A._ Draw a line on the tracing paper enclosing the average area of
_cloud_ (including _rain and snow_), and a second line enclosing the
average area of _precipitation_ (_rain_ or _snow_).

_B._ Determine the percentages of _clear_, _fair_, _cloudy_, and _stormy_
observations for each division of the tracing paper, _i.e._, (_a_) for the
eight sectors of the large circle; (_b_) for the whole of the small
circle; and (_c_) for the portion of the diagram between the circumference
of the inner circle and the circumference of the outer circle.

[Illustration: FIG. 52.]

_C._ Write out in general terms a description of the weather distribution
in cyclones as illustrated by your own investigation.

_B._ =Anticyclones.=——This exercise is done in the same way as the
preceding one, except that anticyclones are substituted for cyclones.

=The association of fair weather with anticyclones and of stormy weather
with cyclones= is one of the most important lessons learned from a study
of the weather maps. The great areas of high and low pressure control our
weather. On land, where daily weather maps are so easily accessible, a
glance at the map serves in most cases to give a fairly accurate idea of
the position and extent of cyclones and anticyclones, and hence also of
the distribution of weather. At sea, on the other hand, the navigator has
no daily weather maps to refer to, and his knowledge of the weather
conditions which he may expect must be gained from his own observations
alone. Of these local observations, the pressure readings are by far the
most important. A falling barometer usually means the approach of a
cyclone, with wind, or storm, or both. A rising barometer, on the other
hand, is usually an indication of the fine weather associated with an
anticyclone. The unsymmetrical distribution of weather, characteristic of
our cyclones in the United States, and also of most cyclones in the
Temperate Zone, is associated with their unsymmetrical form, and the
unsymmetrical distribution of their temperature already studied. Tropical
cyclones have a wonderfully uniform distribution of weather on all sides
of their centers, just as they have a symmetrical form and an even
temperature distribution all around them.



                             CHAPTER XVII.

               PROGRESSION OF CYCLONES AND ANTICYCLONES.


So far no definite study has been made of the changes in the positions of
cyclones and anticyclones. If these areas of stormy and fair weather
always occupied the same geographic positions, the different portions of
the country would always have the same kinds of weather. A knowledge of
the movements of the areas of low and high pressure makes weather
forecasting possible.

_A._ =Cyclones.=——Select a set of daily weather maps for a month. Turn to
the first map of the series. Note the position of the center of low
pressure, and indicate this position on a blank weather map of the United
States by marking down a small circle at the proper place. If there are
two or more areas of low pressure on the map, indicate the position of
each one of them in a similar way. Turn to the second map of the series,
and again enter on the blank map the position of the center of low
pressure. Connect the two positions of each center by a line. This line
may be called the _track_ of the low pressure center. Continue this
process through the whole set of maps, connecting all the new positions
with the last positions of their respective centers. Mark each position
with the appropriate date in small, neat figures. When completed your map
will show at a glance the tracks followed by all the cyclones which
traveled across the United States during the month you selected. Study
these tracks carefully. Notice whether there is any prevailing direction
in which the cyclones move, and whether they show any preference for
particular paths across the country. Can you frame a general rule for the
prevailing direction and path of movement? Are there any cases which do
not accord with the rule? If so, describe them. In what position, with
reference to the cyclonic tracks during the month you are studying, is the
region in which you are now living?

Next determine the _velocities_ with which these cyclones moved. Prepare a
scale of latitude degrees, as described in Chapter V, or of miles, as
given at the bottom of the weather map. Measure the distances, in miles,
between the successive positions of all the cyclonic centers. Divide these
distances by 24 in order to obtain the velocity in _miles per hour_. What
is the highest velocity per hour with which any cyclone moved during the
month? What is the lowest? What is the mean, or average, velocity?

Study the tracks and velocities of cyclones in a similar way during
several other months. Compare the positions of the tracks, and the
velocities of progression, in summer and winter.

_B._ =Anticyclones.=——Study the tracks and velocities of anticyclones in
precisely the same manner. Compare the results derived from your
investigations in the two cases.

=Cyclonic Tracks and the Prevailing Westerly Winds.=——The correspondence
in the direction of movement of most of our cyclones and of the
prevailing westerly winds of the Northern Hemisphere (see Chapter IX)
will readily be noted. Our weather maps show us the atmospheric
conditions over the United States alone, and we can therefore trace the
progression of our cyclones over but a limited area of the Northern
Hemisphere. An examination of the daily weather maps of the North
Atlantic and North Pacific Oceans, which are based on observations made
on board ships, and of the weather maps of other countries, shows us that
these atmospheric disturbances which appear on our maps may often be
traced for long distances across oceans and lands, and that they in
reality form a great procession across the northern portion of this
hemisphere, and towards and around the North Pole.

=The average velocity of cyclones in the United States= was carefully
determined by Loomis. His observations show that the mean hourly velocity
of cyclones for the entire year is 28.4 miles, the maximum (34.2 miles)
coming in February, and the minimum (22.6 miles) in August. Over the North
Atlantic Ocean the hourly velocity is 18 miles; in Europe, 16.7 miles. The
greater velocity of cyclonic movement in the United States in winter
recalls what was said at the close of Chapter X concerning the steeper
barometric gradient and the more rapid movement of the whole atmosphere
over the Northern Hemisphere in winter.



                             CHAPTER XVIII.

                   SEQUENCE OF LOCAL WEATHER CHANGES.


The next, and last, step in our study of the correlation of the various
weather elements concerns the sequence of weather changes at a station
before, during, and after the passage of a cyclone and of an anticyclone.

_A._ =Cyclones.=——I. Select some station which the weather maps show to
have been directly on the track of a well-developed cyclone, _i.e._, to
have been passed over by the center of the cyclone. Note the weather
conditions at this station, before, during, and after the passage of the
storm. Tabulate your observations according to the following scheme:——

                                TABLE V.

Weather Changes at ________________________ during ______________.

  +-------+----------+---------+-----------+----------+---------+----------------+
  | DATES | PRESSURE | TEMPER- |   WIND    |   WIND   | WEATHER |DIREC. AND DIST.|
  |       |          |  ATURE  | DIRECTION | VELOCITY |         |OF STORM CENTER |
  +-------+----------+---------+-----------+----------+---------+----------------+
  |       |          |         |           |          |         |                |
  |       |          |         |           |          |         |                |
  |       |          |         |           |          |         |                |
  |       |          |         |           |          |         |                |
  |       |          |         |           |          |         |                |
  |       |          |         |           |          |         |                |
  +-------+----------+---------+-----------+----------+---------+----------------+

In the last column of the table enter the direction and the distance of
the cyclonic center from the station, at each observation.

II. Select a station which was north of the track of a cyclone, and
tabulate (in a separate table) the weather conditions at that station
before, during, and after the passage of the center.

III. Do the same for a station which was south of the track of a cyclone.
Repeat these observations for several stations.

_B._ =Anticyclones.=——Make a similar series of observations for the
passage of an anticyclone centrally over, north and south of, several
stations.

Study the sequence of the weather changes shown in the various tables.
Deduce a general rule for these changes and write it out.



                              CHAPTER XIX.

                          WEATHER FORECASTING.


In a letter dated at Philadelphia, July 16, 1747, Benjamin Franklin wrote
to his friend Jared Eliot as follows: “We have frequently along the North
American coast storms from the northeast which blow violently sometimes
three or four days. Of these I have had a very singular opinion for some
years, viz.: that, though the course of the wind is from northeast to
southwest, yet the course of the storm is from southwest to northeast; the
air is in violent motion in Virginia before it moves in Connecticut, and
in Connecticut before it moves at Cape Sable, etc. My reason for this
opinion (if the like have not occurred to you) I will give in my next.”

In a second letter to the same correspondent, dated Philadelphia, Feb. 13,
1749-50, Franklin states his reasons as follows: “You desire to know my
thoughts about the northeast storms beginning to leeward. Some years
since, there was an eclipse of the moon at nine o’clock in the evening,
which I intended to observe; but before night a storm blew up at
northeast, and continued violent all night and all the next day; the sky
thick-clouded, dark, and rainy, so that neither moon nor stars could be
seen. The storm did great damage all along the coast, for we had accounts
of it in the newspapers from Boston, Newport, New York, Maryland, and
Virginia; but what surprised us was to find in the Boston newspapers an
account of the observation of that eclipse made there; for I thought as
the storm came from the northeast it must have begun sooner at Boston than
with us, and consequently have prevented such an observation. I wrote to
my brother about it, and he informed me that the eclipse was over there an
hour before the storm began. Since which time I have made inquiries from
time to time of travelers, and observed the accounts in the newspapers
from New England, New York, Maryland, Virginia, and South Carolina; and I
find it to be a constant fact that northeast storms begin to leeward, and
are often more violent there than to windward” (Sparks’s _Life of
Franklin_, VI, 79, 105, 106).

The fact that our northeast storms come from the southwest, which was
first noticed by Benjamin Franklin some years before he put the suggestion
just quoted in writing, was one of the great contributions to meteorology
made by Americans. Modern weather forecasting essentially depends upon the
general eastward movement of cyclones and anticyclones, with their
accompanying weather conditions.

The daily weather map shows us the actual condition of the weather all
over the United States at 8 A.M., “Eastern Standard Time.” The positions
of cyclones and of anticyclones; of areas of clear, fair, cloudy or stormy
weather, and of regions of high or low temperatures, are plainly seen at a
glance. These areas of fair and foul weather, with their accompanying
systems of spiralling winds, move across country in a general easterly
direction. Knowing something of their direction and rate of movement, we
can determine, with greater or less accuracy, their probable positions in
12, 24, 36, or 48 hours. The prediction or foretelling of the weather
which may be expected to prevail at any station or in any district is
_weather forecasting_.

Weather forecasts are usually made on our daily weather maps for 24 hours
in advance. It is by no means an easy thing to make accurate weather
forecasts. Careful study and much practice are required of the forecasters
of the Weather Bureau before they are permitted to make the official
forecasts which are printed on the daily maps and in the newspapers.

A simple extension and application of the principles learned through the
preceding exercises make it possible for us to forecast coming weather
changes in a general way. These suggestions are, however, not at all to be
considered as a complete discussion of this complicated problem.

Weather forecasts include the probable changes in _temperature_, _wind
direction_ and _velocity_, and _weather_. Pressure is not included. Begin
your practice in weather forecasting by considering only the changes that
may be expected at your own point of observation, and at first confine
yourself to predicting temperature changes alone.

=Temperature.=——Provide yourself with a blank weather map. Draw an
isotherm east and west across the map, through your station. Draw a few
other isotherms all the way across the map, parallel with the first one,
and so arranged that they will be equal distances apart, the most
northerly one running through northern Maine and the Northwestern States,
and the most southerly one through southern Florida and Texas. Recalling
what you have already discovered concerning the eastward movement of our
weather conditions, what forecast will you make as to the coming
temperatures at your station? Add some additional east and west isotherms,
so that there will be twice as many on your map as before. What effect
will this change in the temperature distribution on your map have upon
the temperature forecast you make for your station? Formulate a general
rule as to temperature forecasts under the conditions of isothermal
arrangement here suggested.

On a second blank weather map draw an isotherm through your station
inclined from northwest to southeast. Draw a few other isotherms parallel
to the first, and each one representing a temperature 10° higher than that
indicated by the adjacent isotherm on the east. Make a general forecast of
the temperature conditions that may be expected at your station, as to
_kind of change_, if any; _amount of change_, and _rapidity of change_. Of
the isotherms just drawn, erase every second one; still, however, letting
those that are left represent differences of temperature of 10°. What
forecast will you now make as to temperature? How does this forecast
compare with that just made?

Now draw twice as many isotherms on your map as you had in the first
place, still letting these lines represent differences of temperature of
10° in each case. Make a forecast of the kind, amount, and rapidity of
temperature change at your station under the conditions represented on
this map. How does this forecast compare with the two just made? Formulate
a general rule governing temperature forecasts in cases of isothermal
arrangement such as those here considered.

Take another blank map. Draw through your station an isotherm inclined
from northeast to southwest. Draw other isotherms parallel to this, west
of your station, letting each successive isotherm represent a temperature
10° lower than that indicated by the adjacent isotherm on the east. Make a
temperature forecast for your station under these conditions. Diminish and
increase the number of isotherms on your map, as suggested in the
preceding example, making temperature forecasts in each case, and
comparing the three sets of forecasts. Formulate a general rule for
temperature forecasts made under these systems of isotherms.

Make temperature forecasts from the daily weather maps for your own
station, using the knowledge that you have already gained as to the
progression of cyclones and anticyclones (Chapter XVII), and as to the
temperature distribution in these areas (Chapter XIV), to help you in this
work. Study each day’s map carefully before you decide on what you will
say. Then write out your own forecast, and afterwards compare your
forecast with that made by the Weather Bureau. Note also, by reference to
your own instrumental observations, whether the succeeding temperature
conditions are such as you predicted.

=Wind Direction.=——The weather maps already studied taught us that our
winds habitually move in spirals. The composite picture of the wind
circulation around cyclones and anticyclones (Chapter XII) further
emphasized this important fact. Evidently this law of the systematic
circulation of the winds around centers of low and high pressure may be
utilized in making forecasts of wind direction.

Applying the knowledge already gained concerning cyclonic and anticyclonic
wind circulations, ask yourself what winds a station should have which is
within the range of the cyclonic wind system, and is in the following
positions with reference to the center: _northeast_, _north_, _northwest_,
_east_, _at the center_, _west_, _southeast_, _south_, _southwest_. Ask
yourself precisely the same questions with reference to a station within
an anticyclonic wind system. Write out a general rule for the kinds of
wind changes which may be expected to take place under these different
conditions.

When a station is south of the track of a passing cyclone its winds are
said to _veer_, and the change in the direction of its winds is called
_veering_. A station north of the track of a passing cyclone has a change
of direction in its winds which is known as _backing_, the winds
themselves being said to _back_.

=Wind Velocity.=——What general relation between wind velocities and areas
of low and high pressure did you discover in your study of the weather
maps? What was the result of your work on the correlation of the velocity
of the wind and the barometric gradient in Chapter X? And what general
statement as to the relation between the velocity of the wind and its
distance from a cyclonic or anticyclonic center may be made as the result
of your work in Chapter XII, on the correlation of cyclones and
anticyclones with their wind circulation? These results must be borne in
mind in making predictions of coming changes in wind velocities. Forecasts
of wind velocities are made in general terms only,——_light_, _moderate_,
_fresh_, _brisk_, _high_, _gale_, _hurricane_,——and are not given in miles
per hour.

Make forecasts of wind direction and velocity from the daily weather maps
for your own station. Continue these for a week or two, keeping record of
the verification or non-verification of each of your forecasts. Then make
daily forecasts of temperature and of wind direction and velocity
together. Write out your own forecast for each day before you compare it
with the official forecast, and if the two differ, keep note of which one
seemed to you to be the most accurate.

=Weather.=——What general relation between kind of weather and cyclones and
anticyclones was illustrated on the six maps of our series? What is the
average distribution of the different kinds of weather around cyclones and
anticyclones, as shown by your composites? (Chapter XVI.) What changes in
weather will ordinarily be experienced at a station as a cyclone
approaches, passes over, and moves off? What conditions will prevail in an
anticyclone?

Make a series of daily forecasts for your own station of probable weather
changes, omitting temperature and winds at first. Include in your weather
forecasts the state of the sky (_clear_, _fair_, _cloudy_); the changes in
the state of the sky (increasing or decreasing cloudiness); the kind of
precipitation (_rain_ or _snow_) and the amount of precipitation (_light_
or _heavy_). Write out your forecasts; compare them with the official
forecasts, and notice how fully they are verified. Then add temperature
and winds to your forecasts so that you will make a complete prediction of
probable changes in temperature (kind, amount, and rapidity of change),
wind (direction and velocity), and weather. Practice making these complete
forecasts for several weeks, if time allows. Use all the knowledge that
you have gained in the preceding work to aid you in this. Study each
weather map very carefully. Do not write down your forecast until you are
sure that you have done the best you can.

[Illustration: FIG. 53.]

Vary this exercise by extending your forecasts so as to embrace the whole
section of country in which your station is situated (as, _e.g._, New
England, the Gulf States, the Lake region). Pay special attention to
making forecasts of cold waves, of heavy rain or snowstorms, of high
winds over the lakes or along the Atlantic coast, etc. When possible,
obtain from the daily newspapers any particulars as to damage done by
frost or gales, or concerning snow blockades, floods due to heavy rains,
etc.

Fig. 53 summarizes what has thus far been learned as to the distribution
of the various weather elements around a well-developed center of low
pressure. The curved broken lines represent the _isotherms_ (Chapter XIV).
The solid concentric oval lines are the _isobars_ (Chapter XI). The arrows
represent the _winds_, the lengths of the arrows being roughly
proportionate to the wind velocities (Chapter XII). The whole shaded area
represents the region over which the sky is covered by heavy lower clouds.
The smaller shaded area, within the larger, encloses the district over
which rain or snow is falling (Chapter XVI). The lines running out in
front of the cloudy area represent the light upper clouds (_cirrus_ and
_cirro-stratus_) which usually precede an area of low pressure.

Imagine this whole disturbance moving across the United States in a
northeasterly direction, and imagine yourself at a station (1) directly in
the path of the cyclone; (2) south of the track; and (3) north of the
track. In the first case, as the disturbance moved on in its path, you
would successively occupy the positions marked _A_, _B_, and _C_ on the
line _AC_, passing through the center of the cyclone. In the second case
you would be first at _D_, then at _E_, and then at _F_. In the third case
you would be at _G_, _H_, and _J_ in succession. What changes of weather
would you experience in each of these positions as the cyclone passed by
you? Imagine yourself at some station halfway between the lines _AC_ and
_DF_. What weather changes would you have in that position with reference
to the storm track? In what respects would these weather changes differ
from those experienced along the line _DF_? Imagine your station halfway
between the lines _AC_ and _GJ_. What weather changes would you have
there? How would these changes differ from those experienced along the
line _GJ_?

It must be remembered that Fig. 53 is an ideal diagram. It represents
conditions which are not to be expected in every cyclone which appears on
our weather maps. If all cyclones were exactly alike in the weather
conditions around them, weather forecasting would be a very easy task. But
cyclones are not all alike——far from it. Some are well developed, with
strong gradients, high winds, extended cloud areas, heavy precipitation,
and decided temperature contrasts. Others are but poorly developed, with
weak gradients, light winds, small temperature differences, and it may be
without any precipitation whatever. Some cover immense districts of
country; others are small and affect only a limited area. It therefore
becomes necessary to examine the characteristics of each approaching
cyclone, as shown on the daily weather map, very carefully. Notice whether
it is accompanied by heavy rain or snow; whether its winds are violent;
how far ahead of the center the cloudy area extends; how far behind the
outer cloud limit the rain area begins; what is the position of the cloud
and rain area with reference to the center, and other points of equal
importance, and govern yourself, in making your forecast, according to the
special features of each individual cyclone. Well-developed cyclones will
be accompanied by marked weather changes. Weak cyclones will have their
weather changes but faintly marked.

The distance of your station from the center of the cyclone is of great
importance in determining what the weather conditions and changes shall
be, as may easily be seen by examining Fig. 53. If the storm passes far to
the north or far to the south of your station, you may notice none of its
accompanying weather conditions, except, perhaps, a bank of clouds on your
horizon. You may for a few hours be under the cloudy sky of some passing
storm, and yet not be reached by its rainy area. The shifts in the wind
may be marked and the wind velocities high, or the expected veering or
backing may hardly be noticeable, owing to the weakness or the distance of
the controlling cyclone.

Again, the rapidity with which weather conditions will change depends upon
the rate of movement of the cyclone itself. The better developed the
cyclone, the higher its velocity of progression, and the nearer its track
lies to the station, the more emphatic and the more rapid are the weather
changes it causes. On the other hand, the weaker the cyclone, the slower
its rate of progression, and the further away its track, the less marked
and the slower the weather changes. The probable track of a coming storm,
and its probable rate of movement, therefore, need careful study if our
forecasts are to be reliable.

There are many other obstacles in the way which combine to render weather
forecasting extremely difficult. Some of these difficulties you will learn
to overcome more or less successfully by the experience you will gain from
a careful and persevering study of the daily weather maps; others, the
best forecast officials of our Weather Bureau have not yet entirely
overcome. The tracks followed by our cyclones vary more or less from month
to month, and even if the average tracks for each month are known,
individual cyclones may occur which absolutely disregard these tracks.
While the average hourly velocity of cyclones is accurately known for the
year and for each month, the movements of individual storms are often very
capricious. They may move with a fairly uniform velocity throughout the
time of their duration; they may suddenly and unexpectedly increase their
rate of movement, or they may as suddenly come nearly to a standstill. The
characteristics of cyclones vary in different portions of the country and
at different times. Cyclones which have been accompanied by little
precipitation on most of their journey are apt to give increased rain or
snowfall as they near the Atlantic Ocean and Gulf of St. Lawrence.
Cyclones which over one portion of the country were rainy, may give little
or no precipitation in another portion. Cyclones and anticyclones are
found to have considerable influence on one another, retarding or
accelerating one another’s advance, or changing one another’s normal path
of progression. While this mutual interaction is clearly seen, and may be
successfully predicted in many cases, many other cases arise in which,
under apparently similar conditions, the result is very different from the
anticipation. Such are some of the difficulties with which weather
forecasters have to contend, and which prevent the attainment of greater
accuracy in weather prediction.



                   PART V.——PROBLEMS IN OBSERVATIONAL
                              METEOROLOGY.



                              CHAPTER XX.

                              TEMPERATURE.


The chief interest and value of the instrumental work in meteorology are
to be found not only in the taking of the daily observations at stated
hours, but in the working out of numerous simple problems, such as may
readily be undertaken with the help of the instruments already described.
Thus, the temperature of the air (obtained by the sling thermometer,
supplemented by maximum and minimum thermometers, and by the thermograph
if available) can be determined under a variety of conditions, _e.g._,
close to the ground, and at different heights above the ground; at
different hours, by day and night; in different seasons; in sunshine and
in shade; during wind and calms; in clear and cloudy weather; in woods and
in the open; over bare ground, grass, snow, or ice; on hills and in
valleys. Observations may also be made of the temperature of the ground
and of a snow cover, at the surface and at slight depths beneath the
surface, in different seasons and under different weather conditions.
Among the problems which may be worked out by means of such observations
as these are the following:——

_A._ =The Diurnal Range of Temperature under Different Conditions and at
Different Heights above the Ground.=——Under the influence of the sun the
regular normal variation of temperature during 24 hours is as follows: A
gradual increase, with the increasing altitude of the sun, from sunrise
until shortly after noon, and a gradual decrease, with decreasing altitude
of the sun, from the maximum, shortly after noon, until the minimum, about
sunrise. This variation is known as the _diurnal variation of
temperature_. Curve _a_ in Fig. 12 illustrates well the normal diurnal
variation of temperature, as recorded by the thermograph during a period
of clear, warm spring weather (April 27-30, 1889, Nashua, N. H.). The
_diurnal range of temperature_ is the difference between the maximum and
minimum of the diurnal oscillation. The regular normal diurnal variation
in temperature is often much interfered with by other controlling causes
than the sun, _e.g._, cyclonic winds, clouds, etc.

I. Study and compare the diurnal ranges of temperature as indicated by the
maximum and minimum thermometers, or the thermograph, in the instrument
shelter, in clear, fair, cloudy, and stormy weather, during winds and in
calms, in different months. Summarize your results by grouping them
according to the general weather conditions, and according to the months
or seasons in which the observations were made. For example, group
together and average the ranges observed on clear, calm days in winter; on
similar days in early summer or autumn; on clear days with brisk northwest
winds in winter; on similar days in early summer or autumn; on calm days
with overcast sky in the different seasons; on stormy days with strong
winds, etc. Study carefully the weather maps for the days on which your
observations are made. Pay special attention to the relation between the
diurnal ranges and the control exercised over these ranges by cyclones and
anticyclones through their winds and general weather conditions.

II. Observations of diurnal ranges of temperature at different heights
above the ground may be made by means of maximum and minimum thermometers
fastened (temporarily) outside of the windows of different stories of the
school or of some other building. These observations should be made out of
windows facing north, and care should be taken to check, so far as
possible, any draft from within the building out through the window during
the taking of the observation. If a fire escape is provided on the
building, the instruments may often be conveniently fastened to that.

Study the ranges under different conditions of wind and weather at various
heights above the ground, and compare these results with those obtained
under I. Notice the relations of all your results to the cyclonic and
anticyclonic areas of the weather maps.

The diurnal range of temperature in the air over the open ocean from the
equator to latitude 40° has been found to average only 2° to 3°. In
southeastern California and the adjacent portion of Arizona the average
diurnal temperature range in summer is 40° or 45°. Over other arid
regions, such as the Sahara, Arabia, and the interior of Australia, the
range also often amounts to 40°. Observations of temperature above the
earth’s surface, in the free air, made on mountains, in balloons, and by
means of instruments elevated by kites, indicate very clearly that the
diurnal range of temperature decreases with increasing elevation above
sea level. The results obtained at Blue Hill Observatory, Massachusetts,
by means of kites, show that the diurnal range of temperature almost
disappears, on the average, at 3300 feet (1000 meters).

_B._ =Changes of Temperature in the Lower Air, and their Control by the
Condition of the Ground, the Movement of the Air, and Other
Factors.=——Determine the changes of the temperature in the lower air by
making frequent readings of the ordinary thermometer in the instrument
shelter, of the sling thermometer, or by an examination of the thermograph
record. Group these changes, as in Problem _A_, so far as possible
according to the weather conditions under which they occurred, and try to
classify the kinds of change roughly into types. Study the control of
these various types by the wind and other weather conditions accompanying
them, as illustrated on the daily weather maps. The control exercised by
different conditions of the earth’s surface may be studied by means of
observations made with the sling thermometer over different surfaces, such
as grass, bare ground, snow, etc.

Examples of temperature changes in the lower air, under different
conditions of weather, recorded on the thermograph, are given in Fig. 12,
and are briefly referred to their causes in the text accompanying that
figure.

_C._ =Vertical Distribution of Temperature in the Atmosphere.=——The
vertical distribution of temperature in the lower air may be studied by
having ordinary thermometers or thermographs exposed at different heights
above the ground, _e.g._, close to the surface; in an instrument shelter;
out of windows on successive stories of some high building; and on the
roof of the building. They may also, in cases where there is a hill in the
neighborhood, be exposed in a valley at the bottom of the hill and at
successive elevations up the side of the hill. It is, however, usually
much simpler, as well as more practicable, to take these temperature
readings by means of the sling thermometer. In the case of observations
made out of the windows of a building, one observer can take the readings
at different elevations in succession. When the observations are made at
different altitudes on the side of a hill, it is best to have the
coöperation of several observers, who shall all read their thermometers at
the same moment of time. The results obtained in the previous problems
(_A_ and _B_) may, of course, also be utilized in studying the vertical
distribution of temperature in the atmosphere.

Study the vertical distribution of temperature in the lower air under
various conditions of weather and season; at various hours of the day, and
with varying conditions of surface cover. Make your observations
systematically, at regular hours, so that the results may be comparable.
Group together observations made under similar conditions of weather,
season, time, and surface cover. Determine the average vertical
distribution of temperature in the different cases. Note especially any
seeming peculiarities or irregularities in this distribution at certain
times. Study carefully, as in the previous problems, the relation of the
different types of temperature distribution in the atmosphere to the
weather conditions as shown on the daily weather maps.

Observations made in different parts of the world, on mountains and in
balloons, have shown that on the average the temperature decreases from
the earth’s surface upwards at the rate of about 1° in 300 feet of
ascent. The rate of vertical decrease of temperature is known in
meteorology as the _vertical temperature gradient_. When it happens that
there is for a time an _increase in temperature upwards_ from the earth’s
surface, the condition is known as an _inversion of temperature_.

As a result of the decrease of temperature with increasing altitude above
sea level, the tops of many high mountains even in the Torrid Zone are
always covered with snow, while no snow can ever fall at their bases,
owing to the high temperatures which prevail there. Balloons sent up
without aëronauts, but with self-recording instruments, have given us
temperatures of -90° at a height of 10 miles above the earth’s surface. On
Dec. 4, 1896, Berson reached a height of 30,000 feet and noted a
temperature of -52°. Inversions of temperature are quite common,
especially during the clear cold spells of winter. Under such conditions
the tops and sides of hills and mountains are often much warmer than the
valley bottoms at their bases. A good example of an inversion of
temperature occurred in New Hampshire on Dec. 27, 1884. The pressure was
above the normal, the sky clear and the wind light. The observer on the
summit of Mt. Washington reported a temperature of +16° on the morning of
that day, while the thermometers on the neighboring lowlands gave readings
of from -10° to -24°. In Switzerland, the villages and cottages are
generally built on the mountain sides and not down in the valley bottoms,
experience having taught the natives that the greatest cold is found at
the lower levels.



                              CHAPTER XXI.

                                 WINDS.


The determination of the direction of the wind (by means of the wind vane)
and of its velocity (by means of the anemometer, or by estimating its
strength) at different hours, under different conditions of weather and in
different seasons, leads to a number of problems. The following simple
investigations may readily be undertaken in schools:——

_A._ =The Diurnal Variation in Wind Velocity in Fair Weather.=——Observe
and record the velocity of the wind (either estimated or registered by the
anemometer) every hour, or as often as possible, on clear or fair days in
different months. Can you discover any regular change in the velocities
during the day? If so, what is the change? Does the season seem to have
any control over the results obtained? Examine the daily weather maps in
connection with your observations and determine the effect that different
weather conditions have upon the diurnal variation in wind velocity.

The diurnal variation in wind velocity over the open ocean is so slight
as hardly to be noticeable. Over the land, the daytime winds are commonly
strongest in arid regions. Traveling across the desert often becomes
extremely disagreeable, owing to the clouds of dust which these winds
sweep up from the surface.

_B._ =The Variations in Direction and Velocity due to Cyclones and
Anticyclones.=——Record the direction and velocity of the wind at your
station at frequent intervals during the passage of a considerable number
of cyclones and anticyclones. Enter your observations in some form of
table so that they may be readily examined. (See p. 113.) Note the
character of the changes that occur, classifying them into types, so far
as possible. Study the control of wind directions and velocities by the
special features of the individual cyclones and anticyclones as shown on
the daily weather maps. How are the different types of change in
direction and velocity affected by the tracks of cyclones and
anticyclones? By their velocity of progression? By the arrangement of
isobars around them? By the height of the barometer at the center? By the
season in which the cyclones and anticyclones occur?

Frequent changes in the direction and velocity of our winds are one great
characteristic of the Temperate Zones, especially in winter. The
continuous procession of cyclones and anticyclones across the United
States involves continuous shifts of wind. Over much of the earth’s
surface, however, the regularity and constancy of the winds are the
distinguishing feature of the climate. Over a considerable part of the
belts blown over by the northeast and southeast trades, roughly between
latitude 30° N. and S. and the equator, the winds keep very nearly the
same direction and the same velocity day after day and month after month.
Thus the trades are of great benefit to commerce. Sailing ships may
travel for days in the trade wind belts without having their sails
shifted at all, with a fair wind all the time carrying them rapidly on to
their destination.

_C._ =The Occurrence and Characteristics of Local Winds, such as Mountain
and Valley and Land and Sea Breezes.=——If the observer happens to be
living in or near the mouth of a valley or on a mountain side, opportunity
may be given for the observation of the local winds down the mountain
sides and down the valley at night, and up the valley and the mountain
sides by day, known as mountain and valley breezes. Keep a record of wind
direction and velocity during the day, and especially during the morning
and evening hours. Notice any marked changes in direction, and the
relation of these changes to the time of day. Does the velocity of the
daytime up-cast breeze show any systematic variation during the day? Study
the relation of mountain and valley breezes to the general weather
conditions shown on the weather maps. How are these breezes affected by
season? By the presence of a cyclone over the region? Of an anticyclone?
By the state of the sky?

If near the seacoast (_i.e._, within 10 or 15 miles), an interesting study
may be made of local land and sea breezes. The sea breeze is a wind from
the ocean onshore, while the land breeze blows offshore. These breezes
occur only in the warmer months. Take frequent observations during the
day, as in the case of mountain and valley winds, noting especially any
changes in direction and velocity, and the relation of these changes to
the time of day. Study also the control exercised by the prevailing
weather conditions over the occurrence and the strength of development of
the land and sea breezes.

This problem may be considerably extended by adding temperature
observations to the simpler record of wind direction and velocity.

In some of the Swiss valleys the mountain and valley breezes are such
regular daily weather phenomena that it has become a weather proverb that
a failure of the daily change in wind direction indicates a change of
weather. Special names are often given to these breezes where they are
well marked. In a part of the Tyrol sailing boats go up the lakes by day
with the valley breeze, and sail back at night with the mountain breeze.
It is therefore unnecessary for the boats to be rowed either way. Land
and sea breezes, although an unimportant climatic feature in these
northern latitudes, are often of the highest importance in the Torrid
Zone. The fresh pure sea breeze from over the ocean makes it possible for
Europeans to live in many tropical climates where otherwise they would
not keep their health. The land breeze, on the other hand, is apt to be
an unhealthy wind in the tropics, especially when it blows off of swampy
land.



                             CHAPTER XXII.

                       HUMIDITY, DEW, AND FROST.


The humidity of the air, as determined by the wet and dry-bulb
thermometers or the sling psychrometer, and the occurrence or absence of
dew or frost, should be studied together. Observations should be made at
different hours, in different kinds of weather, and in different seasons.
From such observations the following problems may be solved:——

_A._ =Diurnal Variation of Relative Humidity under Different
Conditions.=——Readings of the wet and dry-bulb thermometers in the
instrument shelter, or of the sling psychrometer, several times during the
day, will furnish data for determining the diurnal variation of relative
humidity. Classify your observations according to the weather conditions
under which they were made, and by months or seasons. Summarize the
results of your investigation, paying special attention to the relation
between the diurnal variation of relative humidity and the temperature.

The variations of relative humidity are generally the reverse of those of
absolute humidity. In the case of the latter the average diurnal
variations are small. The fluctuations in the relative humidity during
the day on the northwestern coast of Europe amount to about 7% in
December and 17% in August, while in central Asia they average about 25%
in winter and 50% in summer.

_B._ =Relation of Relative Humidity to the Direction of the
Wind.=——Observations by means of the wet and dry-bulb thermometers in the
shelter, or by means of the sling psychrometer, supplemented by records of
wind direction, will furnish data for the solution of this problem.
Tabulate your observations according to wind directions and seasons.
Determine the characteristics of the different winds as to their relative
humidities. Consider the control of these winds and humidity conditions by
cyclones and anticyclones.

The warm wave, or sirocco, in front of our winter cyclones in the eastern
United States is a damp, disagreeable, irritating wind. In summer, the
sirocco is usually dry, and during the prevalence of such winds we have
our hottest spells, when sunstrokes are not uncommon. In southern Italy
the sirocco has the same position with reference to the controlling
cyclone. There the wind is often so dry as seriously to injure
vegetation. The cold wave, on the rear of our winter cyclones, with its
low temperature and dry air, often comes as a refreshing change after the
enervating warmth of the preceding sirocco. Our feelings of bodily
comfort or discomfort are thus in a large measure dependent upon the
humidity and the movement of the air.

_C._ =The Formation of Dew.=——The formation of dew is to be studied from
the following points of view, viz., as dependent upon: _a_, the
temperature and the humidity of the air; _b_, the exposure and condition
of the ground; _c_, the state of the sky; and _d_, the movement of the
air. The occurrence of dew on any night, as well as the amount, whether
large or small, can readily be ascertained by inspection. Observe the
conditions of temperature, humidity, cloudiness, and wind direction and
velocity, as in previous exercises. Pay special attention to the state of
the sky, the wind movement, and the vertical distribution of temperature
near the ground. Under heading _b_ (exposure and condition of the ground)
make observations of the amounts of dew formed on hilltops, hillsides, and
in valleys; on different kinds of surface covering, as grass, leaves,
pavements, etc., and over different kinds of soil. Classify the results in
accordance with the conditions under which the observations were made.
Compare the results and draw your conclusions from this study. Practise
making predictions of the formation of dew in different places and under
different weather conditions.

Over the greater portion of the earth’s surface the amount of dew which
is deposited is very small. It has been estimated that in Great Britain
the total annual amount would measure only an inch and a half in depth;
and in central Europe the depth is given as hardly one inch. In some
parts of the Torrid Zone, on the other hand, dew is deposited in much
larger quantities. According to Humboldt, the traveler through some of
the South American forests often finds what seems to be a heavy shower
falling under the trees, while the sky is perfectly clear overhead. In
this case dew is formed on the tops of the tree in sufficiently large
quantities to give a shower underneath. It is reported that on the Guinea
coast of Africa the dew sometimes runs off the roofs of the huts like
rain. In many dry regions the dew is an important agency in keeping the
plants alive.

_D._ =The Formation of Frost.=——The formation of frost is to be studied in
the same way as that suggested in the case of dew, _i.e._, as dependent
upon: _a_, the temperature and the humidity of the air; _b_, the exposure
and condition of the ground; _c_, the state of the sky; and _d_, the
movement of the air. Frosts are usually classified as _light_ or _heavy_.
The words _killing frost_ are also used. Study the weather and surface
conditions which are most favorable to the formation of frost. Pay special
attention to the relation of frost and inversions of temperature; to the
frequency of frost on open or sheltered surfaces; on hills or in valleys,
and on the lower and upper branches of trees and shrubs. Determine, as
well as you can, the weather conditions which precede light or heavy
frosts, and make predictions of coming frosts, when the conditions warrant
them.

Our Weather Bureau gives much attention to the prediction of frosts and
to the prompt and widespread distribution of frost warnings. Growing
crops and fruits are often seriously injured by frosts, and farmers are
naturally anxious to have as early warning as possible of their
occurrence. Various methods of protecting crops and trees against frost
are used. The method most commonly employed consists in the building of
fires of brush or other inflammable material on the windward side of the
field or the orchard when a frost is expected. The smoke from the fire is
blown to leeward across the field, and acts as an artificial cloud,
affording protection to the vegetation underneath. Such fires are known
as _smudges_.



                             CHAPTER XXIII.

                     CLOUDS AND UPPER AIR CURRENTS.


Attentive observation of clouds will soon lead to a familiarity with their
common type forms. A series of cloud views,[7] with accompanying
descriptive accounts, will teach the names of the clouds and give
definiteness to the record. The directions of movement of clouds are
determined by means of the nephoscope. Cloud observations should be made
at different hours, in different weather conditions, and in different
seasons. The following problems are concerned with clouds and upper air
currents:——

[Footnote 7: See _Hydrographic Office Cloud Types_, Appendix B.]

_A._ =The Typical Cloud Forms and their Changes.=——Note carefully the
characteristic forms assumed by clouds; their mode of occurrence, whether
in single clots, or in groups, in lines, or all over the sky; their
changes in form and in mode of occurrence. Classify and summarize your
results. Compare the clouds of the warm months with those of the cold
months.

Observations have shown that clouds have certain definite characteristic
forms which are substantially the same in all parts of the world. This
fact makes it possible to give names to the different typical forms, and
these names are used by observers the world over. Hence cloud
observations, wherever made, are comparable. The first classification of
clouds was proposed by Luke Howard, in 1803. The classification at
present in use is known as the _International Classification_, and was
adopted by the International Meteorological Congress in 1896.

_B._ =The Prevailing Direction of Cloud Movements.=——The use of the
nephoscope is necessary in the accurate determination of cloud movements.
Study the prevailing directions of movement of the clouds, by means of
frequent observations with the nephoscope, in different weather
conditions. Separate the upper and lower clouds in this study. Summarize
your results according to the weather conditions and the kinds of clouds.

_C._ =Correlation of Cloud Form and Movement with Surface Winds, with
Cyclones and Anticyclones, and with Weather Changes.=——The results
obtained in the working out of the two preceding problems may be used in
the present problem. Tabulate your observations of cloud forms with
reference to the wind directions which prevailed at the time of making the
observations. Do the same with the directions of cloud movement. Determine
the relation between surface winds and cloud types, and between surface
winds and the direction of the upper air currents, as shown by the
movements of the upper clouds. Study the control exercised by cyclones and
anticyclones over cloud forms and over the direction of the upper air
currents.

_D._ =The Use of Clouds as Weather Prognostics.=——Attentive observation of
the forms and changes of clouds, and of the accompanying and following
weather changes, will lead to the association of certain clouds with
certain coming weather conditions. Make your cloud observations carefully,
taking full notes at the time of observation. Give special attention to
the weather conditions that follow. Continue this investigation through as
long a period as possible, until you have gathered a considerable body of
fact to serve as a basis, and then frame a set of simple rules for
forecasting fair or stormy weather on the basis of the forms and changes
of the clouds. Such local observations as these may be employed as a help
in making forecasts from the daily weather maps.

Clouds were used as weather prognostics long before meteorological
observations and weather maps were thought of. To-day sailors and farmers
still look to the clouds to give them warning of approaching storms. Many
of our common weather proverbs are based on the use of clouds as weather
prognostics.



                             CHAPTER XXIV.

                             PRECIPITATION.


The special study of various problems connected with precipitation
involves detailed observations of the amount and rate of precipitation of
various kinds, measured by the rain gauge during storms in different
seasons. These observations of precipitation should, of course, be
supplemented by the usual record of the other weather elements. The
following problems are suggested:——

A. _The relation of precipitation in general to the other weather
elements, and to cyclones and anticyclones._

B. _The conditions under which special forms of precipitation (rain, snow,
sleet, hail, frozen rain) occur._

C. _The conditions associated with light and heavy, brief and prolonged,
local and general rainfall._

These problems are studied by means of a careful comparison of full
weather records with the daily weather maps during a considerable period
of time.

Rain is the most common form of precipitation the world over, although
snow falls over large portions of both hemispheres. In the Arctic and
Antarctic zones almost all the precipitation, which is small in amount,
comes in the form of snow. In southern Europe snow falls at sea level
during the winter as far south as 36° north latitude on the average. In
eastern Asia snow occasionally falls as far south as 23° north latitude.
The mean annual rainfall varies greatly in different parts of the world.
In desert regions it is practically nothing. At Cherrapunjee, in India,
it reaches 493 inches, or over 40 feet. A fall of 40.8 inches in a single
day occurred at this station on June 14, 1876. In the United States,
Upper Mattole, Cal., had an extraordinary monthly rainfall of 41.63
inches in January, 1888. An excessive daily rainfall of 8 inches occurred
at Syracuse, N. Y., on June 8, 1876. At Washington, D. C., 2.34 inches
fell in 37 minutes on June 27, 1881. A sudden and very heavy fall of rain
occurred at Palmetto, Nevada, in August, 1890. A rain gauge which was not
exposed to the full intensity of the storm caught 8.80 inches of water in
one hour. In August, 1891, an observer at Campo, Cal., measured 11.5
inches as the rainfall in one hour from one very heavy downpour, and from
a portion of a second storm.



                              CHAPTER XXV.

                               PRESSURE.


The variations of atmospheric pressure, although insensible to
non-instrumental observation, are so intimately connected with atmospheric
processes that they deserve careful attention. Their observation leads to
several problems.

_A._ =The Decrease of Pressure with Height, as between Valley and Hill, or
between the Base and Top of a Building.=——Make these observations with the
mercurial barometer, if possible. Note the air temperatures at the two
levels at which the barometer readings are made. Determine the heights of
hill or building by means of the following rule: Multiply by 9 the
difference in barometrical readings at the two stations, given in
hundredths of an inch, and the result will be approximately the difference
in height between the stations in feet. A more accurate result may be
reached by means of the following rule: The difference of level in feet is
equal to the difference of the pressures in inches divided by their sum,
and multiplied by the number 55,761, when the mean of the air temperatures
of the two places is 60°. If the mean temperature is above 60°, the
multiplier must be increased by 117 for every degree by which the mean
exceeds 60°; if less than 60°, the multiplier must be decreased in the
same way. For example, if the lower station has a pressure of 30.00
inches and a temperature of 62°, and the upper station has 29.00 inches
and 58° respectively, the difference of level between the two will be

         (30.00 - 29.00) / (30.00 + 29.00) × 55,761 = 945 feet.

If the lower values are 30.15 inches and 65°, while the upper values are
28.67 inches and 59°, then the formula becomes

 (30.15 - 28.67) / (30.15 + 28.67) × [55,761 + (2 × 117)] = 1409 feet.

The determination of heights by means of the barometer depends upon the
fact that the rate of decrease of pressure upwards is known. As the
weight of a column of air of a given height varies with the temperature
of the air, it is necessary, in accurate work of this sort, to know the
air temperatures at both the lower and upper stations at the time of
observation. From these temperatures the mean temperature of the air
column between the two stations may be determined. Tables have been
published which facilitate the reductions in this work. The heights of
mountains are usually determined, in the first instance, by means of
barometric observations, carried out by scientific expeditions or by
travelers that have been able to reach their summits. More accurate
measurements are later made, when possible, by means of trigonometrical
methods.

_B._ =The Diurnal and Cyclonic Variation of Pressure in Different
Seasons.=——This problem is satisfactorily solved only by a study of the
curves traced by the barograph, or by plotting, as a curve, hourly or
half-hourly readings of the mercurial barometer. The _diurnal variation of
the barometer_ is the name given to a slight double oscillation of
pressure, with two maxima and two minima occurring during the 24 hours.
This oscillation is in some way, not yet understood, connected with the
diurnal variation in temperature. It is most marked in the tropics and
diminishes towards the poles. Fig. 15 illustrates, in the May curve, the
diurnal variation of the barometer at Cambridge, Mass., during a spell of
fair spring weather, May 18-22, 1887. The maxima are marked by + and the
minima by 0. The _cyclonic variation of pressure_ is the name given to
those irregular changes in pressure which are caused by the passage of
cyclones and anticyclones. The second curve in Fig. 15 shows the cyclonic
variations in pressure recorded by the barograph at Cambridge, Mass.,
during a spell of stormy weather, Feb. 23-28, 1887. These curves serve as
good illustrations of these two kinds of pressure variations.

Study your barograph tracings, or your barometer readings, as illustrating
diurnal or cyclonic variations of pressure. Note the character and the
amount of the diurnal and cyclonic variations, and their dependence on
seasons.

Over the greater part of the Torrid Zone the diurnal variation of the
barometer is remarkably distinct and regular. Humboldt first called
attention to the fact that in those latitudes the time of day may be told
within about 15 minutes if the height of the barometer is known.

_C._ =The Relation of Local Pressure Changes to Cyclones and Anticyclones,
and thus to Weather Changes.=——Make a detailed study of the relation of
the local pressure changes at your station, as shown by the barograph
curves, or by frequent readings of the mercurial or aneroid barometers, to
the passage of cyclones and anticyclones, and to their accompanying
weather changes. Classify the simple types of pressure change, so far as
possible, together with the general weather conditions that usually
accompany these types. Apply the knowledge of local weather changes thus
gained when you make your forecast on the basis of the daily weather
maps.



                             CHAPTER XXVI.

                         METEOROLOGICAL TABLES.


The tables which follow are those which are now in use by the United
States Weather Bureau. They were first published in the _Instructions for
Voluntary Observers_ issued in 1892, and were reprinted in 1897. The
following instructions will be found of service in the use of the
tables:——

                          TABLE I.——DEW-POINT.

The figures in heavy type, arranged in vertical columns at each side of
the page, are the _air temperatures_ in degrees Fahrenheit, as recorded by
the dry-bulb thermometer. The figures in heavy type, running across the
page, denote the _differences_, in degrees and tenths of degrees, _between
the dry-bulb and wet-bulb readings_, or, technically, the _depression of
the wet-bulb thermometer_. The figures in the vertical columns denote the
_dew-points_. Make your observation of the wet and dry-bulb thermometers
and note the difference between the two readings. Find, in the vertical
columns of heavy type, the temperature corresponding to your dry-bulb
reading, or the nearest temperature to that. Then look along the
horizontal lines of figures in heavy type for the figure which corresponds
exactly, or most nearly, with the difference between your wet and dry-bulb
readings. Look down the vertical column under this latter figure until you
reach the horizontal line corresponding to your dry-bulb reading. At this
point the figures in the vertical column give the dew-point of the air at
the time of your observation.

_Example_: Air Temperature (dry bulb), 47°; Wet Bulb, 44°; Difference, 3°.
On page 148 will be found the table containing both 47° (dry bulb) and 3°
(depression of the dew-point). In the twenty-eighth line of this table and
in the seventh column will be found the dew-point, viz., 41°.

_Example_: Air Temperature, 61.5°; Wet Bulb, 55.5°; Difference, 6°.

In this case 61.5° is not found in the vertical columns of dry-bulb
readings, but 61° and 62° are found. The dew-point, with a difference
between wet and dry-bulb readings of 6°, for an air temperature of 61°, is
50°; for an air temperature of 62°, it is 52°. Evidently, then, for an air
temperature of 61.5° the dew-point will be 51°, _i.e._, halfway between
50° and 52°. This method of determining dew-points at air temperatures or
with depressions of the wet-bulb thermometer which are not given exactly
in the tables, is known as _interpolation_.

_Example_: Air Temperature, 93°; Wet Bulb, 90.5°; Difference, 2.5°. Our
table gives no dew-points for wet-bulb depressions of 2.5°, with air
temperature 93°, but we find (on page 152) that for air temperature 93°
and depression of wet bulb of 2°, the dew-point is 91°, while for a
wet-bulb depression of 3°, the dew-point is 89°. By the method of
interpolation we can readily determine the dew-point in the special case
under consideration as 90°, _i.e._, halfway between 89° and 91°.

                     TABLE II.——RELATIVE HUMIDITY.

The general plan of this table is the same as that of Table I. The figures
in the vertical columns are the relative humidities (in percentages)
corresponding to the different readings of the wet and dry-bulb
thermometers.

               TABLE III.——REDUCTION OF BAROMETER TO 32°.

The figures in heavy type, arranged in vertical columns at the left of the
page, refer to the temperature in degrees Fahrenheit, as indicated by the
attached thermometer. The figures in heavy type, running across the top
of the page, are the barometer readings in inches and tenths. Make a
reading of the attached thermometer and of the barometer. Find in the
vertical column the temperature corresponding to the reading of the
attached thermometer, and in the horizontal line of heavy figures the
reading corresponding to the height of the barometer. The decimal in the
vertical column, under the appropriate barometer reading, and in the same
horizontal line with the appropriate thermometer reading, is to be
subtracted from the height of the barometer as observed, thus correcting
the reading to freezing. When the attached thermometer reads below 28°,
the correction is additive.

_Example_: Attached Thermometer, 69°; Barometer, 30.00 inches; Correction,
-.110; Corrected reading, 29.890 inches.

_Example_: Attached Thermometer, 73°; Barometer, 29.75 inches; Correction
= ?

We do not find any column corresponding to a barometer reading of 29.75
inches. We do find, however, that with a barometer reading of 29.50, and
an attached thermometer reading of 73, the correction is -.118 inch, and
with a barometer reading of 30.00, the correction is -.120. By
interpolating, as in the case of the humidity table above, we find the
correction for a barometer reading of 29.75 inches, and an attached
thermometer reading of 73°. The correction is -.119, and the corrected
reading is 29.75 - .119 = 29.63 inches.

            TABLE IV.——REDUCTION OF BAROMETER TO SEA LEVEL.

The figures in heavy type, in the left-hand vertical columns, are the
heights, in feet, of the barometer above sea level. The figures in heavy
type at the top of the columns, running across the page, are the readings
of the ordinary thermometer. The numbers of inches and hundredths of
inches to be subtracted from the barometer reading (corrected for
temperature by Table III), for the different heights above sea level, are
given in the vertical columns.

The altitude above sea level of the city or town at which the observation
is made should be ascertained as accurately as possible from some
recognized authority, as, _e.g._, from a railroad survey; from Government
measurements, or from some engineer’s office. The correction to be made is
determined by a simple inspection of the table or by the method of
interpolation.

_Example_: Altitude of Barometer above sea level, 840 feet; Temperature of
the air, 40°; Correction, +.931 inch.

_Example_: Altitude of Barometer above sea level, 205 feet; Temperature of
the air, 45°; Correction = ?

Here 205 feet and 45° are neither of them found in the table. Hence a
double interpolation is necessary. For 200 feet and 40° the correction is
+.224 inch. For 200 feet and 50° the correction is +.220 inch. Hence for
200 feet and 45° the correction is +.222 inch. For 210 feet and 40° the
correction is +.235 inch. For 210 feet and 50° the correction is +.231
inch. Hence for 210 feet and 45° the correction is +.233 inch. Now for 205
feet we should have a correction midway between +.235 inch and +.233 inch
or +.234 inch.

     TABLE I.——TEMPERATURE OF THE DEW-POINT, IN DEGREES FAHRENHEIT.

  ======+================================================================+======
   _t_  |    Difference between the dry and wet thermometers (_t-t′_)    | _t_
  (Dry  +----+----+----+----+----+----+----+----+----+----+----+----+----+(Dry
  ther.)|0°.2|0°.4|0°.6|0°.8|1°.0|1°.2|1°.4|1°.6|1°.8|2°.0|2°.2|2°.4|2°.6|ther.)
  ------+----+----+----+----+----+----+----+----+----+----+----+----+----+------
   -40  | -52|    |    |    |    |    |    |    |    |    |    |    |    | -40
   -39  | -50|    |    |    |    |    |    |    |    |    |    |    |    | -39
   -38  | -49|    |    |    |    |    |    |    |    |    |    |    |    | -38
   -37  | -48|    |    |    |    |    |    |    |    |    |    |    |    | -37
   -36  | -46|    |    |    |    |    |    |    |    |    |    |    |    | -36
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -35  | -44|    |    |    |    |    |    |    |    |    |    |    |    | -35
   -34  | -43| -58|    |    |    |    |    |    |    |    |    |    |    | -34
   -33  | -42| -55|    |    |    |    |    |    |    |    |    |    |    | -33
   -32  | -40| -52|    |    |    |    |    |    |    |    |    |    |    | -32
   -31  | -38| -49|    |    |    |    |    |    |    |    |    |    |    | -31
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -30  | -36| -47|    |    |    |    |    |    |    |    |    |    |    | -30
   -29  | -35| -44|    |    |    |    |    |    |    |    |    |    |    | -29
   -28  | -33| -42| -56|    |    |    |    |    |    |    |    |    |    | -28
   -27  | -32| -40| -52|    |    |    |    |    |    |    |    |    |    | -27
   -26  | -30| -37| -48|    |    |    |    |    |    |    |    |    |    | -26
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -25  | -29| -35| -45|    |    |    |    |    |    |    |    |    |    | -25
   -24  | -28| -34| -43| -58|    |    |    |    |    |    |    |    |    | -24
   -23  | -27| -32| -40| -53|    |    |    |    |    |    |    |    |    | -23
   -22  | -26| -30| -37| -49|    |    |    |    |    |    |    |    |    | -22
   -21  | -25| -29| -35| -45|    |    |    |    |    |    |    |    |    | -21
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -20  | -23| -28| -33| -41| -55|    |    |    |    |    |    |    |    | -20
   -19  | -22| -26| -31| -38| -50|    |    |    |    |    |    |    |    | -19
   -18  | -21| -25| -29| -35| -45|    |    |    |    |    |    |    |    | -18
   -17  | -20| -23| -27| -32| -41| -55|    |    |    |    |    |    |    | -17
   -16  | -19| -22| -26| -30| -37| -49|    |    |    |    |    |    |    | -16
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -15  | -17| -20| -24| -28| -34| -44|    |    |    |    |    |    |    | -15
   -14  | -16| -19| -22| -26| -31| -39| -52|    |    |    |    |    |    | -14
   -13  | -15| -18| -21| -25| -29| -35| -46|    |    |    |    |    |    | -13
   -12  | -14| -17| -20| -23| -27| -32| -41| -55|    |    |    |    |    | -12
   -11  | -13| -16| -18| -21| -25| -30| -36| -48|    |    |    |    |    | -11
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -10  | -12| -14| -17| -20| -23| -27| -33| -42| -58|    |    |    |    | -10
    -9  | -11| -13| -15| -18| -21| -25| -30| -37| -48|    |    |    |    |  -9
    -8  | -10| -12| -14| -17| -20| -23| -27| -33| -42| -58|    |    |    |  -8
    -7  |  -9| -11| -13| -15| -18| -21| -25| -30| -36| -48|    |    |    |  -7
    -6  |  -8| -10| -12| -14| -16| -19| -23| -27| -32| -41| -56|    |    |  -6
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
    -5  |  -7|  -8| -10| -12| -15| -17| -21| -24| -29| -35| -47|    |    |  -5
    -4  |  -6|  -7|  -9| -11| -13| -16| -19| -22| -26| -31| -39| -54|    |  -4
    -3  |  -4|  -6|  -8| -10| -12| -14| -17| -20| -23| -28| -33| -44|    |  -3
    -2  |  -3|  -5|  -6|  -8| -10| -12| -15| -18| -21| -24| -29| -36| -48|  -2
    -1  |  -2|  -4|  -5|  -7|  -9| -11| -13| -16| -18| -22| -26| -31| -39|  -1
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
    ±0  |  -1|  -3|  -4|  -6|  -7|  -9| -11| -14| -16| -19| -23| -27| -33|   0
    +1  |  -0|  -2|  -3|  -4|  -6|  -8| -10| -12| -14| -17| -20| -24| -28|  +1
     2  |  +1|  -1|  -2|  -3|  -5|  -6|  -8| -10| -12| -15| -17| -21| -25|   2
     3  |   2|  +1|  -1|  -2|  -3|  -5|  -7|  -8| -10| -13| -15| -18| -21|   3
     4  |   3|  +2|   0|  -1|  -2|  -4|  -5|  -7|  -9| -11| -13| -16| -19|   4
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
     5  |   4|   3|  +1|   0|  -1|  -2|  -4|  -5|  -7|  -9| -11| -14| -16|   5
     6  |   5|   4|   3|  +1|   0|  -1|  -3|  -4|  -6|  -7|  -9| -12| -14|   6
     7  |   6|   5|   4|   3|  +1|   0|  -1|  -3|  -4|  -6|  -8| -10| -12|   7
     8  |   7|   6|   5|   4|   3|  +1|   0|  -1|  -3|  -4|  -6|  -8| -10|   8
     9  |   8|   7|   6|   5|   4|   3|  +1|   0|  -1|  -3|  -4|  -6|  -8|   9
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
    10  |   9|   8|   7|   6|   5|   4|   3|  +1|   0|  -1|  -3|  -4|  -6|  10
    11  |  10|   9|   8|   7|   6|   5|   4|   3|  +2|   0|  -1|  -3|  -4|  11
    12  |  11|  10|   9|   9|   8|   7|   5|   4|   3|  +2|   0|  -1|  -2|  12
    13  |  12|  11|  11|  10|   9|   8|   7|   6|   5|   3|  +2|  +1|  -1|  13
    14  |  13|  12|  12|  11|  10|   9|   8|   7|   6|   5|   4|   2|  +1|  14
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
    15  |  14|  13|  13|  12|  11|  10|   9|   8|   7|   6|   5|   4|   3|  15
    16  |  15|  15|  14|  13|  12|  11|  10|  10|   9|   8|   7|   5|   4|  16
    17  |  16|  16|  15|  14|  13|  12|  12|  11|  10|   9|   8|   7|   6|  17
    18  |  17|  17|  16|  15|  14|  14|  13|  12|  11|  10|   9|   8|   7|  18
    19  |  18|  18|  17|  16|  15|  15|  14|  13|  12|  11|  10|  10|   9|  19
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
    20  |  19|  19|  18|  17|  17|  16|  15|  14|  13|  13|  12|  11|  10|  20
  ------+----+----+----+----+----+----+----+----+----+----+----+----+----+ ——————
   _t._ |0°.2|0°.4|0°.6|0°.8|1°.0|1°.2|1°.4|1°.6|1°.8|2°.0|2°.2|2°.4|2°.6| _t._
  ======+====+====+====+====+====+====+====+====+====+====+====+====+====+======

     TABLE I.——TEMPERATURE OF THE DEW-POINT, IN DEGREES FAHRENHEIT.

  ======+===========================================================+======
   _t_  | Difference between the dry and wet thermometers (_t-t′_). | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)| 2°.6| 2°.8| 3°.0| 3°.2| 3°.4| 3°.6| 3°.8| 4°.0| 4°.2| 4°.4|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   -2   | -48 |     |     |     |     |     |     |     |     |     |  -2
   -1   | -39 | -54 |     |     |     |     |     |     |     |     |  -1
        |     |     |     |     |     |     |     |     |     |     |
    0   | -33 | -43 |     |     |     |     |     |     |     |     |   0
   +1   | -28 | -35 | -46 |     |     |     |     |     |     |     |  +1
    2   | -25 | -30 | -37 | -50 |     |     |     |     |     |     |   2
    3   | -21 | -26 | -31 | -39 | -54 |     |     |     |     |     |   3
    4   | -19 | -22 | -27 | -32 | -42 | -60 |     |     |     |     |   4
        |     |     |     |     |     |     |     |     |     |     |
    5   | -16 | -19 | -23 | -28 | -34 | -45 |     |     |     |     |   5
    6   | -14 | -17 | -20 | -24 | -29 | -35 | -47 |     |     |     |   6
    7   | -12 | -14 | -17 | -20 | -24 | -29 | -37 | -50 |     |     |   7
    8   | -10 | -12 | -15 | -17 | -21 | -25 | -30 | -38 | -53 |     |   8
    9   |  -8 | -10 | -12 | -15 | -18 | -21 | -25 | -31 | -39 | -55 |   9
        |     |     |     |     |     |     |     |     |     |     |
   10   |  -6 |  -8 | -10 | -12 | -15 | -18 | -21 | -26 | -31 | -40 |  10
   11   |  -4 |  -6 |  -8 | -10 | -12 | -15 | -18 | -21 | -26 | -31 |  11
   12   |  -2 |  -4 |  -6 |  -8 | -10 | -12 | -15 | -18 | -21 | -26 |  12
   13   |  -1 |  -2 |  -4 |  -5 |  -7 |  -9 | -12 | -14 | -17 | -21 |  13
   14   |  +1 |   0 |  -2 |  -3 |  -5 |  -7 |  -9 | -11 | -14 | -17 |  14
        |     |     |     |     |     |     |     |     |     |     |
   15   |   3 |  +1 |   0 |  -2 |  -3 |  -5 |  -7 |  -9 | -11 | -14 |  15
   16   |   4 |   3 |  +2 |   0 |  -1 |  -3 |  -4 |  -6 |  -8 | -10 |  16
   17   |   6 |   5 |   3 |  +2 |  +1 |  -1 |  -2 |  -4 |  -6 |  -8 |  17
   18   |   7 |   6 |   5 |   4 |   2 |  +1 |   0 |  -2 |  -3 |  -5 |  18
   19   |   9 |   8 |   7 |   5 |   4 |   3 |  +1 |   0 |  -1 |  -3 |  19
        |     |     |     |     |     |     |     |     |     |     |
   20   |  10 |   9 |   8 |   7 |   6 |   5 |   3 |  +2 |  +1 |  -1 |  20
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ | 2°.6| 2°.8| 3°.0| 3°.2| 3°.4| 3°.6| 3°.8| 4°.0| 4°.2| 4°.4| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======
   _t._ | 4°.6| 4°.8| 5°.0| 5°.2| 5°.4| 5°.6| 5°.8| 6°.0| 6°.2| 6°.4| _t._
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    8   |     |     |     |     |     |     |     |     |     |     |   8
    9   |     |     |     |     |     |     |     |     |     |     |   9
        |     |     |     |     |     |     |     |     |     |     |
   10   | -57 |     |     |     |     |     |     |     |     |     |  10
   11   | -41 | -60 |     |     |     |     |     |     |     |     |  11
   12   | -31 | -41 | -59 |     |     |     |     |     |     |     |  12
   13   | -25 | -31 | -40 | -58 |     |     |     |     |     |     |  13
   14   | -20 | -25 | -30 | -39 | -56 |     |     |     |     |     |  14
        |     |     |     |     |     |     |     |     |     |     |
   15   | -16 | -20 | -24 | -30 | -38 | -53 |     |     |     |     |  15
   16   | -13 | -16 | -19 | -23 | -28 | -36 | -50 |     |     |     |  16
   17   | -10 | -12 | -15 | -18 | -22 | -27 | -34 | -47 |     |     |  17
   18   |  -7 |  -9 | -12 | -14 | -17 | -21 | -26 | -32 | -44 |     |  18
   19   |  -5 |  -7 |  -9 | -11 | -14 | -17 | -20 | -25 | -30 | -40 |  19
        |     |     |     |     |     |     |     |     |     |     |
   20   |  -2 |  -4 |  -6 |  -8 | -10 | -13 | -16 | -19 | -23 | -29 |  20
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ | 4°.6| 4°.8| 5°.0| 5°.2| 5°.4| 5°.6| 5°.8| 6°.0| 6°.2| 6°.4| _t_.
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

     TABLE I.——TEMPERATURE OF THE DEW-POINT, IN DEGREES FAHRENHEIT.

  ======+===========================================================+======
   _t_  | Difference between the dry and wet thermometers (_t-t′_). | _t_
  (Dry  +----+----+----+----+----+----+----+----+----+----+----+----+(Dry
  ther.)|0°.5|1°.0|1°.5|2°.0|2°.5|3°.0|3°.5|4°.0|4°.5|5°.0|5°.5|6°.0|ther.)
  ------+----+----+----+----+----+----+----+----+----+----+----+----+------
   20   | 18 | 17 | 15 | 13 | 10 |  8 |  5 |  2 | -2 | -6 |-12 |-19 | 20
   21   | 19 | 18 | 16 | 14 | 12 |  9 |  7 |  4 |  0 | -4 | -8 |-15 | 21
   22   | 20 | 19 | 17 | 15 | 13 | 11 |  8 |  6 | +2 | -1 | -6 |-11 | 22
   23   | 22 | 20 | 18 | 16 | 14 | 12 | 10 |  7 |  4 | +1 | -3 | -8 | 23
   24   | 23 | 21 | 19 | 18 | 16 | 14 | 11 |  9 |  6 |  3 | -1 | -5 | 24
        |    |    |    |    |    |    |    |    |    |    |    |    |
   25   | 24 | 22 | 21 | 19 | 17 | 15 | 13 | 11 |  8 |  5 | +2 | -2 | 25
   26   | 25 | 23 | 22 | 20 | 18 | 16 | 14 | 12 | 10 |  7 |  4 |  0 | 26
   27   | 26 | 24 | 23 | 21 | 20 | 18 | 16 | 14 | 11 |  9 |  6 | +3 | 27
   28   | 27 | 25 | 24 | 22 | 21 | 19 | 17 | 15 | 13 | 11 |  8 |  5 | 28
   29   | 28 | 26 | 25 | 24 | 22 | 20 | 19 | 17 | 14 | 12 | 10 |  7 | 29
        |    |    |    |    |    |    |    |    |    |    |    |    |
   30   | 29 | 27 | 26 | 25 | 23 | 22 | 20 | 18 | 16 | 14 | 11 |  9 | 30
   31   | 30 | 29 | 27 | 26 | 24 | 23 | 21 | 19 | 18 | 15 | 13 | 11 | 31
   32   | 31 | 30 | 28 | 27 | 26 | 24 | 22 | 21 | 19 | 17 | 15 | 13 | 32
   33   | 31 | 31 | 29 | 28 | 26 | 25 | 23 | 22 | 19 | 18 | 16 | 14 | 33
   34   | 32 | 32 | 30 | 29 | 27 | 26 | 24 | 24 | 21 | 20 | 18 | 16 | 34
        |    |    |    |    |    |    |    |    |    |    |    |    |
   35   | 33 | 32 | 31 | 30 | 29 | 28 | 26 | 25 | 23 | 22 | 20 | 18 | 35
   36   | 35 | 34 | 32 | 31 | 30 | 29 | 27 | 26 | 24 | 23 | 21 | 19 | 36
   37   | 36 | 35 | 33 | 32 | 31 | 30 | 28 | 27 | 26 | 24 | 22 | 21 | 37
   38   | 37 | 36 | 34 | 33 | 32 | 31 | 30 | 28 | 27 | 26 | 24 | 22 | 38
   39   | 38 | 37 | 35 | 34 | 33 | 32 | 30 | 29 | 28 | 27 | 25 | 24 | 39
        |    |    |    |    |    |    |    |    |    |    |    |    |
   40   | 39 | 38 | 36 | 35 | 34 | 33 | 31 | 30 | 29 | 28 | 26 | 25 | 40
   41   | 40 | 39 | 37 | 36 | 35 | 34 | 32 | 32 | 30 | 29 | 28 | 26 | 41
   42   | 41 | 40 | 39 | 38 | 36 | 35 | 34 | 33 | 31 | 30 | 29 | 27 | 42
   43   | 42 | 41 | 40 | 39 | 37 | 36 | 35 | 34 | 32 | 31 | 30 | 29 | 43
   44   | 43 | 42 | 41 | 40 | 38 | 37 | 36 | 35 | 33 | 32 | 31 | 30 | 44
        |    |    |    |    |    |    |    |    |    |    |    |    |
   45   | 44 | 43 | 42 | 41 | 40 | 39 | 37 | 36 | 34 | 33 | 32 | 31 | 45
   46   | 45 | 44 | 43 | 42 | 41 | 40 | 38 | 37 | 36 | 35 | 33 | 32 | 46
   47   | 46 | 45 | 44 | 43 | 42 | 41 | 40 | 39 | 37 | 36 | 34 | 33 | 47
   48   | 47 | 46 | 45 | 44 | 43 | 42 | 41 | 40 | 38 | 37 | 36 | 35 | 48
   49   | 48 | 47 | 46 | 45 | 44 | 43 | 42 | 41 | 39 | 38 | 37 | 36 | 49
        |    |    |    |    |    |    |    |    |    |    |    |    |
   50   | 49 | 48 | 47 | 46 | 45 | 44 | 43 | 42 | 41 | 40 | 38 | 37 | 50
   51   | 50 | 49 | 48 | 47 | 46 | 45 | 44 | 43 | 42 | 41 | 39 | 38 | 51
   52   | 51 | 50 | 49 | 48 | 47 | 46 | 45 | 44 | 43 | 42 | 41 | 40 | 52
   53   | 52 | 51 | 50 | 49 | 48 | 47 | 46 | 45 | 44 | 43 | 42 | 41 | 53
   54   | 53 | 52 | 51 | 50 | 50 | 49 | 47 | 46 | 45 | 44 | 43 | 42 | 54
        |    |    |    |    |    |    |    |    |    |    |    |    |
   55   | 54 | 53 | 53 | 52 | 51 | 50 | 49 | 48 | 47 | 46 | 44 | 43 | 55
   56   | 55 | 54 | 54 | 53 | 52 | 51 | 50 | 49 | 48 | 47 | 45 | 44 | 56
   57   | 56 | 55 | 55 | 54 | 53 | 52 | 51 | 50 | 49 | 48 | 47 | 46 | 57
   58   | 57 | 56 | 56 | 55 | 54 | 53 | 52 | 51 | 50 | 49 | 48 | 47 | 58
   59   | 58 | 57 | 57 | 56 | 55 | 54 | 53 | 52 | 51 | 50 | 49 | 48 | 59
        |    |    |    |    |    |    |    |    |    |    |    |    |
   60   | 59 | 58 | 58 | 57 | 56 | 55 | 54 | 53 | 52 | 51 | 50 | 49 | 60
   61   | 60 | 59 | 59 | 58 | 57 | 56 | 55 | 54 | 53 | 52 | 51 | 50 | 61
   62   | 61 | 60 | 60 | 59 | 58 | 57 | 56 | 55 | 54 | 53 | 52 | 52 | 62
   63   | 62 | 61 | 61 | 60 | 59 | 58 | 57 | 56 | 55 | 55 | 54 | 53 | 63
   64   | 63 | 62 | 62 | 61 | 60 | 59 | 58 | 57 | 56 | 56 | 55 | 54 | 64
        |    |    |    |    |    |    |    |    |    |    |    |    |
   65   | 64 | 63 | 63 | 62 | 61 | 60 | 59 | 59 | 58 | 57 | 56 | 55 | 65
   66   | 65 | 64 | 64 | 63 | 62 | 61 | 60 | 60 | 59 | 58 | 57 | 56 | 66
   67   | 67 | 66 | 65 | 64 | 63 | 62 | 61 | 61 | 60 | 59 | 58 | 57 | 67
   68   | 68 | 67 | 66 | 65 | 64 | 63 | 62 | 62 | 61 | 60 | 59 | 58 | 68
   69   | 69 | 68 | 67 | 66 | 65 | 64 | 63 | 63 | 62 | 61 | 60 | 59 | 69
        |    |    |    |    |    |    |    |    |    |    |    |    |
   70   | 70 | 69 | 68 | 67 | 67 | 66 | 65 | 64 | 63 | 62 | 61 | 61 | 70
   71   | 71 | 70 | 69 | 68 | 68 | 67 | 66 | 65 | 64 | 63 | 62 | 62 | 71
   72   | 72 | 71 | 70 | 69 | 69 | 68 | 67 | 66 | 65 | 64 | 63 | 63 | 72
   73   | 73 | 72 | 71 | 70 | 70 | 69 | 68 | 67 | 66 | 66 | 65 | 64 | 73
   74   | 74 | 73 | 72 | 71 | 71 | 70 | 69 | 68 | 67 | 67 | 66 | 65 | 74
        |    |    |    |    |    |    |    |    |    |    |    |    |
   75   | 75 | 74 | 73 | 72 | 72 | 71 | 70 | 69 | 68 | 68 | 67 | 66 | 75
   76   | 76 | 75 | 74 | 73 | 73 | 72 | 71 | 70 | 69 | 69 | 68 | 67 | 76
   77   | 77 | 76 | 75 | 74 | 74 | 73 | 72 | 71 | 70 | 70 | 69 | 68 | 77
   78   | 78 | 77 | 76 | 75 | 75 | 74 | 73 | 72 | 71 | 71 | 70 | 69 | 78
   79   | 79 | 78 | 77 | 76 | 76 | 75 | 74 | 73 | 72 | 72 | 71 | 70 | 79
        |    |    |    |    |    |    |    |    |    |    |    |    |
   80   | 80 | 79 | 78 | 77 | 77 | 76 | 75 | 74 | 73 | 73 | 72 | 72 | 80
  ------+----+----+----+----+----+----+----+----+----+----+----+----+------
   _t._ |0°.5|1°.0|1°.5|2°.0|2°.5|3°.0|3°.5|4°.0|4°.5|5°.0|5°.5|6°.0| _t._
  ======+====+====+====+====+====+====+====+====+====+====+====+====+======

     TABLE I.——TEMPERATURE OF THE DEW-POINT, IN DEGREES FAHRENHEIT.

  ======+=============================================================================+======
   _t_  |         Difference between the dry and wet thermometers (_t-t′_).           | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+ (Dry
  ther.)| 6°.0| 6°.5| 7°.0| 7°.5| 8°.0| 8°.5| 9°.0| 9°.5|10°.0|10°.5|11°.0|11°.5|12°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    19  | -25 |     |     |     |     |     |     |     |     |     |     |     |     |  19
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    20  | -19 | -32 |     |     |     |     |     |     |     |     |     |     |     |  20
    21  | -15 | -24 | -47 |     |     |     |     |     |     |     |     |     |     |  21
    22  | -11 | -19 | -31 |     |     |     |     |     |     |     |     |     |     |  22
    23  |  -8 | -14 | -24 | -45 |     |     |     |     |     |     |     |     |     |  23
    24  |  -5 | -10 | -18 | -30 |     |     |     |     |     |     |     |     |     |  24
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    25  |  -2 |  -7 | -13 | -22 | -42 |     |     |     |     |     |     |     |     |  25
    26  |   0 |  -4 |  -9 | -17 | -28 |     |     |     |     |     |     |     |     |  26
    27  |  +3 |  -1 |  -6 | -12 | -20 | -37 |     |     |     |     |     |     |     |  27
    28  |   5 |  +1 |  -3 |  -8 | -15 | -25 | -54 |     |     |     |     |     |     |  28
    29  |   7 |   4 |   0 |  -4 | -10 | -18 | -32 |     |     |     |     |     |     |  29
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    30  |   9 |   6 |  +2 |  -2 |  -6 | -13 | -22 | -43 |     |     |     |     |     |  30
    31  |  11 |   8 |   5 |  +1 |  -3 |  -8 | -15 | -27 |     |     |     |     |     |  31
    32  |  13 |  10 |   7 |   4 |   0 |  -4 | -10 | -18 | -33 |     |     |     |     |  32
    33  |  14 |  12 |   9 |   6 |  +3 |  -1 |  -6 | -12 | -22 | -44 |     |     |     |  33
    34  |  16 |  14 |  11 |   8 |   6 |  +2 |  -2 |  -8 | -15 | -27 |     |     |     |  34
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    35  |  18 |  15 |  13 |  10 |   8 |   5 |  +1 |  -4 |  -9 | -18 | -32 |     |     |  35
    36  |  19 |  17 |  15 |  12 |  10 |   8 |   4 |   0 |  -5 | -12 | -20 | -42 |     |  36
    37  |  21 |  19 |  17 |  14 |  12 |   9 |   6 |  +3 |  -2 |  -6 | -14 | -25 | -52 |  37
    38  |  22 |  20 |  19 |  16 |  14 |  11 |   9 |   6 |  +2 |  -2 |  -8 | -16 | -29 |  38
    39  |  24 |  22 |  20 |  18 |  16 |  14 |  11 |   8 |   5 |  +1 |  -4 | -10 | -18 |  39
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    40  |  25 |  23 |  22 |  20 |  18 |  16 |  13 |  11 |   8 |   4 |   0 |  -5 | -12 |  40
    41  |  26 |  25 |  23 |  21 |  20 |  17 |  15 |  13 |  10 |   7 |  +4 |  -1 |  -6 |  41
    42  |  27 |  26 |  24 |  23 |  21 |  19 |  18 |  15 |  12 |  10 |   7 |  +3 |  -2 |  42
    43  |  29 |  27 |  26 |  24 |  23 |  21 |  19 |  17 |  14 |  12 |   9 |   6 |  +2 |  43
    44  |  30 |  28 |  27 |  26 |  24 |  22 |  20 |  18 |  16 |  14 |  12 |   9 |   6 |  44
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    45  |  31 |  30 |  28 |  27 |  25 |  24 |  22 |  20 |  18 |  16 |  13 |  11 |   8 |  45
    46  |  32 |  31 |  30 |  28 |  27 |  25 |  24 |  22 |  20 |  18 |  16 |  13 |  11 |  46
    47  |  33 |  32 |  31 |  29 |  28 |  26 |  25 |  23 |  22 |  20 |  18 |  15 |  13 |  47
    48  |  35 |  33 |  32 |  30 |  29 |  28 |  26 |  25 |  23 |  21 |  20 |  17 |  15 |  48
    49  |  36 |  34 |  33 |  32 |  31 |  29 |  28 |  26 |  25 |  23 |  21 |  19 |  17 |  49
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    50  |  37 |  35 |  34 |  33 |  32 |  31 |  29 |  28 |  26 |  24 |  23 |  21 |  19 |  50
    51  |  38 |  37 |  36 |  34 |  33 |  32 |  31 |  29 |  28 |  26 |  24 |  22 |  21 |  51
    52  |  40 |  38 |  37 |  36 |  34 |  33 |  32 |  30 |  29 |  28 |  26 |  24 |  23 |  52
    53  |  41 |  39 |  38 |  37 |  36 |  34 |  33 |  32 |  30 |  29 |  28 |  26 |  24 |  53
    54  |  42 |  41 |  40 |  39 |  37 |  36 |  34 |  33 |  32 |  30 |  29 |  27 |  26 |  54
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    55  |  43 |  42 |  41 |  40 |  39 |  37 |  36 |  34 |  33 |  32 |  30 |  29 |  28 |  55
    56  |  44 |  43 |  42 |  41 |  40 |  39 |  37 |  36 |  34 |  33 |  32 |  30 |  29 |  56
    57  |  46 |  45 |  44 |  42 |  41 |  40 |  39 |  37 |  36 |  35 |  33 |  32 |  30 |  57
    58  |  47 |  46 |  45 |  44 |  42 |  41 |  40 |  39 |  37 |  36 |  35 |  33 |  32 |  58
    59  |  48 |  47 |  46 |  45 |  44 |  43 |  41 |  40 |  39 |  38 |  36 |  35 |  33 |  59
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    60  |  49 |  48 |  47 |  46 |  45 |  44 |  43 |  41 |  40 |  39 |  38 |  36 |  35 |  60
    61  |  50 |  49 |  48 |  47 |  46 |  45 |  44 |  43 |  42 |  41 |  39 |  38 |  36 |  61
    62  |  52 |  51 |  50 |  49 |  48 |  47 |  45 |  44 |  43 |  42 |  41 |  39 |  38 |  62
    63  |  53 |  52 |  51 |  50 |  49 |  48 |  47 |  45 |  44 |  43 |  42 |  41 |  39 |  63
    64  |  54 |  53 |  52 |  51 |  50 |  49 |  48 |  47 |  46 |  45 |  43 |  42 |  41 |  64
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    65  |  55 |  54 |  53 |  52 |  51 |  50 |  49 |  48 |  47 |  46 |  45 |  43 |  42 |  65
    66  |  56 |  55 |  54 |  53 |  52 |  51 |  50 |  49 |  48 |  47 |  46 |  45 |  44 |  66
    67  |  57 |  56 |  55 |  55 |  54 |  53 |  52 |  51 |  50 |  48 |  47 |  46 |  45 |  67
    68  |  58 |  57 |  57 |  56 |  55 |  54 |  53 |  52 |  51 |  50 |  49 |  47 |  46 |  68
    69  |  59 |  58 |  58 |  57 |  56 |  55 |  54 |  53 |  52 |  51 |  50 |  49 |  48 |  69
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    70  |  61 |  60 |  59 |  58 |  57 |  56 |  55 |  54 |  53 |  52 |  51 |  50 |  49 |  70
    71  |  62 |  61 |  60 |  59 |  58 |  57 |  56 |  55 |  55 |  54 |  53 |  52 |  51 |  71
    72  |  63 |  62 |  61 |  60 |  59 |  59 |  58 |  57 |  56 |  55 |  54 |  53 |  52 |  72
    73  |  64 |  63 |  62 |  62 |  61 |  60 |  59 |  58 |  57 |  56 |  55 |  54 |  53 |  73
    74  |  65 |  64 |  63 |  63 |  62 |  61 |  60 |  59 |  58 |  57 |  56 |  55 |  54 |  74
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    75  |  66 |  65 |  64 |  64 |  63 |  62 |  61 |  60 |  59 |  58 |  57 |  56 |  56 |  75
    76  |  67 |  66 |  65 |  65 |  64 |  63 |  62 |  61 |  61 |  60 |  59 |  58 |  57 |  76
    77  |  68 |  67 |  67 |  66 |  65 |  64 |  63 |  62 |  62 |  61 |  60 |  59 |  58 |  77
    78  |  69 |  68 |  68 |  67 |  66 |  66 |  65 |  64 |  63 |  62 |  61 |  60 |  59 |  78
    79  |  70 |  69 |  69 |  68 |  67 |  67 |  66 |  65 |  64 |  63 |  62 |  61 |  61 |  79
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    80  |  72 |  71 |  70 |  69 |  68 |  68 |  67 |  66 |  65 |  64 |  63 |  62 |  62 |  80
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ | 6°.0| 6°.5| 7°.0| 7°.5| 8°.0| 8°.5| 9°.0| 9°.5|10°.0|10°.5|11°.0|11°.5|12°.0| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

     TABLE I.——TEMPERATURE OF THE DEW-POINT, IN DEGREES FAHRENHEIT.

  ======+=============================================================================+======
   _t_  |     Difference between the dry and wet thermometers (_t-t′_).               | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)|12°.0|12°.5|13°.0|13°.5|14°.0|14°.5|15°.0|15°.5|16°.0|16°.5|17°.0|17°.5|18°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
  40    | -12 | -22 | -44 |     |     |     |     |     |     |     |     |     |     | 40
  41    |  -6 | -13 | -25 |     |     |     |     |     |     |     |     |     |     | 41
  42    |  -2 |  -7 | -15 | -28 |     |     |     |     |     |     |     |     |     | 42
  43    |  +2 |  -3 |  -8 | -17 | -33 |     |     |     |     |     |     |     |     | 43
  44    |   6 |  +1 |  -4 | -10 | -19 | -40 |     |     |     |     |     |     |     | 44
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
  45    |   8 |   5 |   0 |  -4 | -11 | -22 | -48 |     |     |     |     |     |     | 45
  46    |  11 |   8 |  +4 |   0 |  -5 | -13 | -24 |     |     |     |     |     |     | 46
  47    |  13 |  10 |   7 |  +3 |  -1 |  -6 | -14 | -27 |     |     |     |     |     | 47
  48    |  15 |  12 |  10 |   6 |  +2 |  -2 |  -8 | -16 | -30 |     |     |     |     | 48
  49    |  17 |  14 |  12 |   9 |   6 |  +2 |  -3 |  -9 | -18 | -35 |     |     |     | 49
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
  50    |  19 |  16 |  14 |  12 |   9 |   5 |  +1 |  -4 | -10 | -20 | -42 |     |     | 50
  51    |  21 |  18 |  17 |  14 |  11 |   8 |   5 |   0 |  -5 | -12 | -22 | -52 |     | 51
  52    |  23 |  21 |  19 |  16 |  14 |  11 |   8 |  +4 |   0 |  -6 | -13 | -25 |     | 52
  53    |  24 |  22 |  20 |  18 |  16 |  14 |  11 |   8 |  +4 |  -1 |  -6 | -14 | -28 | 53
  54    |  26 |  24 |  22 |  20 |  18 |  16 |  13 |  10 |   7 |  +3 |  -2 |  -8 | -16 | 54
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
  55    |  28 |  26 |  24 |  22 |  20 |  18 |  16 |  13 |  10 |   7 |  +3 |  -2 |  -8 | 55
  56    |  29 |  27 |  26 |  24 |  22 |  20 |  18 |  15 |  13 |  10 |   6 |  +2 |  -2 | 56
  57    |  30 |  29 |  28 |  26 |  24 |  22 |  20 |  18 |  15 |  13 |  10 |   6 |  +2 | 57
  58    |  32 |  30 |  29 |  27 |  26 |  24 |  22 |  20 |  18 |  15 |  12 |   9 |   6 | 58
  59    |  33 |  32 |  31 |  29 |  27 |  26 |  24 |  22 |  20 |  18 |  15 |  12 |   9 | 59
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
  60    |  35 |  33 |  32 |  30 |  29 |  27 |  26 |  24 |  22 |  20 |  18 |  15 |  12 | 60
  61    |  36 |  35 |  33 |  32 |  31 |  29 |  28 |  26 |  24 |  22 |  20 |  18 |  15 | 61
  62    |  38 |  37 |  35 |  34 |  32 |  31 |  29 |  28 |  26 |  24 |  22 |  20 |  18 | 62
  63    |  39 |  38 |  37 |  35 |  34 |  32 |  31 |  29 |  28 |  26 |  24 |  22 |  20 | 63
  64    |  41 |  39 |  38 |  37 |  35 |  34 |  32 |  31 |  29 |  28 |  26 |  24 |  22 | 64
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
  65    |  42 |  41 |  40 |  38 |  37 |  35 |  34 |  32 |  31 |  29 |  28 |  26 |  24 | 65
  66    |  44 |  43 |  41 |  40 |  38 |  37 |  35 |  34 |  32 |  31 |  30 |  28 |  26 | 66
  67    |  45 |  44 |  43 |  41 |  40 |  39 |  37 |  36 |  34 |  32 |  31 |  30 |  28 | 67
  68    |  46 |  45 |  44 |  43 |  42 |  40 |  39 |  38 |  36 |  34 |  33 |  31 |  30 | 68
  69    |  48 |  47 |  46 |  45 |  43 |  42 |  40 |  39 |  38 |  36 |  34 |  33 |  32 | 69
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
  70    |  49 |  48 |  47 |  46 |  45 |  43 |  42 |  41 |  39 |  38 |  36 |  35 |  33 | 70
  71    |  51 |  49 |  48 |  47 |  46 |  45 |  43 |  42 |  41 |  39 |  38 |  36 |  35 | 71
  72    |  52 |  51 |  50 |  49 |  47 |  46 |  45 |  44 |  43 |  41 |  40 |  38 |  37 | 72
  73    |  53 |  52 |  51 |  50 |  49 |  48 |  46 |  45 |  44 |  43 |  41 |  40 |  38 | 73
  74    |  54 |  53 |  52 |  51 |  50 |  49 |  48 |  47 |  45 |  44 |  43 |  41 |  40 | 74
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
  75    |  56 |  55 |  54 |  53 |  52 |  50 |  49 |  48 |  47 |  45 |  44 |  43 |  42 | 75
  76    |  57 |  56 |  55 |  54 |  53 |  52 |  50 |  49 |  48 |  47 |  46 |  45 |  43 | 76
  77    |  58 |  57 |  56 |  55 |  54 |  53 |  52 |  51 |  50 |  49 |  48 |  46 |  45 | 77
  78    |  59 |  58 |  57 |  56 |  55 |  54 |  53 |  52 |  51 |  50 |  49 |  48 |  47 | 78
  79    |  61 |  60 |  59 |  58 |  57 |  56 |  55 |  54 |  53 |  52 |  51 |  49 |  48 | 79
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
  80    |  62 |  61 |  60 |  59 |  58 |  57 |  56 |  55 |  54 |  53 |  52 |  51 |  50 | 80
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ |12°.0|12°.5|13°.0|13°.5|14°.0|14°.5|15°.0|15°.5|16°.0|16°.5|17°.0|17°.5|18°.0| _t._
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------

     TABLE I.——TEMPERATURE OF THE DEW-POINT, IN DEGREES FAHRENHEIT.

  ======+=============================================================================+======
   _t_  |          Difference between the dry and wet thermometers (_t-t′_).          | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)|18°.0|19°.0|20°.0|21°.0|22°.0|23°.0|24°.0|25°.0|26°.0|27°.0|28°.0|29°.0|30°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    55  |  -8 |     |     |     |     |     |     |     |     |     |     |     |     |  55
    56  |  -2 | -19 |     |     |     |     |     |     |     |     |     |     |     |  56
    57  |  +2 | -10 | -48 |     |     |     |     |     |     |     |     |     |     |  57
    58  |   6 |  -3 | -22 |     |     |     |     |     |     |     |     |     |     |  58
    59  |   9 |  +1 | -12 |     |     |     |     |     |     |     |     |     |     |  59
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    60  |  12 |   5 |  -5 | -25 |     |     |     |     |     |     |     |     |     |  60
    61  |  15 |   9 |   0 | -14 |     |     |     |     |     |     |     |     |     |  61
    62  |  18 |  12 |  +5 |  -6 | -28 |     |     |     |     |     |     |     |     |  62
    63  |  20 |  15 |   9 |   0 | -14 |     |     |     |     |     |     |     |     |  63
    64  |  22 |  18 |  12 |  +4 |  -6 | -32 |     |     |     |     |     |     |     |  64
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    65  |  24 |  20 |  15 |   9 |   0 | -16 |     |     |     |     |     |     |     |  65
    66  |  26 |  22 |  18 |  12 |  +4 |  -7 | -34 |     |     |     |     |     |     |  66
    67  |  28 |  24 |  20 |  15 |   9 |  -1 | -16 |     |     |     |     |     |     |  67
    68  |  30 |  26 |  23 |  18 |  12 |  +4 |  -7 | -37 |     |     |     |     |     |  68
    69  |  32 |  28 |  25 |  20 |  15 |   8 |   0 | -17 |     |     |     |     |     |  69
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    70  |  33 |  30 |  27 |  23 |  19 |  12 |  +5 |  -7 | -39 |     |     |     |     |  70
    71  |  35 |  32 |  29 |  25 |  21 |  16 |   9 |   0 | -17 |     |     |     |     |  71
    72  |  37 |  33 |  31 |  27 |  23 |  18 |  13 |  +5 |  -6 | -39 |     |     |     |  72
    73  |  38 |  35 |  32 |  29 |  25 |  21 |  16 |  10 |   0 | -16 |     |     |     |  73
    74  |  40 |  37 |  34 |  31 |  28 |  24 |  19 |  13 |  +6 |  -6 | -37 |     |     |  74
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    75  |  42 |  39 |  36 |  32 |  30 |  26 |  22 |  16 |  10 |   0 | -16 |     |     |  75
    76  |  43 |  41 |  38 |  34 |  32 |  28 |  24 |  20 |  14 |  +6 |  -6 | -34 |     |  76
    77  |  45 |  42 |  40 |  36 |  33 |  30 |  26 |  22 |  17 |  11 |  +1 | -14 |     |  77
    78  |  47 |  44 |  41 |  38 |  35 |  32 |  28 |  24 |  20 |  14 |   7 |  -4 | -30 |  78
    79  |  48 |  46 |  43 |  40 |  37 |  34 |  31 |  27 |  23 |  18 |  11 |  +2 | -13 |  79
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    80  |  50 |  47 |  45 |  42 |  39 |  36 |  32 |  29 |  25 |  21 |  15 |   8 |  -3 |  80
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t_  |18°.0|19°.0|20°.0|21°.0|22°.0|23°.0|24°.0|25°.0|26°.0|27°.0|28°.0|29°.0|30°.0| _t_
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

     TABLE I.——TEMPERATURE OF THE DEW-POINT, IN DEGREES FAHRENHEIT.

  ======+==============================================================+======
   _t_  |   Difference between the dry and wet thermometers (_t-t′_).  | _t_
  (Dry  +----+----+----+----+----+----+----+----+----+-----+-----+-----+(Dry
  ther.)|1°.0|2°.0|3°.0|4°.0|5°.0|6°.0|7°.0|8°.0|9°.0|10°.0|11°.0|12°.0|ther.)
  ------+----+----+----+----+----+----+----+----+----+-----+-----+-----+------
    80  | 79 | 77 | 76 | 74 | 73 | 72 | 70 | 68 | 67 |  65 |  63 |  62 |  80
    81  | 80 | 78 | 77 | 75 | 74 | 73 | 71 | 70 | 68 |  66 |  65 |  63 |  81
    82  | 81 | 79 | 78 | 77 | 75 | 74 | 72 | 71 | 69 |  68 |  66 |  64 |  82
    83  | 82 | 80 | 79 | 78 | 76 | 75 | 73 | 72 | 70 |  69 |  67 |  65 |  83
    84  | 83 | 81 | 80 | 79 | 77 | 76 | 74 | 73 | 71 |  70 |  68 |  67 |  84
        |    |    |    |    |    |    |    |    |    |     |     |     |
    85  | 84 | 82 | 81 | 80 | 78 | 77 | 75 | 74 | 72 |  71 |  69 |  68 |  85
    86  | 85 | 83 | 82 | 81 | 79 | 78 | 76 | 75 | 73 |  72 |  71 |  69 |  86
    87  | 86 | 84 | 83 | 82 | 80 | 79 | 78 | 76 | 74 |  73 |  72 |  70 |  87
    88  | 87 | 85 | 84 | 83 | 81 | 80 | 79 | 77 | 75 |  74 |  73 |  71 |  88
    89  | 88 | 86 | 85 | 84 | 82 | 81 | 80 | 78 | 76 |  76 |  74 |  72 |  89
        |    |    |    |    |    |    |    |    |    |     |     |     |
    90  | 89 | 87 | 86 | 85 | 84 | 82 | 81 | 79 | 78 |  77 |  75 |  74 |  90
    91  | 90 | 88 | 87 | 86 | 85 | 83 | 82 | 80 | 79 |  78 |  76 |  75 |  91
    92  | 91 | 89 | 88 | 87 | 86 | 84 | 83 | 82 | 80 |  79 |  77 |  76 |  92
    93  | 92 | 91 | 89 | 88 | 87 | 85 | 84 | 83 | 81 |  80 |  78 |  77 |  93
    94  | 93 | 92 | 90 | 89 | 88 | 86 | 85 | 84 | 82 |  81 |  80 |  78 |  94
        |    |    |    |    |    |    |    |    |    |     |     |     |
    95  | 94 | 93 | 91 | 90 | 89 | 87 | 86 | 85 | 83 |  82 |  81 |  79 |  95
    96  | 95 | 94 | 92 | 91 | 90 | 88 | 87 | 86 | 84 |  83 |  82 |  80 |  96
    97  | 96 | 95 | 93 | 92 | 91 | 90 | 88 | 87 | 86 |  84 |  83 |  81 |  97
    98  | 97 | 96 | 94 | 93 | 92 | 91 | 89 | 88 | 87 |  85 |  84 |  83 |  98
    99  | 98 | 97 | 95 | 94 | 93 | 92 | 90 | 89 | 88 |  86 |  85 |  84 |  99
        |    |    |    |    |    |    |    |    |    |     |     |     |
   100  | 99 | 98 | 96 | 95 | 94 | 93 | 91 | 90 | 89 |  87 |  86 |  85 | 100
   101  |100 | 99 | 97 | 96 | 95 | 94 | 92 | 91 | 90 |  88 |  87 |  86 | 101
   102  |101 |100 | 98 | 97 | 96 | 95 | 93 | 92 | 91 |  90 |  88 |  87 | 102
   103  |102 |101 | 99 | 98 | 97 | 96 | 94 | 93 | 92 |  91 |  89 |  88 | 103
   104  |103 |102 |100 | 99 | 98 | 97 | 96 | 94 | 93 |  92 |  90 |  89 | 104
        |    |    |    |    |    |    |    |    |    |     |     |     |
   105  |104 |103 |101 |100 | 99 | 98 | 97 | 95 | 94 |  93 |  91 |  90 | 105
   106  |105 |104 |102 |101 |100 | 99 | 98 | 96 | 95 |  94 |  93 |  91 | 106
   107  |106 |105 |103 |102 |101 |100 | 99 | 97 | 96 |  95 |  94 |  92 | 107
   108  |107 |106 |104 |103 |102 |101 |100 | 98 | 97 |  96 |  95 |  93 | 108
   109  |108 |107 |105 |104 |103 |102 |101 | 99 | 98 |  97 |  96 |  94 | 109
        |    |    |    |    |    |    |    |    |    |     |     |     |
   110  |109 |108 |107 |105 |104 |103 |102 |101 | 99 |  98 |  97 |  96 | 110
  ------+----+----+----+----+----+----+----+----+----+-----+-----+-----+------
   _t._ |1°.0|2°.0|3°.0|4°.0|5°.0|6°.0|7°.0|8°.0|9°.0|10°.0|11°.0|12°.0| _t._
  ======+====+====+====+====+====+====+====+====+====+=====+=====+=====+======

     TABLE I.——TEMPERATURE OF THE DEW-POINT, IN DEGREES FAHRENHEIT.

  ======+=============================================================================+======
   _t_  |          Difference between the dry and wet thermometers (_t-t′_).          | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)|12°.0|13°.0|14°.0|15°.0|16°.0|17°.0|18°.0|19°.0|20°.0|21°.0|22°.0|23°.0|24°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    80  |  62 |  60 |  58 |  56 |  54 |  52 |  50 |  47 |  45 |  42 |  39 |  36 |  32 |  80
    81  |  63 |  61 |  59 |  57 |  55 |  53 |  51 |  49 |  47 |  44 |  41 |  38 |  35 |  81
    82  |  64 |  62 |  61 |  59 |  57 |  55 |  53 |  50 |  48 |  45 |  43 |  40 |  37 |  82
    83  |  65 |  64 |  62 |  60 |  58 |  56 |  54 |  52 |  50 |  47 |  44 |  42 |  39 |  83
    84  |  67 |  65 |  63 |  61 |  59 |  57 |  55 |  53 |  51 |  49 |  46 |  43 |  41 |  84
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    85  |  68 |  66 |  64 |  62 |  61 |  59 |  57 |  55 |  53 |  50 |  48 |  45 |  42 |  85
    86  |  69 |  67 |  66 |  64 |  62 |  60 |  58 |  56 |  54 |  52 |  49 |  47 |  44 |  86
    87  |  70 |  68 |  67 |  65 |  63 |  61 |  59 |  57 |  55 |  53 |  51 |  48 |  46 |  87
    88  |  71 |  70 |  68 |  66 |  64 |  63 |  61 |  59 |  57 |  55 |  53 |  50 |  48 |  88
    89  |  72 |  71 |  69 |  67 |  66 |  64 |  62 |  60 |  58 |  56 |  54 |  52 |  49 |  89
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    90  |  74 |  72 |  70 |  69 |  67 |  65 |  63 |  62 |  60 |  58 |  56 |  53 |  51 |  90
    91  |  75 |  73 |  72 |  70 |  68 |  67 |  65 |  63 |  61 |  59 |  57 |  55 |  53 |  91
    92  |  76 |  74 |  73 |  71 |  69 |  68 |  66 |  64 |  62 |  60 |  58 |  56 |  54 |  92
    93  |  77 |  75 |  74 |  72 |  71 |  69 |  67 |  66 |  64 |  62 |  60 |  58 |  56 |  93
    94  |  78 |  77 |  75 |  73 |  72 |  70 |  69 |  67 |  65 |  63 |  61 |  59 |  57 |  94
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    95  |  79 |  78 |  76 |  75 |  73 |  71 |  70 |  68 |  66 |  64 |  63 |  61 |  59 |  95
    96  |  80 |  79 |  77 |  76 |  74 |  73 |  71 |  69 |  68 |  66 |  64 |  62 |  60 |  96
    97  |  81 |  80 |  78 |  77 |  75 |  74 |  72 |  71 |  69 |  67 |  65 |  63 |  61 |  97
    98  |  83 |  81 |  80 |  78 |  77 |  75 |  73 |  72 |  70 |  68 |  67 |  65 |  63 |  98
    99  |  84 |  82 |  81 |  79 |  78 |  76 |  75 |  73 |  71 |  70 |  68 |  66 |  64 |  99
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   100  |  85 |  83 |  82 |  80 |  79 |  77 |  76 |  74 |  73 |  71 |  69 |  67 |  66 | 100
   101  |  86 |  84 |  83 |  82 |  80 |  79 |  77 |  75 |  74 |  72 |  71 |  69 |  67 | 101
   102  |  87 |  85 |  84 |  83 |  81 |  80 |  78 |  77 |  75 |  73 |  72 |  70 |  68 | 102
   103  |  88 |  87 |  85 |  84 |  82 |  81 |  79 |  78 |  76 |  75 |  73 |  71 |  70 | 103
   104  |  89 |  88 |  86 |  85 |  83 |  82 |  81 |  79 |  78 |  76 |  74 |  73 |  71 | 104
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   105  |  90 |  89 |  87 |  86 |  85 |  83 |  82 |  80 |  79 |  77 |  76 |  74 |  72 | 105
   106  |  91 |  90 |  89 |  87 |  86 |  84 |  83 |  81 |  80 |  78 |  77 |  75 |  74 | 106
   107  |  92 |  91 |  90 |  88 |  87 |  85 |  84 |  83 |  81 |  80 |  78 |  76 |  75 | 107
   108  |  93 |  92 |  91 |  89 |  88 |  87 |  85 |  84 |  82 |  81 |  79 |  78 |  76 | 108
   109  |  94 |  93 |  92 |  90 |  89 |  88 |  86 |  85 |  83 |  82 |  80 |  79 |  77 | 109
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   110  |  96 |  94 |  93 |  92 |  90 |  89 |  87 |  86 |  85 |  83 |  82 |  80 |  79 | 110
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ |12°.0|13°.0|14°.0|15°.0|16°.0|17°.0|18°.0|19°.0|20°.0|21°.0|22°.0|23°.0|24°.0| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

     TABLE I.——TEMPERATURE OF THE DEW-POINT, IN DEGREES FAHRENHEIT.

  ======+=============================================================================+======
   _t_  |          Difference between the dry and wet thermometers (_t-t′_).          | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)|24°.0|25°.0|26°.0|27°.0|28°.0|29°.0|30°.0|31°.0|32°.0|33°.0|34°.0|35°.0|36°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    80  |  32 |  29 |  25 |  21 |  15 |   8 |  -3 | -27 |     |     |     |     |     |  80
    81  |  35 |  31 |  28 |  24 |  18 |  12 |  +3 | -11 |     |     |     |     |     |  81
    82  |  37 |  33 |  30 |  26 |  22 |  16 |   9 |  -2 | -24 |     |     |     |     |  82
    83  |  39 |  35 |  32 |  28 |  24 |  19 |  13 |  +5 |  -9 |     |     |     |     |  83
    84  |  41 |  37 |  34 |  30 |  27 |  22 |  17 |  10 |   0 | -20 |     |     |     |  84
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    85  |  42 |  39 |  36 |  32 |  29 |  25 |  20 |  14 |  +6 |  -7 | -54 |     |     |  85
    86  |  44 |  41 |  38 |  35 |  31 |  28 |  23 |  18 |  11 |  +1 | -17 |     |     |  86
    87  |  46 |  43 |  40 |  37 |  33 |  30 |  26 |  21 |  15 |   7 |  -5 | -38 |     |  87
    88  |  48 |  45 |  42 |  39 |  35 |  32 |  28 |  24 |  19 |  12 |  +3 | -13 |     |  88
    89  |  49 |  47 |  44 |  41 |  38 |  34 |  31 |  27 |  22 |  16 |   9 |  -2 | -28 |  89
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    90  |  51 |  48 |  46 |  43 |  40 |  36 |  32 |  29 |  25 |  20 |  13 |  +4 | -10 |  90
    91  |  53 |  50 |  47 |  45 |  42 |  38 |  35 |  32 |  28 |  23 |  18 |  10 |   0 |  91
    92  |  54 |  52 |  49 |  46 |  44 |  41 |  37 |  34 |  30 |  26 |  21 |  15 |  +7 |  92
    93  |  56 |  53 |  51 |  48 |  46 |  43 |  39 |  36 |  32 |  29 |  24 |  19 |  12 |  93
    94  |  57 |  55 |  53 |  50 |  47 |  45 |  42 |  38 |  35 |  31 |  27 |  22 |  16 |  94
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    95  |  59 |  56 |  54 |  52 |  49 |  46 |  44 |  40 |  37 |  33 |  30 |  25 |  20 |  95
    96  |  60 |  58 |  56 |  53 |  51 |  48 |  46 |  43 |  39 |  36 |  32 |  28 |  24 |  96
    97  |  61 |  59 |  57 |  55 |  53 |  50 |  47 |  45 |  41 |  38 |  34 |  31 |  26 |  97
    98  |  63 |  61 |  59 |  57 |  54 |  52 |  49 |  47 |  44 |  40 |  37 |  33 |  29 |  98
    99  |  64 |  62 |  60 |  58 |  56 |  54 |  51 |  48 |  46 |  43 |  39 |  35 |  32 |  99
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   100  |  66 |  64 |  62 |  60 |  57 |  55 |  53 |  50 |  48 |  45 |  41 |  38 |  34 | 100
   101  |  67 |  65 |  63 |  61 |  59 |  57 |  54 |  52 |  49 |  47 |  44 |  40 |  37 | 101
   102  |  68 |  66 |  65 |  63 |  61 |  58 |  56 |  54 |  51 |  49 |  46 |  43 |  39 | 102
   103  |  70 |  68 |  66 |  64 |  62 |  60 |  58 |  55 |  53 |  50 |  48 |  45 |  41 | 103
   104  |  71 |  69 |  67 |  65 |  63 |  61 |  59 |  57 |  55 |  52 |  50 |  47 |  44 | 104
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   105  |  72 |  70 |  69 |  67 |  65 |  63 |  61 |  59 |  56 |  54 |  52 |  49 |  46 | 105
   106  |  74 |  72 |  70 |  68 |  66 |  64 |  62 |  60 |  58 |  56 |  53 |  51 |  48 | 106
   107  |  75 |  73 |  71 |  70 |  68 |  66 |  64 |  62 |  60 |  57 |  55 |  52 |  50 | 107
   108  |  76 |  74 |  73 |  71 |  69 |  67 |  65 |  63 |  61 |  59 |  57 |  54 |  52 | 108
   109  |  77 |  76 |  74 |  72 |  71 |  69 |  67 |  65 |  63 |  61 |  58 |  56 |  54 | 109
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   110  |  79 |  77 |  75 |  74 |  72 |  70 |  68 |  66 |  64 |  62 |  60 |  58 |  55 | 110
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ |24°.0|25°.0|26°.0|27°.0|28°.0|29°.0|30°.0|31°.0|32°.0|33°.0|34°.0|35°.0|36°.0| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

     TABLE I.——TEMPERATURE OF THE DEW-POINT, IN DEGREES FAHRENHEIT.

  ======+=============================================================================+======
   _t_  |          Difference between the dry and wet thermometers (_t-t′_).          | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)|36°.0|37°.0|38°.0|39°.0|40°.0|41°.0|42°.0|43°.0|44°.0|45°.0|46°.0|47°.0|48°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    89  | -28 |     |     |     |     |     |     |     |     |     |     |     |     |  89
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    90  | -10 |     |     |     |     |     |     |     |     |     |     |     |     |  90
    91  |   0 | -22 |     |     |     |     |     |     |     |     |     |     |     |  91
    92  |  +7 |  -7 | -16 |     |     |     |     |     |     |     |     |     |     |  92
    93  |  12 |  +2 |  -4 |     |     |     |     |     |     |     |     |     |     |  93
    94  |  16 |   8 |  +4 | -37 |     |     |     |     |     |     |     |     |     |  94
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    95  |  20 |  13 |  10 | -12 |     |     |     |     |     |     |     |     |     |  95
    96  |  24 |  15 |  15 |  -1 | -25 |     |     |     |     |     |     |     |     |  96
    97  |  26 |  21 |  19 |  +7 |  -8 |     |     |     |     |     |     |     |     |  97
    98  |  29 |  25 |  23 |  12 |  +2 | -18 |     |     |     |     |     |     |     |  98
    99  |  32 |  28 |  26 |  17 |   9 |  -4 | -42 |     |     |     |     |     |     |  99
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   100  |  34 |  30 |  29 |  21 |  14 |  +5 | -12 |     |     |     |     |     |     | 100
   101  |  37 |  32 |  32 |  24 |  18 |  11 |   0 | -25 |     |     |     |     |     | 101
   102  |  39 |  35 |  34 |  27 |  22 |  16 |  +7 |  -7 |     |     |     |     |     | 102
   103  |  41 |  38 |  34 |  30 |  26 |  20 |  13 |  +3 | -16 |     |     |     |     | 103
   104  |  44 |  40 |  37 |  32 |  29 |  24 |  18 |  10 |  -2 | -38 |     |     |     | 104
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   105  |  46 |  43 |  39 |  35 |  31 |  27 |  22 |  15 |  +6 | -10 |     |     |     | 105
   106  |  48 |  45 |  42 |  38 |  34 |  30 |  25 |  20 |  12 |  +1 | -22 |     |     | 106
   107  |  50 |  47 |  44 |  40 |  37 |  32 |  28 |  24 |  17 |   9 |  -5 |     |     | 107
   108  |  52 |  49 |  46 |  43 |  39 |  35 |  31 |  27 |  21 |  14 |  +5 | -13 |     | 108
   109  |  54 |  51 |  48 |  45 |  42 |  38 |  34 |  30 |  25 |  19 |  12 |   0 | -28 | 109
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   110  |  55 |  53 |  50 |  47 |  44 |  41 |  37 |  33 |  28 |  23 |  17 |  +8 |  -7 | 110
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ |36°.0|37°.0|38°.0|39°.0|40°.0|41°.0|42°.0|43°.0|44°.0|45°.0|46°.0|47°.0|48°.0| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

                TABLE II.——RELATIVE HUMIDITY, PER CENT.

  ======+================================================================+=======
   _t_  |  Difference between the dry and wet thermometers (_t_——_t′_).  |  _t_
  (Dry  +----+----+----+----+----+----+----+----+----+----+----+----+----+ (Dry
  ther.)|0°.2|0°.4|0°.6|0°.8|1°.0|1°.2|1°.4|1°.6|1°.8|2°.0|2°.2|2°.4|2°.6| ther.)
  ------+----+----+----+----+----+----+----+----+----+----+----+----+----+-------
   -40  | 46 |    |    |    |    |    |    |    |    |    |    |    |    |  -40
   -39  | 49 |    |    |    |    |    |    |    |    |    |    |    |    |  -39
   -38  | 51 |    |    |    |    |    |    |    |    |    |    |    |    |  -38
   -37  | 54 |    |    |    |    |    |    |    |    |    |    |    |    |  -37
   -36  | 56 |    |    |    |    |    |    |    |    |    |    |    |    |  -36
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -35  | 59 |    |    |    |    |    |    |    |    |    |    |    |    |  -35
   -34  | 61 | 22 |    |    |    |    |    |    |    |    |    |    |    |  -34
   -33  | 63 | 25 |    |    |    |    |    |    |    |    |    |    |    |  -33
   -32  | 65 | 30 |    |    |    |    |    |    |    |    |    |    |    |  -32
   -31  | 67 | 34 |    |    |    |    |    |    |    |    |    |    |    |  -31
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -30  | 69 | 38 |    |    |    |    |    |    |    |    |    |    |    |  -30
   -29  | 71 | 42 |    |    |    |    |    |    |    |    |    |    |    |  -29
   -28  | 72 | 45 | 17 |    |    |    |    |    |    |    |    |    |    |  -28
   -27  | 74 | 48 | 22 |    |    |    |    |    |    |    |    |    |    |  -27
   -26  | 76 | 51 | 26 |    |    |    |    |    |    |    |    |    |    |  -26
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -25  | 77 | 53 | 31 |    |    |    |    |    |    |    |    |    |    |  -25
   -24  | 78 | 56 | 34 | 12 |    |    |    |    |    |    |    |    |    |  -24
   -23  | 79 | 58 | 37 | 16 |    |    |    |    |    |    |    |    |    |  -23
   -22  | 80 | 60 | 40 | 20 |    |    |    |    |    |    |    |    |    |  -22
   -21  | 81 | 62 | 44 | 25 |    |    |    |    |    |    |    |    |    |  -21
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -20  | 82 | 64 | 47 | 29 | 11 |    |    |    |    |    |    |    |    |  -20
   -19  | 83 | 66 | 49 | 33 | 16 |    |    |    |    |    |    |    |    |  -19
   -18  | 84 | 68 | 52 | 36 | 20 |    |    |    |    |    |    |    |    |  -18
   -17  | 85 | 70 | 54 | 39 | 24 |  9 |    |    |    |    |    |    |    |  -17
   -16  | 86 | 71 | 57 | 43 | 28 | 14 |    |    |    |    |    |    |    |  -16
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -15  | 86 | 73 | 59 | 46 | 32 | 19 |    |    |    |    |    |    |    |  -15
   -14  | 87 | 74 | 61 | 48 | 36 | 23 | 10 |    |    |    |    |    |    |  -14
   -13  | 88 | 76 | 63 | 51 | 39 | 27 | 15 |    |    |    |    |    |    |  -13
   -12  | 88 | 77 | 65 | 53 | 42 | 30 | 19 |  7 |    |    |    |    |    |  -12
   -11  | 89 | 78 | 67 | 56 | 45 | 34 | 23 | 12 |    |    |    |    |    |  -11
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
   -10  | 90 | 79 | 68 | 58 | 48 | 37 | 26 | 16 |  5 |    |    |    |    |  -10
    -9  | 90 | 80 | 70 | 60 | 50 | 40 | 30 | 20 | 10 |    |    |    |    |   -9
    -8  | 90 | 81 | 71 | 62 | 52 | 43 | 33 | 24 | 14 |  5 |    |    |    |   -8
    -7  | 91 | 82 | 73 | 63 | 54 | 45 | 36 | 27 | 18 |  9 |    |    |    |   -7
    -6  | 91 | 83 | 74 | 65 | 56 | 48 | 39 | 31 | 22 | 13 |  5 |    |    |   -6
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
    -5  | 92 | 83 | 75 | 67 | 58 | 50 | 42 | 34 | 25 | 17 |  9 |    |    |   -5
    -4  | 92 | 84 | 76 | 68 | 60 | 52 | 45 | 37 | 29 | 21 | 13 |  5 |    |   -4
    -3  | 92 | 85 | 77 | 70 | 62 | 55 | 47 | 40 | 32 | 25 | 17 | 10 |    |   -3
    -2  | 93 | 86 | 78 | 71 | 64 | 57 | 50 | 42 | 35 | 28 | 21 | 14 |  7 |   -2
    -1  | 93 | 86 | 79 | 72 | 66 | 59 | 52 | 45 | 38 | 31 | 25 | 18 | 11 |   -1
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
     0  | 93 | 87 | 80 | 74 | 67 | 61 | 54 | 48 | 41 | 35 | 28 | 22 | 15 |    0
    +1  | 94 | 87 | 81 | 75 | 69 | 63 | 56 | 50 | 44 | 38 | 32 | 25 | 19 |   +1
     2  | 94 | 88 | 82 | 76 | 70 | 64 | 58 | 52 | 46 | 40 | 35 | 29 | 23 |    2
     3  | 94 | 88 | 83 | 77 | 71 | 66 | 60 | 54 | 49 | 43 | 37 | 32 | 26 |    3
     4  | 94 | 89 | 83 | 78 | 73 | 67 | 62 | 56 | 51 | 45 | 40 | 34 | 29 |    4
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
     5  | 95 | 89 | 84 | 79 | 74 | 68 | 63 | 58 | 53 | 48 | 42 | 37 | 32 |    5
     6  | 95 | 90 | 85 | 80 | 75 | 70 | 65 | 60 | 54 | 50 | 44 | 39 | 34 |    6
     7  | 95 | 90 | 85 | 80 | 76 | 71 | 66 | 61 | 56 | 51 | 47 | 42 | 37 |    7
     8  | 95 | 91 | 86 | 81 | 76 | 72 | 67 | 62 | 58 | 53 | 49 | 44 | 39 |    8
     9  | 96 | 91 | 86 | 82 | 77 | 73 | 68 | 64 | 59 | 55 | 51 | 46 | 42 |    9
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
    10  | 96 | 91 | 87 | 83 | 78 | 74 | 69 | 65 | 61 | 57 | 52 | 48 | 44 |   10
    11  | 96 | 92 | 87 | 83 | 79 | 75 | 71 | 66 | 62 | 58 | 54 | 50 | 46 |   11
    12  | 96 | 92 | 88 | 84 | 80 | 76 | 72 | 68 | 64 | 60 | 56 | 52 | 48 |   12
    13  | 96 | 92 | 88 | 84 | 81 | 77 | 73 | 69 | 65 | 61 | 58 | 54 | 50 |   13
    14  | 96 | 93 | 89 | 85 | 81 | 78 | 74 | 70 | 67 | 63 | 59 | 56 | 52 |   14
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
    15  | 96 | 93 | 89 | 86 | 82 | 79 | 75 | 71 | 68 | 64 | 61 | 57 | 54 |   15
    16  | 97 | 93 | 90 | 86 | 83 | 79 | 76 | 73 | 69 | 66 | 62 | 59 | 56 |   16
    17  | 97 | 93 | 90 | 87 | 83 | 80 | 77 | 74 | 70 | 67 | 64 | 60 | 57 |   17
    18  | 97 | 94 | 90 | 87 | 84 | 81 | 78 | 74 | 71 | 68 | 65 | 62 | 59 |   18
    19  | 97 | 94 | 91 | 88 | 84 | 81 | 78 | 75 | 72 | 69 | 66 | 63 | 60 |   19
        |    |    |    |    |    |    |    |    |    |    |    |    |    |
    20  | 97 | 94 | 91 | 88 | 85 | 82 | 79 | 76 | 73 | 70 | 67 | 64 | 61 |   20
  ------+----+----+----+----+----+----+----+----+----+----+----+----+----+-------
   _t._ |0°.2|0°.4|0°.6|0°.8|1°.0|1°.2|1°.4|1°.6|1°.8|2°.0|2°.2|2°.4|2°.6| _t._
  ======+====+====+====+====+====+====+====+====+====+====+====+====+====+=======

                TABLE II.——RELATIVE HUMIDITY, PER CENT.

  ======+===========================================================+======
   _t_  | Difference between the dry and wet thermometers (_t-t′_).| _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)| 2°.6| 2°.8| 3°.0| 3°.2| 3°.4| 3°.6| 3°.8| 4°.0| 4°.2| 4°.4|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    -2  |   7 |     |     |     |     |     |     |     |     |     |  -2
    -1  |  11 |   4 |     |     |     |     |     |     |     |     |  -1
        |     |     |     |     |     |     |     |     |     |     |
     0  |  15 |   9 |     |     |     |     |     |     |     |     |   0
    +1  |  19 |  13 |   7 |     |     |     |     |     |     |     |  +1
     2  |  23 |  17 |  11 |   5 |     |     |     |     |     |     |   2
     3  |  26 |  20 |  15 |   9 |   4 |     |     |     |     |     |   3
     4  |  29 |  24 |  18 |  13 |   8 |   2 |     |     |     |     |   4
        |     |     |     |     |     |     |     |     |     |     |
     5  |  32 |  27 |  22 |  16 |  11 |   6 |     |     |     |     |   5
     6  |  34 |  29 |  25 |  20 |  15 |  10 |   5 |     |     |     |   6
     7  |  37 |  32 |  28 |  23 |  18 |  13 |   9 |   4 |     |     |   7
     8  |  39 |  35 |  30 |  26 |  21 |  17 |  12 |   8 |   3 |     |   8
     9  |  42 |  37 |  33 |  28 |  24 |  20 |  15 |  11 |   7 |   2 |   9
        |     |     |     |     |     |     |     |     |     |     |
    10  |  44 |  40 |  35 |  31 |  27 |  23 |  19 |  14 |  10 |   6 |  10
    11  |  46 |  42 |  38 |  34 |  30 |  26 |  22 |  18 |  14 |  10 |  11
    12  |  48 |  44 |  40 |  36 |  32 |  28 |  25 |  21 |  17 |  13 |  12
    13  |  50 |  46 |  42 |  39 |  35 |  31 |  27 |  24 |  20 |  16 |  13
    14  |  52 |  48 |  45 |  41 |  37 |  34 |  30 |  27 |  23 |  19 |  14
        |     |     |     |     |     |     |     |     |     |     |
    15  |  54 |  50 |  47 |  43 |  40 |  36 |  33 |  29 |  26 |  23 |  15
    16  |  56 |  52 |  49 |  46 |  42 |  39 |  36 |  32 |  29 |  25 |  16
    17  |  57 |  54 |  51 |  48 |  44 |  41 |  38 |  35 |  31 |  28 |  17
    18  |  59 |  56 |  53 |  49 |  46 |  43 |  40 |  37 |  34 |  31 |  18
    19  |  60 |  57 |  54 |  51 |  48 |  45 |  42 |  39 |  36 |  33 |  19
        |     |     |     |     |     |     |     |     |     |     |
    20  |  61 |  58 |  56 |  53 |  50 |  47 |  44 |  41 |  38 |  35 |  20
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ | 2°.6| 2°.8| 3°.0| 3°.2| 3°.4| 3°.6| 3°.8| 4°.0| 4°.2| 4°.4| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======
   _t._ | 4°.6| 4°.8| 5°.0| 5°.2| 5°.4| 5°.6| 5°.8| 6°.0| 6°.2| 6°.4| _t._
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
     8  |     |     |     |     |     |     |     |     |     |     |   8
     9  |     |     |     |     |     |     |     |     |     |     |   9
        |     |     |     |     |     |     |     |     |     |     |
    10  |   2 |     |     |     |     |     |     |     |     |     |  10
    11  |   6 |   2 |     |     |     |     |     |     |     |     |  11
    12  |   9 |   5 |   2 |     |     |     |     |     |     |     |  12
    13  |  13 |   9 |   5 |   2 |     |     |     |     |     |     |  13
    14  |  16 |  12 |   9 |   5 |   2 |     |     |     |     |     |  14
        |     |     |     |     |     |     |     |     |     |     |
    15  |  19 |  16 |  12 |   9 |   5 |   2 |     |     |     |     |  15
    16  |  22 |  19 |  16 |  12 |   9 |   6 |   2 |     |     |     |  16
    17  |  25 |  22 |  19 |  16 |  12 |   9 |   6 |   3 |     |     |  17
    18  |  28 |  25 |  22 |  19 |  16 |  13 |   9 |   6 |   3 |     |  18
    19  |  30 |  27 |  24 |  21 |  19 |  16 |  13 |  10 |   7 |   4 |  19
        |     |     |     |     |     |     |     |     |     |     |
    20  |  33 |  30 |  27 |  24 |  21 |  19 |  16 |  13 |  10 |   7 |  20
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ | 4°.6| 4°.8| 5°.0| 5°.2| 5°.4| 5°.6| 5°.8| 6°.0| 6°.2| 6°.4| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

                TABLE II.——RELATIVE HUMIDITY, PER CENT.

  ======+=======================================================================+======
   _t_  |          Difference between the dry and wet thermometers (_t-t′_).    | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)| 0°.5| 1°.0| 1°.5| 2°.0| 2°.5| 3°.0| 3°.5| 4°.0| 4°.5| 5°.0| 5°.5| 6°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    20  |  92 |  85 |  77 |  70 |  63 |  56 |  48 |  41 |  34 |  27 |  20 |  13 |  20
    21  |  93 |  85 |  78 |  71 |  64 |  57 |  50 |  43 |  36 |  29 |  23 |  16 |  21
    22  |  93 |  86 |  79 |  72 |  65 |  58 |  51 |  45 |  38 |  32 |  25 |  19 |  22
    23  |  93 |  86 |  80 |  73 |  66 |  60 |  53 |  46 |  40 |  34 |  27 |  21 |  23
    24  |  93 |  87 |  80 |  74 |  67 |  61 |  54 |  48 |  42 |  36 |  30 |  24 |  24
        |     |     |     |     |     |     |     |     |     |     |     |     |
    25  |  94 |  87 |  81 |  74 |  68 |  62 |  56 |  50 |  44 |  38 |  32 |  26 |  25
    26  |  94 |  88 |  81 |  75 |  69 |  63 |  57 |  51 |  45 |  40 |  34 |  28 |  26
    27  |  94 |  88 |  82 |  76 |  70 |  64 |  59 |  53 |  47 |  42 |  36 |  30 |  27
    28  |  94 |  88 |  82 |  77 |  71 |  65 |  60 |  54 |  49 |  43 |  38 |  33 |  28
    29  |  94 |  89 |  83 |  77 |  72 |  66 |  61 |  56 |  50 |  45 |  40 |  35 |  29
        |     |     |     |     |     |     |     |     |     |     |     |     |
    30  |  94 |  89 |  84 |  78 |  73 |  67 |  62 |  57 |  52 |  47 |  41 |  36 |  30
    31  |  95 |  89 |  84 |  79 |  74 |  68 |  63 |  58 |  53 |  48 |  43 |  38 |  31
    32  |  95 |  90 |  84 |  79 |  74 |  69 |  64 |  59 |  54 |  50 |  45 |  40 |  32
    33  |  95 |  90 |  85 |  80 |  75 |  70 |  65 |  60 |  56 |  51 |  47 |  42 |  33
    34  |  95 |  91 |  86 |  81 |  75 |  72 |  67 |  62 |  57 |  53 |  48 |  44 |  34
        |     |     |     |     |     |     |     |     |     |     |     |     |
    35  |  95 |  91 |  86 |  82 |  76 |  73 |  69 |  65 |  59 |  54 |  50 |  45 |  35
    36  |  96 |  91 |  86 |  82 |  77 |  73 |  70 |  66 |  61 |  56 |  51 |  47 |  36
    37  |  96 |  91 |  87 |  82 |  78 |  74 |  70 |  66 |  62 |  57 |  52 |  48 |  37
    38  |  96 |  92 |  87 |  83 |  79 |  75 |  71 |  67 |  63 |  58 |  54 |  50 |  38
    39  |  96 |  92 |  88 |  83 |  79 |  75 |  72 |  68 |  63 |  59 |  55 |  52 |  39
        |     |     |     |     |     |     |     |     |     |     |     |     |
    40  |  96 |  92 |  88 |  84 |  80 |  76 |  72 |  68 |  64 |  60 |  56 |  53 |  40
    41  |  96 |  92 |  88 |  84 |  80 |  76 |  72 |  69 |  65 |  61 |  57 |  54 |  41
    42  |  96 |  92 |  88 |  84 |  81 |  77 |  73 |  69 |  65 |  62 |  58 |  55 |  42
    43  |  96 |  92 |  88 |  85 |  81 |  77 |  74 |  70 |  66 |  63 |  59 |  56 |  43
    44  |  96 |  92 |  88 |  85 |  81 |  78 |  74 |  70 |  67 |  63 |  60 |  57 |  44
        |     |     |     |     |     |     |     |     |     |     |     |     |
    45  |  96 |  92 |  89 |  85 |  82 |  78 |  75 |  71 |  67 |  64 |  61 |  58 |  45
    46  |  96 |  93 |  89 |  85 |  82 |  79 |  75 |  72 |  68 |  65 |  61 |  58 |  46
    47  |  96 |  93 |  89 |  86 |  83 |  79 |  76 |  72 |  69 |  66 |  62 |  59 |  47
    48  |  96 |  93 |  89 |  86 |  83 |  79 |  76 |  73 |  69 |  66 |  63 |  60 |  48
    49  |  97 |  93 |  90 |  86 |  83 |  80 |  76 |  73 |  70 |  67 |  63 |  60 |  49
        |     |     |     |     |     |     |     |     |     |     |     |     |
    50  |  97 |  93 |  90 |  87 |  83 |  80 |  77 |  74 |  70 |  67 |  64 |  61 |  50
    51  |  97 |  93 |  90 |  87 |  84 |  81 |  77 |  74 |  71 |  68 |  65 |  62 |  51
    52  |  97 |  94 |  90 |  87 |  84 |  81 |  78 |  75 |  72 |  69 |  66 |  63 |  52
    53  |  97 |  94 |  91 |  87 |  84 |  81 |  78 |  75 |  72 |  69 |  66 |  63 |  53
    54  |  97 |  94 |  91 |  88 |  85 |  82 |  79 |  76 |  73 |  70 |  67 |  64 |  54
        |     |     |     |     |     |     |     |     |     |     |     |     |
    55  |  97 |  94 |  91 |  88 |  85 |  82 |  79 |  76 |  73 |  70 |  68 |  65 |  55
    56  |  97 |  94 |  91 |  88 |  85 |  82 |  80 |  77 |  74 |  71 |  68 |  65 |  56
    57  |  97 |  94 |  91 |  88 |  86 |  83 |  80 |  77 |  74 |  71 |  69 |  66 |  57
    58  |  97 |  94 |  91 |  89 |  86 |  83 |  80 |  78 |  75 |  72 |  69 |  67 |  58
    59  |  97 |  94 |  92 |  89 |  86 |  83 |  81 |  78 |  75 |  72 |  70 |  67 |  59
        |     |     |     |     |     |     |     |     |     |     |     |     |
    60  |  97 |  94 |  92 |  89 |  86 |  84 |  81 |  78 |  75 |  73 |  70 |  68 |  60
    61  |  97 |  94 |  92 |  89 |  87 |  84 |  81 |  78 |  76 |  73 |  71 |  68 |  61
    62  |  97 |  95 |  92 |  89 |  87 |  84 |  81 |  79 |  76 |  74 |  71 |  69 |  62
    63  |  97 |  95 |  92 |  89 |  87 |  84 |  82 |  79 |  77 |  74 |  72 |  69 |  63
    64  |  97 |  95 |  92 |  90 |  87 |  85 |  82 |  79 |  77 |  74 |  72 |  70 |  64
        |     |     |     |     |     |     |     |     |     |     |     |     |
    65  |  97 |  95 |  92 |  90 |  87 |  85 |  82 |  80 |  77 |  75 |  72 |  70 |  65
    66  |  97 |  95 |  92 |  90 |  87 |  85 |  82 |  80 |  78 |  75 |  73 |  71 |  66
    67  |  98 |  95 |  93 |  90 |  88 |  85 |  83 |  80 |  78 |  76 |  73 |  71 |  67
    68  |  98 |  95 |  93 |  90 |  88 |  85 |  83 |  81 |  78 |  76 |  74 |  71 |  68
    69  |  98 |  95 |  93 |  90 |  88 |  86 |  83 |  81 |  78 |  76 |  74 |  72 |  69
        |     |     |     |     |     |     |     |     |     |     |     |     |
    70  |  98 |  95 |  93 |  90 |  88 |  86 |  83 |  81 |  79 |  77 |  74 |  72 |  70
    71  |  98 |  95 |  93 |  91 |  88 |  86 |  84 |  81 |  79 |  77 |  75 |  72 |  71
    72  |  98 |  95 |  93 |  91 |  88 |  86 |  84 |  82 |  79 |  77 |  75 |  73 |  72
    73  |  98 |  95 |  93 |  91 |  88 |  86 |  84 |  82 |  80 |  78 |  75 |  73 |  73
    74  |  98 |  95 |  93 |  91 |  88 |  86 |  84 |  82 |  80 |  78 |  76 |  74 |  74
        |     |     |     |     |     |     |     |     |     |     |     |     |
    75  |  98 |  95 |  93 |  91 |  89 |  87 |  84 |  82 |  80 |  78 |  76 |  74 |  75
    76  |  98 |  95 |  93 |  91 |  89 |  87 |  85 |  82 |  80 |  78 |  76 |  74 |  76
    77  |  98 |  95 |  93 |  91 |  89 |  87 |  85 |  83 |  80 |  78 |  76 |  74 |  77
    78  |  98 |  96 |  93 |  91 |  89 |  87 |  85 |  83 |  81 |  79 |  77 |  75 |  78
    79  |  98 |  96 |  94 |  91 |  89 |  87 |  85 |  83 |  81 |  79 |  77 |  75 |  79
        |     |     |     |     |     |     |     |     |     |     |     |     |
    80  |  98 |  96 |  94 |  92 |  89 |  87 |  85 |  83 |  81 |  79 |  77 |  75 |  80
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ | 0°.5| 1°.0| 1°.5| 2°.0| 2°.5| 3°.0| 3°.5| 4°.0| 4°.5| 5°.0| 5°.5| 6°.0| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

                TABLE II.——RELATIVE HUMIDITY, PER CENT.

  ======+=====================================================================+=====
   _t_  |        Difference between the dry and wet thermometers (_t-t′_).    |_t_
  (Dry  +----+----+----+----+----+----+----+----+-----+-----+-----+-----+-----+(Dry
  ther.)|6°.0|6°.5|7°.0|7°.5|8°.0|8°.5|9°.0|9°.5|10°.0|10°.5|11°.0|11°.5|12°.0|ther.)
  ------+----+----+----+----+----+----+----+----+-----+-----+-----+-----+-----+------
    19  | 10 |    |    |    |    |    |    |    |     |     |     |     |     | 19
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    20  | 13 |  6 |    |    |    |    |    |    |     |     |     |     |     | 20
    21  | 16 |  9 |  2 |    |    |    |    |    |     |     |     |     |     | 21
    22  | 19 | 12 |  6 |    |    |    |    |    |     |     |     |     |     | 22
    23  | 21 | 15 |  9 |  2 |    |    |    |    |     |     |     |     |     | 23
    24  | 24 | 17 | 11 |  6 |    |    |    |    |     |     |     |     |     | 24
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    25  | 26 | 20 | 14 |  8 |  3 |    |    |    |     |     |     |     |     | 25
    26  | 28 | 23 | 17 | 11 |  6 |    |    |    |     |     |     |     |     | 26
    27  | 30 | 25 | 19 | 14 |  9 |  3 |    |    |     |     |     |     |     | 27
    28  | 33 | 27 | 22 | 17 | 11 |  6 |  1 |    |     |     |     |     |     | 28
    29  | 35 | 29 | 24 | 19 | 14 |  9 |  4 |    |     |     |     |     |     | 29
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    30  | 36 | 31 | 26 | 22 | 17 | 12 |  7 |  2 |     |     |     |     |     | 30
    31  | 38 | 33 | 29 | 24 | 19 | 14 | 10 |  5 |     |     |     |     |     | 31
    32  | 40 | 35 | 31 | 26 | 21 | 17 | 12 |  8 |  3  |     |     |     |     | 32
    33  | 42 | 37 | 33 | 28 | 24 | 19 | 15 | 10 |  6  |  2  |     |     |     | 33
    34  | 44 | 39 | 35 | 30 | 26 | 21 | 17 | 13 |  9  |  4  |     |     |     | 34
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    35  | 45 | 41 | 37 | 32 | 28 | 24 | 19 | 15 | 12  |  7  |  3  |     |     | 35
    36  | 47 | 43 | 38 | 34 | 30 | 26 | 22 | 18 | 14  | 10  |  6  |  2  |     | 36
    37  | 48 | 44 | 40 | 36 | 32 | 28 | 24 | 20 | 16  | 12  |  8  |  5  |  1  | 37
    38  | 50 | 46 | 42 | 38 | 34 | 30 | 26 | 22 | 18  | 15  | 11  |  7  |  3  | 38
    39  | 52 | 48 | 44 | 40 | 36 | 32 | 28 | 24 | 20  | 17  | 13  |  9  |  6  | 39
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    40  | 53 | 49 | 45 | 41 | 38 | 34 | 30 | 26 | 22  | 19  | 16  | 12  |  8  | 40
    41  | 54 | 50 | 46 | 43 | 39 | 36 | 32 | 29 | 24  | 21  | 18  | 14  | 10  | 41
    42  | 55 | 51 | 48 | 44 | 40 | 37 | 34 | 30 | 27  | 23  | 20  | 16  | 13  | 42
    43  | 56 | 52 | 49 | 46 | 42 | 38 | 35 | 32 | 29  | 25  | 22  | 19  | 15  | 43
    44  | 57 | 53 | 50 | 47 | 43 | 40 | 37 | 33 | 30  | 27  | 24  | 21  | 17  | 44
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    45  | 58 | 54 | 51 | 48 | 44 | 41 | 38 | 35 | 32  | 29  | 25  | 22  | 19  | 45
    46  | 58 | 55 | 52 | 49 | 46 | 42 | 39 | 36 | 33  | 30  | 27  | 23  | 21  | 46
    47  | 59 | 56 | 53 | 50 | 47 | 44 | 40 | 38 | 34  | 31  | 28  | 25  | 22  | 47
    48  | 60 | 56 | 53 | 51 | 48 | 45 | 42 | 39 | 36  | 33  | 30  | 27  | 24  | 48
    49  | 60 | 57 | 54 | 52 | 49 | 46 | 43 | 40 | 37  | 34  | 31  | 29  | 26  | 49
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    50  | 61 | 58 | 55 | 52 | 50 | 47 | 44 | 41 | 38  | 36  | 33  | 30  | 27  | 50
    51  | 62 | 59 | 56 | 53 | 50 | 48 | 45 | 42 | 39  | 37  | 34  | 31  | 28  | 51
    52  | 63 | 60 | 57 | 54 | 51 | 48 | 46 | 43 | 40  | 38  | 35  | 33  | 30  | 52
    53  | 63 | 61 | 58 | 55 | 52 | 49 | 47 | 44 | 42  | 39  | 36  | 34  | 31  | 53
    54  | 64 | 61 | 59 | 56 | 53 | 50 | 48 | 45 | 43  | 40  | 38  | 35  | 32  | 54
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    55  | 65 | 62 | 59 | 57 | 54 | 51 | 49 | 46 | 43  | 41  | 39  | 36  | 34  | 55
    56  | 65 | 63 | 60 | 57 | 55 | 52 | 50 | 47 | 44  | 42  | 40  | 37  | 35  | 56
    57  | 66 | 64 | 61 | 58 | 55 | 53 | 50 | 48 | 45  | 43  | 40  | 38  | 36  | 57
    58  | 67 | 64 | 61 | 59 | 56 | 53 | 51 | 49 | 46  | 44  | 42  | 39  | 37  | 58
    59  | 67 | 65 | 62 | 60 | 57 | 54 | 52 | 49 | 47  | 45  | 43  | 40  | 38  | 59
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    60  | 68 | 65 | 63 | 60 | 58 | 55 | 53 | 50 | 48  | 46  | 44  | 41  | 39  | 60
    61  | 68 | 66 | 63 | 61 | 58 | 56 | 54 | 51 | 49  | 47  | 44  | 42  | 40  | 61
    62  | 69 | 66 | 64 | 61 | 59 | 57 | 54 | 52 | 50  | 47  | 45  | 43  | 41  | 62
    63  | 69 | 67 | 64 | 62 | 60 | 57 | 55 | 53 | 51  | 48  | 46  | 44  | 42  | 63
    64  | 70 | 67 | 65 | 62 | 60 | 58 | 56 | 53 | 51  | 49  | 47  | 45  | 43  | 64
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    65  | 70 | 68 | 65 | 63 | 61 | 59 | 56 | 54 | 52  | 50  | 48  | 46  | 44  | 65
    66  | 71 | 68 | 66 | 63 | 61 | 59 | 57 | 55 | 53  | 51  | 49  | 47  | 45  | 66
    67  | 71 | 69 | 66 | 64 | 62 | 60 | 58 | 55 | 53  | 51  | 49  | 47  | 45  | 67
    68  | 71 | 69 | 67 | 65 | 63 | 60 | 58 | 56 | 54  | 52  | 50  | 48  | 46  | 68
    69  | 72 | 70 | 67 | 65 | 63 | 61 | 59 | 57 | 55  | 53  | 51  | 49  | 47  | 69
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    70  | 72 | 70 | 68 | 66 | 64 | 62 | 60 | 57 | 55  | 53  | 52  | 50  | 48  | 70
    71  | 72 | 70 | 68 | 66 | 64 | 62 | 60 | 58 | 56  | 54  | 52  | 50  | 48  | 71
    72  | 73 | 71 | 69 | 67 | 65 | 63 | 61 | 59 | 57  | 55  | 53  | 51  | 49  | 72
    73  | 73 | 71 | 69 | 67 | 65 | 63 | 61 | 59 | 57  | 55  | 53  | 52  | 50  | 73
    74  | 74 | 72 | 70 | 68 | 66 | 64 | 62 | 60 | 58  | 56  | 54  | 52  | 50  | 74
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    75  | 74 | 72 | 70 | 68 | 66 | 64 | 62 | 60 | 58  | 56  | 55  | 53  | 51  | 75
    76  | 74 | 72 | 70 | 68 | 66 | 64 | 63 | 61 | 59  | 57  | 55  | 53  | 52  | 76
    77  | 74 | 73 | 71 | 69 | 67 | 65 | 63 | 61 | 59  | 57  | 56  | 54  | 52  | 77
    78  | 75 | 73 | 71 | 69 | 67 | 65 | 63 | 62 | 60  | 58  | 56  | 54  | 53  | 78
    79  | 75 | 73 | 71 | 70 | 68 | 66 | 64 | 62 | 60  | 58  | 57  | 55  | 53  | 79
        |    |    |    |    |    |    |    |    |     |     |     |     |     |
    80  | 75 | 73 | 72 | 70 | 68 | 66 | 64 | 63 | 61  | 59  | 57  | 55  | 54  | 80
  ------+----+----+----+----+----+----+----+----+-----+-----+-----+-----+-----+------
   _t._ |6°.0|6°.5|7°.0|7°.5|8°.0|8°.5|9°.0|9°.5|10°.0|10°.5|11°.0|11°.5|12°0|_t._
  ======+====+====+====+====+====+====+====+====+=====+=====+=====+=====+=====+======

                TABLE II.——RELATIVE HUMIDITY, PER CENT.

  ======+=============================================================================+======
   _t_  | Difference between the dry and wet thermometers (_t-t′_).                   | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)|12°.0|12°.5|13°.0|13°.5|14°.0|14°.5|15°.0|15°.5|16°.0|16°.5|17°.0|17°.5|18°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    40  |   8 |   5 |   1 |     |     |     |     |     |     |     |     |     |     |  40
    41  |  10 |   7 |   4 |     |     |     |     |     |     |     |     |     |     |  41
    42  |  13 |  10 |   6 |   3 |     |     |     |     |     |     |     |     |     |  42
    43  |  15 |  12 |   9 |   5 |   2 |     |     |     |     |     |     |     |     |  43
    44  |  17 |  14 |  11 |   8 |   5 |   1 |     |     |     |     |     |     |     |  44
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    45  |  19 |  16 |  13 |  10 |   7 |   4 |   1 |     |     |     |     |     |     |  45
    46  |  21 |  18 |  15 |  12 |   9 |   6 |   3 |     |     |     |     |     |     |  46
    47  |  22 |  20 |  16 |  14 |  11 |   8 |   5 |   3 |     |     |     |     |     |  47
    48  |  24 |  21 |  19 |  16 |  13 |  10 |   7 |   5 |   2 |     |     |     |     |  48
    49  |  26 |  23 |  20 |  17 |  15 |  12 |   9 |   7 |   4 |   1 |     |     |     |  49
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    50  |  27 |  24 |  22 |  19 |  16 |  14 |  11 |   9 |   6 |   4 |   1 |     |     |  50
    51  |  28 |  26 |  23 |  21 |  18 |  16 |  13 |  10 |   8 |   5 |   3 |     |     |  51
    52  |  30 |  27 |  24 |  22 |  20 |  17 |  15 |  12 |  10 |   7 |   5 |     |     |  52
    53  |  31 |  29 |  26 |  24 |  21 |  19 |  16 |  14 |  12 |   9 |   7 |   4 |   2 |  53
    54  |  32 |  30 |  28 |  25 |  23 |  20 |  18 |  15 |  13 |  11 |   8 |   6 |   4 |  54
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    55  |  34 |  31 |  29 |  26 |  24 |  22 |  19 |  17 |  15 |  12 |  10 |   8 |   6 |  55
    56  |  35 |  33 |  30 |  28 |  25 |  23 |  21 |  19 |  16 |  14 |  12 |  10 |   8 |  56
    57  |  36 |  34 |  32 |  29 |  27 |  24 |  22 |  20 |  18 |  16 |  13 |  11 |   9 |  57
    58  |  37 |  35 |  33 |  30 |  28 |  26 |  24 |  21 |  19 |  17 |  15 |  13 |  11 |  58
    59  |  38 |  36 |  34 |  31 |  29 |  27 |  25 |  23 |  21 |  18 |  16 |  14 |  12 |  59
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    60  |  39 |  37 |  34 |  32 |  30 |  28 |  26 |  24 |  22 |  20 |  18 |  16 |  14 |  60
    61  |  40 |  38 |  35 |  33 |  32 |  29 |  27 |  25 |  23 |  21 |  19 |  17 |  15 |  61
    62  |  41 |  39 |  37 |  34 |  32 |  30 |  28 |  26 |  24 |  22 |  20 |  18 |  16 |  62
    63  |  42 |  40 |  38 |  35 |  33 |  31 |  29 |  28 |  26 |  24 |  22 |  20 |  18 |  63
    64  |  43 |  41 |  38 |  36 |  34 |  32 |  30 |  29 |  27 |  25 |  23 |  21 |  19 |  64
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    65  |  44 |  42 |  39 |  37 |  35 |  33 |  31 |  29 |  28 |  26 |  24 |  22 |  20 |  65
    66  |  45 |  42 |  40 |  38 |  36 |  34 |  32 |  30 |  29 |  27 |  25 |  23 |  22 |  66
    67  |  45 |  43 |  41 |  39 |  37 |  35 |  33 |  32 |  30 |  28 |  26 |  25 |  23 |  67
    68  |  46 |  44 |  42 |  40 |  38 |  36 |  34 |  33 |  31 |  29 |  27 |  26 |  24 |  68
    69  |  47 |  45 |  43 |  41 |  39 |  37 |  35 |  33 |  32 |  30 |  28 |  26 |  25 |  69
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    70  |  48 |  46 |  44 |  42 |  40 |  38 |  36 |  34 |  33 |  31 |  29 |  27 |  26 |  70
    71  |  48 |  46 |  45 |  43 |  41 |  39 |  37 |  35 |  34 |  32 |  30 |  28 |  27 |  71
    72  |  49 |  47 |  45 |  43 |  42 |  40 |  38 |  36 |  35 |  33 |  31 |  30 |  28 |  72
    73  |  50 |  48 |  46 |  44 |  42 |  41 |  39 |  37 |  35 |  34 |  32 |  30 |  29 |  73
    74  |  50 |  48 |  47 |  45 |  43 |  41 |  40 |  38 |  36 |  35 |  33 |  31 |  30 |  74
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    75  |  51 |  49 |  47 |  46 |  44 |  42 |  40 |  39 |  37 |  35 |  34 |  32 |  31 |  75
    76  |  52 |  50 |  48 |  46 |  45 |  43 |  41 |  39 |  38 |  36 |  35 |  33 |  31 |  76
    77  |  52 |  50 |  49 |  47 |  45 |  44 |  42 |  40 |  39 |  37 |  35 |  34 |  32 |  77
    78  |  53 |  51 |  49 |  48 |  46 |  44 |  43 |  41 |  39 |  38 |  36 |  35 |  33 |  78
    79  |  53 |  52 |  50 |  48 |  47 |  45 |  43 |  42 |  40 |  39 |  37 |  36 |  34 |  79
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    80  |  54 |  52 |  51 |  49 |  47 |  45 |  44 |  42 |  41 |  39 |  38 |  36 |  35 |  80
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ |12°.0|12°.5|13°.0|13°.5|14°.0|14°.5|15°.0|15°.5|16°.0|16°.5|17°.0|17°.5|18°.0| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

                TABLE II.——RELATIVE HUMIDITY, PER CENT.

  ======+=============================================================================+======
   _t_  | Difference between the dry and wet thermometers (_t-t′_).                   | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)|18°.0|19°.0|20°.0|21°.0|22°.0|23°.0|24°.0|25°.0|26°.0|27°.0|28°.0|29°.0|30°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-------
    55  |   6 |   1 |     |     |     |     |     |     |     |     |     |     |     | 55
    56  |   8 |   3 |     |     |     |     |     |     |     |     |     |     |     | 56
    57  |   9 |   5 |     |     |     |     |     |     |     |     |     |     |     | 57
    58  |  11 |   7 |   2 |     |     |     |     |     |     |     |     |     |     | 58
    59  |  12 |   8 |   4 |     |     |     |     |     |     |     |     |     |     | 59
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    60  |  14 |  10 |   6 |   2 |     |     |     |     |     |     |     |     |     | 60
    61  |  15 |  11 |   7 |   3 |     |     |     |     |     |     |     |     |     | 61
    62  |  16 |  13 |   9 |   5 |   1 |     |     |     |     |     |     |     |     | 62
    63  |  18 |  14 |  10 |   7 |   3 |     |     |     |     |     |     |     |     | 63
    64  |  19 |  15 |  12 |   8 |   5 |   1 |     |     |     |     |     |     |     | 64
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    65  |  20 |  17 |  13 |  10 |   6 |   3 |     |     |     |     |     |     |     | 65
    66  |  22 |  18 |  14 |  11 |   8 |   4 |   1 |     |     |     |     |     |     | 66
    67  |  23 |  19 |  16 |  12 |   9 |   6 |   2 |     |     |     |     |     |     | 67
    68  |  24 |  20 |  17 |  14 |  10 |   7 |   4 |   1 |     |     |     |     |     | 68
    69  |  25 |  22 |  18 |  15 |  12 |   8 |   5 |   2 |     |     |     |     |     | 69
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    70  |  26 |  23 |  19 |  16 |  13 |  10 |   7 |   4 |   1 |     |     |     |     | 70
    71  |  27 |  24 |  20 |  17 |  14 |  11 |   8 |   5 |   2 |     |     |     |     | 71
    72  |  28 |  24 |  22 |  18 |  15 |  12 |   9 |   6 |   3 |   1 |     |     |     | 72
    73  |  29 |  25 |  22 |  19 |  16 |  13 |  10 |   8 |   5 |   2 |     |     |     | 73
    74  |  30 |  26 |  23 |  20 |  18 |  15 |  12 |   9 |   6 |   3 |   1 |     |     | 74
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    75  |  31 |  27 |  24 |  21 |  19 |  16 |  13 |  10 |   7 |   5 |   2 |     |     | 75
    76  |  31 |  28 |  25 |  22 |  20 |  17 |  14 |  11 |   8 |   6 |   3 |   1 |     | 76
    77  |  32 |  29 |  26 |  23 |  20 |  18 |  15 |  12 |  10 |   7 |   4 |   2 |     | 77
    78  |  33 |  30 |  27 |  24 |  21 |  19 |  16 |  13 |  11 |   8 |   6 |   3 |   1 | 78
    79  |  34 |  31 |  28 |  25 |  22 |  19 |  17 |  14 |  12 |   9 |   7 |   4 |   2 | 79
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    80  |  35 |  32 |  29 |  26 |  23 |  20 |  18 |  15 |  13 |  10 |   8 |   6 |   3 | 80
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ |18°.0|19°.0|20°.0|21°.0|22°.0|23°.0|24°.0|25°.0|26°.0|27°.0|28°.0|29°.0|30°.0| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

                TABLE II.——RELATIVE HUMIDITY, PER CENT.

  ======+=======================================================================+======
   _t_  | Difference between the dry and wet thermometers (_t-t′_).             | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)| 1°.0| 2°.0| 3°.0| 4°.0| 5°.0| 6°.0| 7°.0| 8°.0| 9°.0|10°.0|11°.0|12°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-------
    80  |  96 |  92 |  87 |  83 |  79 |  75 |  72 |  68 |  64 |  61 |  57 |  54 |  80
    81  |  96 |  92 |  88 |  84 |  80 |  76 |  72 |  68 |  65 |  61 |  58 |  54 |  81
    82  |  96 |  92 |  88 |  84 |  80 |  76 |  72 |  69 |  65 |  62 |  58 |  55 |  82
    83  |  96 |  92 |  88 |  84 |  80 |  76 |  73 |  69 |  66 |  62 |  59 |  55 |  83
    84  |  96 |  92 |  88 |  84 |  80 |  77 |  73 |  69 |  66 |  63 |  59 |  56 |  84
        |     |     |     |     |     |     |     |     |     |     |     |     |
    85  |  96 |  92 |  88 |  84 |  80 |  77 |  73 |  70 |  66 |  63 |  60 |  56 |  85
    86  |  96 |  92 |  88 |  84 |  81 |  77 |  73 |  70 |  67 |  63 |  60 |  57 |  86
    87  |  96 |  92 |  88 |  84 |  81 |  77 |  74 |  70 |  67 |  64 |  60 |  57 |  87
    88  |  96 |  92 |  88 |  85 |  81 |  77 |  74 |  71 |  67 |  64 |  61 |  58 |  88
    89  |  96 |  92 |  88 |  85 |  81 |  78 |  74 |  71 |  68 |  64 |  61 |  58 |  89
        |     |     |     |     |     |     |     |     |     |     |     |     |
    90  |  96 |  92 |  88 |  85 |  81 |  78 |  75 |  71 |  68 |  65 |  62 |  59 |  90
    91  |  96 |  92 |  89 |  85 |  82 |  78 |  75 |  71 |  68 |  65 |  62 |  59 |  91
    92  |  96 |  92 |  89 |  85 |  82 |  78 |  75 |  72 |  69 |  65 |  62 |  59 |  92
    93  |  96 |  93 |  89 |  85 |  82 |  78 |  75 |  72 |  69 |  66 |  63 |  60 |  93
    94  |  96 |  93 |  89 |  86 |  82 |  79 |  75 |  72 |  69 |  66 |  63 |  60 |  94
        |     |     |     |     |     |     |     |     |     |     |     |     |
    95  |  96 |  93 |  89 |  86 |  82 |  79 |  76 |  72 |  69 |  66 |  63 |  60 |  95
    96  |  96 |  93 |  89 |  86 |  82 |  79 |  76 |  73 |  70 |  67 |  64 |  61 |  96
    97  |  96 |  93 |  89 |  86 |  82 |  79 |  76 |  73 |  70 |  67 |  64 |  61 |  97
    98  |  96 |  93 |  89 |  86 |  83 |  79 |  76 |  73 |  70 |  67 |  64 |  61 |  98
    99  |  96 |  93 |  89 |  86 |  83 |  80 |  76 |  73 |  70 |  68 |  65 |  62 |  99
        |     |     |     |     |     |     |     |     |     |     |     |     |
   100  |  97 |  93 |  90 |  86 |  83 |  80 |  77 |  74 |  71 |  68 |  65 |  62 | 100
   101  |  97 |  93 |  90 |  86 |  83 |  80 |  77 |  74 |  71 |  68 |  65 |  62 | 101
   102  |  97 |  93 |  90 |  86 |  83 |  80 |  77 |  74 |  71 |  68 |  65 |  63 | 102
   103  |  97 |  93 |  90 |  87 |  83 |  80 |  77 |  74 |  71 |  69 |  66 |  63 | 103
   104  |  97 |  93 |  90 |  87 |  83 |  80 |  77 |  74 |  72 |  69 |  66 |  63 | 104
        |     |     |     |     |     |     |     |     |     |     |     |     |
   105  |  97 |  93 |  90 |  87 |  84 |  81 |  78 |  75 |  72 |  69 |  66 |  64 | 105
   106  |  97 |  93 |  90 |  87 |  84 |  81 |  78 |  75 |  72 |  69 |  66 |  64 | 106
   107  |  97 |  93 |  90 |  87 |  84 |  81 |  78 |  75 |  72 |  69 |  67 |  64 | 107
   108  |  97 |  93 |  90 |  87 |  84 |  81 |  78 |  75 |  72 |  70 |  67 |  64 | 108
   109  |  97 |  93 |  90 |  87 |  84 |  81 |  78 |  75 |  73 |  70 |  67 |  65 | 109
        |     |     |     |     |     |     |     |     |     |     |     |     |
   110  |  97 |  94 |  90 |  87 |  84 |  81 |  78 |  76 |  73 |  70 |  67 |  65 | 110
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ | 1°.0| 2°.0| 3°.0| 4°.0| 5°.0| 6°.0| 7°.0| 8°.0| 9°.0|10°.0|11°.0|12°.0| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

                TABLE II.——RELATIVE HUMIDITY, PER CENT.

  ======+=============================================================================+======
   _t_  |          Difference between the dry and wet thermometers (_t-t′_).          | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)|12°.0|13°.0|14°.0|15°.0|16°.0|17°.0|18°.0|19°.0|20°.0|21°.0|22°.0|23°.0|24°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    80  |  54 |  51 |  47 |  44 |  41 |  38 |  35 |  32 |  29 |  26 |  23 |  20 |  18 |  80
    81  |  54 |  51 |  48 |  44 |  41 |  38 |  35 |  33 |  30 |  27 |  24 |  21 |  19 |  81
    82  |  55 |  52 |  48 |  45 |  42 |  39 |  36 |  33 |  31 |  28 |  25 |  22 |  20 |  82
    83  |  55 |  52 |  49 |  46 |  43 |  40 |  37 |  34 |  31 |  29 |  26 |  23 |  21 |  83
    84  |  56 |  53 |  49 |  46 |  44 |  41 |  38 |  35 |  32 |  29 |  27 |  24 |  22 |  84
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    85  |  56 |  53 |  50 |  47 |  44 |  41 |  38 |  36 |  33 |  30 |  28 |  25 |  22 |  85
    86  |  57 |  54 |  51 |  48 |  45 |  42 |  39 |  36 |  34 |  31 |  29 |  26 |  23 |  86
    87  |  57 |  54 |  51 |  48 |  45 |  42 |  40 |  37 |  34 |  32 |  30 |  27 |  24 |  87
    88  |  58 |  55 |  52 |  49 |  46 |  43 |  40 |  38 |  35 |  32 |  30 |  27 |  25 |  88
    89  |  58 |  55 |  52 |  49 |  46 |  44 |  41 |  38 |  36 |  33 |  31 |  28 |  26 |  89
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    90  |  59 |  56 |  53 |  50 |  47 |  44 |  41 |  39 |  36 |  34 |  32 |  29 |  26 |  90
    91  |  59 |  56 |  53 |  50 |  47 |  45 |  42 |  39 |  37 |  35 |  33 |  30 |  27 |  91
    92  |  59 |  56 |  54 |  51 |  48 |  45 |  43 |  40 |  37 |  35 |  33 |  30 |  28 |  92
    93  |  60 |  57 |  54 |  51 |  48 |  46 |  43 |  41 |  38 |  36 |  34 |  31 |  29 |  93
    94  |  60 |  57 |  54 |  52 |  49 |  46 |  44 |  41 |  39 |  36 |  34 |  31 |  29 |  94
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    95  |  60 |  58 |  55 |  52 |  49 |  47 |  44 |  42 |  39 |  37 |  35 |  32 |  30 |  95
    96  |  61 |  58 |  55 |  53 |  50 |  47 |  45 |  42 |  40 |  37 |  36 |  33 |  30 |  96
    97  |  61 |  58 |  56 |  53 |  50 |  48 |  45 |  43 |  40 |  38 |  36 |  33 |  31 |  97
    98  |  61 |  59 |  56 |  53 |  51 |  48 |  46 |  43 |  41 |  38 |  37 |  34 |  32 |  98
    99  |  62 |  59 |  56 |  54 |  51 |  49 |  46 |  44 |  41 |  39 |  37 |  34 |  32 |  99
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   100  |  62 |  59 |  57 |  54 |  51 |  49 |  47 |  44 |  42 |  39 |  37 |  35 |  33 | 100
   101  |  62 |  60 |  57 |  54 |  52 |  49 |  47 |  45 |  42 |  40 |  38 |  36 |  33 | 101
   102  |  63 |  60 |  57 |  55 |  52 |  50 |  47 |  45 |  43 |  40 |  38 |  36 |  34 | 102
   103  |  63 |  60 |  58 |  55 |  53 |  50 |  48 |  45 |  43 |  41 |  39 |  37 |  34 | 103
   104  |  63 |  61 |  58 |  55 |  53 |  51 |  48 |  46 |  44 |  41 |  39 |  37 |  35 | 104
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   105  |  64 |  61 |  58 |  56 |  53 |  51 |  49 |  46 |  44 |  42 |  40 |  38 |  35 | 105
   106  |  64 |  61 |  59 |  56 |  54 |  51 |  49 |  47 |  44 |  42 |  40 |  38 |  36 | 106
   107  |  64 |  62 |  59 |  57 |  54 |  52 |  49 |  47 |  45 |  43 |  41 |  38 |  36 | 107
   108  |  64 |  62 |  59 |  57 |  54 |  52 |  50 |  47 |  45 |  43 |  41 |  39 |  37 | 108
   109  |  65 |  62 |  60 |  57 |  55 |  52 |  50 |  48 |  46 |  44 |  41 |  39 |  37 | 109
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   110  |  65 |  62 |  60 |  57 |  55 |  53 |  50 |  48 |  46 |  44 |  42 |  40 |  38 | 110
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ |12°.0|13°.0|14°.0|15°.0|16°.0|17°.0|18°.0|19°.0|20°.0|21°.0|22°.0|23°.0|24°.0| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

                TABLE II.——RELATIVE HUMIDITY, PER CENT.

  ======+=============================================================================+======
   _t_  |          Difference between the dry and wet thermometers (_t-t′_).          | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)|24°.0|25°.0|26°.0|27°.0|28°.0|29°.0|30°.0|31°.0|32°.0|33°.0|34°.0|35°.0|36°.0|ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    80  |  18 |  15 |  13 |  10 |   8 |   6 |   3 |   1 |     |     |     |     |     |  80
    81  |  19 |  16 |  14 |  11 |   9 |   7 |   4 |   2 |     |     |     |     |     |  81
    82  |  20 |  17 |  15 |  12 |  10 |   8 |   5 |   3 |   1 |     |     |     |     |  82
    83  |  21 |  18 |  16 |  13 |  11 |   9 |   6 |   4 |   2 |     |     |     |     |  83
    84  |  22 |  19 |  17 |  14 |  12 |  10 |   8 |   5 |   3 |   1 |     |     |     |  84
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    85  |  22 |  20 |  17 |  15 |  13 |  11 |   9 |   6 |   4 |   2 |     |     |     |  85
    86  |  23 |  21 |  18 |  16 |  14 |  12 |  10 |   7 |   5 |   3 |   1 |     |     |  86
    87  |  24 |  22 |  19 |  17 |  15 |  13 |  11 |   8 |   6 |   4 |   2 |     |     |  87
    88  |  25 |  22 |  20 |  18 |  16 |  14 |  12 |   9 |   7 |   5 |   3 |   1 |     |  88
    89  |  26 |  23 |  21 |  19 |  16 |  14 |  12 |  10 |   8 |   6 |   4 |   2 |   1 |  89
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    90  |  26 |  24 |  22 |  20 |  17 |  15 |  13 |  11 |   9 |   7 |   5 |   3 |   2 |  90
    91  |  27 |  25 |  23 |  20 |  18 |  16 |  14 |  12 |  10 |   8 |   6 |   4 |   3 |  91
    92  |  28 |  26 |  23 |  21 |  19 |  17 |  15 |  13 |  11 |   9 |   7 |   5 |   3 |  92
    93  |  29 |  26 |  24 |  22 |  20 |  18 |  16 |  14 |  12 |  10 |   8 |   6 |   4 |  93
    94  |  29 |  27 |  25 |  23 |  21 |  18 |  16 |  14 |  13 |  11 |   9 |   7 |   5 |  94
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
    95  |  30 |  28 |  25 |  23 |  21 |  19 |  17 |  15 |  13 |  11 |  10 |   8 |   6 |  95
    96  |  30 |  28 |  26 |  24 |  22 |  20 |  18 |  16 |  14 |  12 |  10 |   9 |   7 |  96
    97  |  31 |  29 |  27 |  25 |  23 |  21 |  19 |  17 |  15 |  13 |  11 |  10 |   8 |  97
    98  |  32 |  29 |  27 |  25 |  23 |  21 |  19 |  18 |  16 |  14 |  12 |  10 |   9 |  98
    99  |  32 |  30 |  28 |  26 |  24 |  22 |  20 |  18 |  16 |  15 |  13 |  11 |  10 |  99
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   100  |  33 |  31 |  29 |  27 |  25 |  23 |  21 |  19 |  17 |  15 |  14 |  12 |  10 | 100
   101  |  33 |  31 |  29 |  27 |  25 |  23 |  21 |  20 |  18 |  16 |  14 |  13 |  11 | 101
   102  |  34 |  32 |  30 |  28 |  26 |  24 |  22 |  20 |  19 |  17 |  15 |  13 |  12 | 102
   103  |  34 |  32 |  30 |  28 |  26 |  25 |  23 |  21 |  19 |  17 |  16 |  14 |  12 | 103
   104  |  35 |  33 |  31 |  29 |  27 |  25 |  23 |  22 |  20 |  18 |  16 |  15 |  13 | 104
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   105  |  35 |  33 |  31 |  30 |  28 |  26 |  24 |  22 |  20 |  19 |  17 |  15 |  14 | 105
   106  |  36 |  34 |  32 |  30 |  28 |  26 |  25 |  23 |  21 |  19 |  18 |  16 |  14 | 106
   107  |  36 |  34 |  32 |  31 |  29 |  27 |  25 |  23 |  22 |  20 |  18 |  17 |  15 | 107
   108  |  37 |  35 |  33 |  31 |  29 |  27 |  26 |  24 |  22 |  21 |  19 |  17 |  16 | 108
   109  |  37 |  35 |  33 |  32 |  30 |  28 |  26 |  25 |  23 |  21 |  20 |  18 |  16 | 109
        |     |     |     |     |     |     |     |     |     |     |     |     |     |
   110  |  38 |  36 |  34 |  32 |  30 |  28 |  27 |  25 |  23 |  22 |  20 |  19 |  17 | 110
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ |24°.0|25°.0|26°.0|27°.0|28°.0|29°.0|30°.0|31°.0|32°.0|33°.0|34°.0|35°.0|36°.0| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

                TABLE II.——RELATIVE HUMIDITY, PER CENT.

  ======+=======================================================================+======
   _t_  |       Difference between the dry and wet thermometers (_t-t′_).       | _t_
  (Dry  +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+(Dry
  ther.)|36°.0|37°.0|38°.0|39°.0|40°.0|41°.0|42°.0|43°.0|44°.0|45°.0|46°.0|48°.0|Ther.)
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
    89  |   1 |     |     |     |     |     |     |     |     |     |     |     |  89
        |     |     |     |     |     |     |     |     |     |     |     |     |
    90  |   2 |     |     |     |     |     |     |     |     |     |     |     |  90
    91  |   3 |   1 |     |     |     |     |     |     |     |     |     |     |  91
    92  |   3 |   2 |   1 |     |     |     |     |     |     |     |     |     |  92
    93  |   4 |   3 |   2 |     |     |     |     |     |     |     |     |     |  93
    94  |   5 |   4 |   3 |   0 |     |     |     |     |     |     |     |     |  94
        |     |     |     |     |     |     |     |     |     |     |     |     |
    95  |   6 |   5 |   4 |   1 |     |     |     |     |     |     |     |     |  95
    96  |   7 |   5 |   5 |   2 |   1 |     |     |     |     |     |     |     |  96
    97  |   8 |   6 |   6 |   3 |   1 |     |     |     |     |     |     |     |  97
    98  |   9 |   7 |   7 |   4 |   2 |   1 |     |     |     |     |     |     |  98
    99  |  10 |   8 |   7 |   5 |   3 |   2 |   0 |     |     |     |     |     |  99
        |     |     |     |     |     |     |     |     |     |     |     |     |
   100  |  10 |   9 |   8 |   6 |   4 |   3 |   1 |     |     |     |     |     | 100
   101  |  11 |   9 |   9 |   6 |   5 |   3 |   2 |   0 |     |     |     |     | 101
   102  |  12 |  10 |   9 |   7 |   6 |   4 |   3 |   1 |     |     |     |     | 102
   103  |  12 |  11 |   9 |   8 |   6 |   5 |   4 |   2 |   1 |     |     |     | 103
   104  |  13 |  12 |  10 |   8 |   7 |   6 |   5 |   3 |   2 |     |     |     | 104
        |     |     |     |     |     |     |     |     |     |     |     |     |
   105  |  14 |  12 |  11 |   9 |   8 |   6 |   5 |   4 |   2 |   1 |     |     | 105
   106  |  14 |  13 |  11 |  10 |   8 |   7 |   6 |   4 |   3 |   2 |   0 |     | 106
   107  |  15 |  14 |  12 |  11 |   9 |   8 |   6 |   5 |   4 |   3 |   1 |     | 107
   108  |  16 |  14 |  13 |  11 |  10 |   9 |   7 |   6 |   5 |   3 |   2 |     | 108
   109  |  16 |  15 |  13 |  12 |  10 |   9 |   8 |   7 |   5 |   4 |   3 |     | 109
        |     |     |     |     |     |     |     |     |     |     |     |     |
   110  |  17 |  15 |  14 |  13 |  11 |  10 |   8 |   7 |   6 |   5 |   3 |   1 | 110
  ------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------
   _t._ |36°.0|37°.0|38°.0|39°.0|40°.0|41°.0|42°.0|43°.0|44°.0|45°.0|46°.0|48°.0| _t._
  ======+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+======

           TABLE III.——REDUCTION OF BAROMETER READING TO 32°.

  ===+=========================================================================================
     |                                         Inches.
     +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
  °F | 24.0| 24.5| 25.0| 25.5| 26.0| 26.5| 27.0| 27.5| 28.0| 28.5| 29.0| 29.5| 30.0| 30.5| 31.0
  ---+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
   30|-.003|-.003|-.003|-.003|-.003|-.003|-.003|-.003|-.003|-.004|-.004|-.004|-.004|-.004|-.004
   31| .005| .005| .005| .005| .006| .006| .006| .006| .006| .006| .006| .006| .006| .007| .007
   32| .007| .008| .008| .008| .008| .008| .008| .008| .009| .009| .009| .009| .009| .009| .009
   33| .010| .010| .010| .010| .010| .010| .011| .011| .011| .011| .012| .012| .012| .012| .012
   34| .012| .012| .012| .012| .013| .013| .013| .013| .014| .014| .014| .014| .015| .015| .015
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   35| .014| .014| .014| .015| .015| .015| .016| .016| .016| .016| .017| .017| .017| .018| .018
   36| .016| .016| .017| .017| .017| .018| .018| .018| .019| .019| .019| .020| .020| .020| .021
   37| .018| .019| .019| .019| .020| .020| .021| .021| .021| .022| .022| .022| .023| .023| .024
   38| .020| .021| .021| .022| .022| .022| .023| .023| .024| .024| .025| .025| .026| .026| .026
   39| .023| .023| .024| .024| .024| .025| .025| .026| .026| .027| .027| .028| .028| .029| .029
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   40| .025| .025| .026| .026| .027| .027| .028| .028| .029| .030| .030| .030| .031| .031| .032
   41| .027| .027| .028| .029| .029| .030| .030| .031| .031| .032| .033| .033| .034| .034| .035
   42| .029| .030| .030| .031| .032| .032| .033| .033| .034| .034| .035| .036| .036| .037| .038
   43| .031| .032| .033| .033| .034| .035| .035| .036| .036| .037| .038| .038| .039| .040| .040
   44| .033| .034| .035| .035| .036| .037| .038| .038| .039| .040| .040| .041| .042| .042| .043
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   45| .036| .037| .037| .038| .039| .039| .040| .041| .042| .042| .043| .044| .045| .045| .046
   46| .038| .038| .039| .040| .041| .042| .043| .043| .044| .045| .046| .046| .047| .048| .049
   47| .040| .041| .042| .042| .043| .044| .045| .046| .047| .048| .048| .049| .050| .051| .052
   48| .042| .043| .044| .045| .046| .047| .047| .048| .049| .050| .051| .052| .053| .053| .054
   49| .044| .045| .046| .047| .048| .049| .050| .051| .052| .052| .054| .054| .055| .056| .057
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   50| .046| .047| .048| .049| .050| .051| .052| .053| .054| .055| .056| .057| .058| .059| .060
   51| .049| .050| .051| .052| .053| .054| .055| .056| .057| .058| .059| .060| .061| .062| .063
   52| .051| .052| .053| .054| .055| .056| .057| .058| .059| .060| .061| .062| .064| .065| .066
   53| .053| .054| .055| .056| .057| .058| .060| .061| .062| .063| .064| .065| .066| .067| .068
   54| .055| .056| .057| .058| .060| .061| .062| .063| .064| .065| .067| .068| .069| .070| .071
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   55| .057| .058| .060| .061| .062| .063| .064| .065| .066| .068| .069| .070| .071| .073| .074
   56| .060| .061| .062| .063| .064| .065| .067| .068| .069| .070| .072| .073| .074| .075| .077
   57| .062| .063| .064| .065| .067| .068| .069| .070| .072| .073| .075| .076| .077| .078| .080
   58| .064| .065| .066| .068| .069| .070| .071| .073| .074| .076| .077| .078| .080| .081| .082
   59| .066| .068| .069| .070| .072| .073| .074| .075| .077| .078| .080| .081| .083| .084| .085
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   60| .068| .070| .071| .072| .074| .076| .077| .078| .079| .081| .082| .084| .085| .086| .088
   61| .070| .072| .073| .074| .076| .077| .079| .080| .082| .083| .085| .086| .088| .089| .091
   62| .073| .074| .076| .077| .079| .080| .082| .083| .085| .086| .088| .089| .091| .092| .094
   63| .075| .076| .078| .079| .081| .082| .084| .085| .087| .088| .090| .091| .093| .095| .096
   64| .077| .078| .080| .081| .083| .085| .086| .088| .090| .091| .093| .094| .096| .097| .099
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   65|-.079|-.080|-.082|-.084|-.086|-.087|-.089|-.090|-.092|-.093|-.095|-.097|-.099|-.100|-.102
  ===+=========================================================================================

    TABLE III.——REDUCTION OF BAROMETER READING TO 32°.——_Continued._

  ===+=========================================================================================
     |                                         Inches.
     +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
  °F | 24.0| 24.5| 25.0| 25.5| 26.0| 26.5| 27.0| 27.5| 28.0| 28.5| 29.0| 29.5| 30.0| 30.5| 31.0
  ---+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
   65|-.079|-.080|-.082|-.084|-.086|-.087|-.089|-.090|-.092|-.093|-.095|-.097|-.099|-.100|-.102
   66| .081| .083| .085| .086| .088| .089| .091| .093| .095| .096| .098| .099| .101| .103| .105
   67| .083| .085| .087| .088| .090| .092| .094| .095| .097| .099| .101| .102| .104| .106| .108
   68| .085| .087| .089| .090| .093| .094| .096| .098| .100| .101| .103| .105| .107| .108| .110
   69| .088| .089| .091| .093| .095| .097| .099| .100| .102| .104| .106| .107| .110| .111| .113
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   70| .090| .092| .094| .096| .097| .099| .101| .103| .105| .106| .109| .110| .112| .114| .116
   71| .092| .094| .096| .098| .100| .101| .103| .105| .107| .109| .111| .113| .115| .116| .119
   72| .094| .096| .098| .100| .102| .104| .106| .108| .110| .112| .114| .116| .118| .120| .122
   73| .096| .098| .100| .102| .104| .106| .108| .110| .112| .114| .116| .118| .120| .122| .124
   74| .098| .100| .103| .105| .107| .109| .111| .113| .115| .117| .119| .121| .123| .125| .127
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   75| .101| .102| .105| .106| .109| .111| .113| .115| .117| .119| .122| .124| .126| .128| .130
   76| .103| .104| .107| .109| .111| .113| .116| .118| .120| .122| .124| .126| .128| .130| .133
   77| .105| .107| .109| .111| .114| .116| .118| .120| .122| .124| .127| .129| .131| .133| .136
   78| .107| .109| .112| .113| .116| .118| .120| .122| .125| .127| .129| .131| .134| .136| .138
   79| .109| .111| .114| .116| .118| .120| .123| .125| .127| .129| .132| .134| .137| .139| .141
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   80| .111| .113| .116| .118| .121| .123| .125| .127| .130| .132| .135| .137| .139| .141| .144
   81| .114| .116| .118| .120| .123| .125| .128| .130| .132| .134| .137| .139| .142| .144| .147
   82| .116| .118| .121| .122| .125| .128| .130| .132| .135| .137| .140| .142| .145| .147| .149
   83| .118| .120| .123| .125| .128| .130| .133| .135| .138| .140| .142| .145| .147| .149| .152
   84| .120| .122| .125| .127| .130| .132| .135| .138| .140| .142| .145| .147| .150| .152| .155
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   85| .122| .124| .127| .129| .132| .134| .137| .139| .143| .145| .148| .150| .153| .155| .158
   86| .124| .126| .128| .130| .135| .137| .140| .143| .145| .148| .150| .153| .155| .158| .161
   87| .126| .129| .132| .134| .137| .139| .142| .144| .148| .150| .153| .155| .158| .161| .163
   88| .129| .131| .134| .137| .139| .142| .145| .147| .150| .152| .155| .158| .161| .163| .166
   89| .131| .133| .136| .139| .142| .144| .147| .150| .153| .155| .158| .161| .164| .166| .169
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   90| .133| .136| .138| .141| .144| .147| .150| .153| .155| .157| .161| .164| .166| .169| .172
   91| .135| .138| .141| .143| .146| .149| .152| .155| .158| .160| .163| .166| .169| .172| .175
   92| .137| .140| .143| .146| .149| .152| .154| .157| .160| .163| .166| .169| .172| .175| .177
   93| .139| .142| .145| .148| .151| .154| .157| .160| .163| .166| .168| .171| .174| .177| .180
   94| .142| .145| .147| .150| .153| .156| .159| .162| .165| .168| .171| .174| .177| .180| .183
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
   95| .144| .147| .150| .153| .156| .159| .162| .165| .168| .171| .174| .177| .180| .183| .186
   96| .146| .149| .152| .155| .158| .161| .164| .167| .170| .173| .176| .179| .182| .185| .188
   97| .148| .151| .154| .157| .160| .164| .167| .170| .173| .176| .179| .182| .185| .188| .191
   98| .150| .153| .156| .160| .163| .166| .169| .172| .175| .178| .181| .185| .188| .191| .194
   99| .152| .155| .159| .162| .165| .168| .171| .175| .178| .181| .184| .187| .190| .194| .197
     |     |     |     |     |     |     |     |     |     |     |     |     |     |     |
  100|-.154|-.157|-.161|-.164|-.167|-.171|-.174|-.177|-.180|-.184|-.187|-.190|-.193|-.197|-.200
  ---+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----

     TABLE IV.——TABLE FOR REDUCING OBSERVATIONS OF THE BAROMETER TO
                    SEA LEVEL, CORRECTION ADDITIVE.

  ==========+=======================================================================
  Height, in|          Temperature of external air——degrees Fahrenheit.
  feet.     |
      +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
      | -20°| -10°|  0° |  10°|  20°|  30°|  40°|  50°|  60°|  70°|  80°|  90°| 100°
  ----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
    10| .013| .013| .012| .012| .012| .012| .011| .011| .011| .011| .010| .010| .010
    20| .026| .025| .025| .024| .023| .023| .023| .022| .022| .021| .021| .020| .020
    30| .039| .038| .037| .036| .035| .034| .034| .033| .032| .032| .031| .030| .030
    40| .052| .050| .049| .048| .047| .046| .045| .044| .043| .042| .041| .040| .040
      |     |     |     |     |     |     |     |     |     |     |     |     |
    50| .065| .063| .061| .060| .059| .058| .056| .055| .054| .053| .052| .051| .050
    60| .077| .076| .074| .072| .070| .069| .068| .066| .065| .063| .062| .061| .059
    70| .090| .088| .086| .084| .082| .081| .078| .077| .076| .074| .072| .071| .069
    80| .103| .101| .098| .096| .094| .092| .090| .088| .086| .084| .082| .081| .079
    90| .116| .113| .111| .108| .105| .104| .101| .099| .097| .095| .093| .091| .089
      |     |     |     |     |     |     |     |     |     |     |     |     |
   100| .129| .126| .123| .120| .117| .115| .112| .110| .108| .105| .103| .101| .099
   110| .142| .139| .135| .132| .129| .126| .123| .121| .119| .116| .113| .111| .109
   120| .155| .151| .148| .144| .140| .138| .134| .132| .129| .126| .124| .121| .119
   130| .168| .164| .160| .156| .152| .149| .146| .143| .140| .137| .134| .131| .129
   140| .181| .176| .172| .168| .164| .161| .157| .154| .151| .147| .144| .141| .139
      |     |     |     |     |     |     |     |     |     |     |     |     |
   150| .194| .189| .185| .180| .176| .172| .168| .165| .162| .158| .155| .152| .149
   160| .206| .201| .197| .192| .187| .183| .179| .176| .172| .168| .165| .162| .158
   170| .219| .214| .209| .204| .199| .195| .190| .187| .183| .179| .175| .172| .168
   180| .232| .227| .222| .216| .211| .206| .202| .198| .194| .189| .185| .182| .178
   190| .245| .239| .234| .228| .222| .218| .213| .209| .204| .200| .196| .192| .188
      |     |     |     |     |     |     |     |     |     |     |     |     |
   200| .258| .252| .246| .240| .234| .229| .224| .220| .215| .210| .206| .202| .198
   210| .271| .264| .258| .252| .246| .240| .235| .231| .226| .221| .216| .212| .208
   220| .284| .277| .270| .264| .257| .252| .246| .242| .236| .231| .227| .222| .218
   230| .296| .289| .283| .276| .269| .263| .257| .253| .247| .242| .237| .232| .228
   240| .309| .302| .295| .288| .281| .275| .269| .264| .258| .252| .248| .242| .238
      |     |     |     |     |     |     |     |     |     |     |     |     |
   250| .322| .314| .307| .300| .293| .286| .280| .275| .269| .263| .258| .253| .248
   260| .335| .327| .319| .311| .304| .297| .291| .285| .279| .273| .268| .263| .257
   270| .348| .339| .331| .323| .316| .309| .302| .296| .290| .284| .278| .273| .267
   280| .360| .352| .344| .335| .328| .320| .314| .307| .301| .294| .288| .283| .277
   290| .373| .364| .356| .347| .339| .332| .325| .318| .311| .305| .299| .293| .287
      |     |     |     |     |     |     |     |     |     |     |     |     |
   300| .386| .377| .368| .359| .351| .343| .336| .329| .322| .315| .309| .303| .297
   310| .399| .389| .380| .371| .363| .354| .347| .340| .333| .326| .319| .313| .307
   320| .412| .402| .392| .383| .374| .366| .358| .351| .343| .336| .329| .323| .317
   330| .424| .414| .404| .395| .386| .377| .369| .362| .354| .347| .340| .333| .326
   340| .437| .427| .416| .407| .397| .389| .380| .373| .365| .357| .350| .343| .336
      |     |     |     |     |     |     |     |     |     |     |     |     |
   350| .450| .439| .429| .419| .409| .400| .392| .384| .376| .368| .360| .353| .346
   360| .463| .451| .441| .430| .421| .411| .403| .394| .386| .378| .370| .363| .356
   370| .476| .464| .453| .442| .432| .423| .414| .405| .397| .389| .380| .373| .366
   380| .488| .476| .465| .454| .444| .434| .425| .416| .408| .399| .391| .383| .375
   390| .501| .489| .477| .466| .455| .446| .436| .427| .418| .410| .401| .393| .385
      |     |     |     |     |     |     |     |     |     |     |     |     |
   400| .514| .501| .489| .478| .467| .457| .447| .438| .429| .420| .411| .403| .395
   410| .527| .513| .501| .490| .479| .468| .458| .449| .440| .430| .421| .413| .405
   420| .539| .526| .513| .502| .490| .480| .469| .460| .450| .441| .431| .423| .415
   430| .552| .538| .525| .513| .502| .491| .480| .470| .461| .451| .442| .433| .425
   440| .565| .551| .537| .525| .513| .502| .491| .481| .471| .462| .452| .443| .434
      |     |     |     |     |     |     |     |     |     |     |     |     |
   450| .578| .563| .550| .537| .525| .513| .503| .492| .482| .472| .462| .453| .444
   460| .590| .575| .562| .549| .537| .525| .514| .503| .493| .482| .472| .463| .454
   470| .603| .588| .574| .561| .548| .536| .525| .514| .503| .493| .482| .473| .464
   480| .616| .600| .586| .572| .560| .547| .536| .524| .514| .503| .493| .483| .474
   490| .628| .613| .598| .584| .571| .559| .547| .535| .524| .514| .503| .493| .483
      |     |     |     |     |     |     |     |     |     |     |     |     |
   500| .641| .625| .610| .596| .583| .570| .558| .546| .535| .524| .513| .503| .493
   510| .654| .637| .622| .608| .594| .581| .569| .557| .545| .534| .523| .513| .503
   520| .666| .650| .634| .620| .606| .593| .580| .568| .556| .545| .533| .523| .513
   530| .679| .662| .646| .631| .617| .604| .591| .578| .566| .555| .544| .533| .522
   540| .691| .675| .658| .643| .629| .615| .602| .589| .577| .565| .554| .543| .532
  ====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====

      TABLE IV.——FOR REDUCING OBSERVATIONS OF THE BAROMETER TO SEA
                          LEVEL.——_Continued._

  ==========+=======================================================================
  Height, in|          Temperature of external air——degrees Fahrenheit.
  feet.     |
      +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
      | -20°| -10°|  0° |  10°|  20°|  30°|  40°|  50°|  60°|  70°|  80°|  90°| 100°
  ----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
   550| .704| .687| .670| .655| .640| .626| .613| .600| .587| .575| .564| .553| .542
   560| .717| .699| .683| .667| .652| .638| .624| .611| .598| .586| .574| .563| .552
   570| .729| .712| .695| .679| .663| .649| .635| .622| .608| .596| .584| .573| .562
   580| .742| .724| .707| .690| .675| .660| .646| .632| .619| .606| .595| .583| .571
   590| .754| .737| .719| .702| .686| .672| .657| .643| .629| .617| .605| .593| .581
      |     |     |     |     |     |     |     |     |     |     |     |     |
   600| .767| .749| .731| .714| .698| .683| .668| .654| .640| .627| .615| .603| .591
   610| .780| .761| .743| .726| .709| .694| .679| .665| .650| .637| .625| .613| .601
   620| .792| .774| .755| .738| .721| .705| .690| .675| .661| .648| .635| .623| .611
   630| .805| .786| .767| .749| .732| .717| .701| .686| .671| .658| .645| .633| .620
   640| .817| .798| .779| .761| .744| .728| .712| .697| .682| .668| .655| .643| .630
      |     |     |     |     |     |     |     |     |     |     |     |     |
   650| .830| .811| .791| .773| .755| .739| .723| .708| .692| .679| .666| .653| .640
   660| .843| .823| .803| .785| .767| .750| .734| .718| .703| .689| .676| .662| .650
   670| .855| .835| .815| .797| .778| .761| .745| .729| .713| .699| .686| .672| .660
   680| .868| .847| .827| .808| .790| .773| .756| .740| .724| .709| .696| .682| .669
   690| .880| .860| .839| .820| .801| .784| .767| .750| .734| .720| .706| .692| .679
      |     |     |     |     |     |     |     |     |     |     |     |     |
   700| .893| .872| .851| .832| .813| .795| .778| .761| .745| .730| .716| .702| .689
   710| .905| .884| .863| .844| .824| .806| .789| .772| .755| .740| .726| .712| .698
   720| .918| .896| .875| .855| .836| .817| .800| .782| .766| .751| .736| .722| .708
   730| .930| .909| .887| .867| .847| .829| .811| .793| .776| .761| .746| .732| .718
   740| .943| .921| .899| .879| .859| .840| .822| .804| .787| .771| .756| .742| .728
      |     |     |     |     |     |     |     |     |     |     |     |     |
   750| .955| .933| .911| .891| .870| .851| .833| .815| .797| .782| .767| .752| .738
   760| .968| .945| .922| .902| .881| .862| .843| .825| .808| .792| .777| .761| .747
   770| .980| .957| .934| .914| .893| .873| .854| .836| .818| .802| .787| .771| .757
   780| .993| .970| .946| .926| .904| .885| .865| .847| .829| .812| .797| .781| .767
   790|1.005| .982| .958| .937| .916| .896| .876| .857| .839| .823| .807| .791| .776
      |     |     |     |     |     |     |     |     |     |     |     |     |
   800|1.018| .994| .970| .949| .927| .907| .887| .868| .850| .833| .817| .801| .786
   810|1.030|1.006| .982| .961| .938| .918| .898| .878| .860| .843| .827| .811| .796
   820|1.043|1.018| .994| .972| .950| .929| .909| .889| .871| .854| .937| .821| .805
   830|1.055|1.031|1.006| .984| .961| .940| .920| .900| .881| .864| .847| .831| .815
   840|1.068|1.043|1.018| .995| .973| .951| .931| .911| .892| .874| .857| .841| .825
      |     |     |     |     |     |     |     |     |     |     |     |     |
   850|1.080|1.055|1.030|1.007| .984| .962| .942| .922| .902| .885| .867| .851| .835
   860|1.093|1.067|1.041|1.019| .995| .974| .952| .932| .913| .895| .877| .860| .844
   870|1.105|1.079|1.053|1.030|1.007| .985| .963| .943| .923| .905| .887| .870| .854
   880|1.118|1.092|1.065|1.042|1.018| .996| .974| .954| .934| .915| .897| .880| .864
   890|1.130|1.104|1.077|1.053|1.030|1.007| .985| .904| .944| .926| .907| .890| .873
      |     |     |     |     |     |     |     |     |     |     |     |     |
   900|1.143|1.116|1.089|1.065|1.041|1.018| .996| .975| .955| .936| .917| .900| .883
   910|1.155|1.128|1.101|1.077|1.052|1.029|1.007| .986| .965| .946| .927| .910| .893
   920|1.168|1.140|1.113|1.088|1.064|1.040|1.018| .996| .976| .956| .937| .920| .902
   930|1.180|1.152|1.125|1.100|1.075|1.051|1.029|1.007| .986| .967| .947| .929| .912
   940|1.193|1.164|1.137|1.111|1.086|1.062|1.040|1.017| .997| .977| .957| .939| .921
      |     |     |     |     |     |     |     |     |     |     |     |     |
   950|1.205|1.177|1.149|1.123|1.098|1.074|1.051|1.028|1.007| .987| .967| .949| .931
   960|1.217|1.189|1.160|1.135|1.109|1.085|1.061|1.039|1.017| .997| .977| .959| .941
   970|1.230|1.201|1.172|1.146|1.120|1.096|1.072|1.049|1.028|1.007| .987| .969| .950
   980|1.242|1.213|1.184|1.158|1.131|1.107|1.083|1.060|1.038|1.018| .997| .978| .960
   990|1.255|1.225|1.196|1.169|1.143|1.118|1.094|1.070|1.049|1.028|1.007| .988| .969
      |     |     |     |     |     |     |     |     |     |     |     |     |
  1000|1.267|1.237|1.208|1.181|1.154|1.129|1.105|1.081|1.059|1.038|1.017| .998| .979
  1010|1.279|1.249|1.220|1.192|1.165|1.140|1.116|1.092|1.069|1.048|1.027|1.008| .989
  1020|1.292|1.261|1.232|1.204|1.177|1.151|1.127|1.102|1.080|1.058|1.037|1.018| .998
  1030|1.304|1.273|1.243|1.215|1.188|1.162|1.137|1.113|1.090|1.069|1.047|1.027|1.008
  1040|1.317|1.285|1.255|1.227|1.199|1.173|1.148|1.123|1.101|1.079|1.057|1.037|1.017
      |     |     |     |     |     |     |     |     |     |     |     |     |
  1050|1.329|1.298|1.267|1.238|1.211|1.184|1.159|1.134|1.111|1.089|1.067|1.047|1.027
  1060|1.341|1.310|1.279|1.250|1.222|1.195|1.170|1.145|1.121|1.099|1.077|1.057|1.037
  1070|1.354|1.322|1.291|1.261|1.233|1.206|1.181|1.155|1.132|1.109|1.087|1.067|1.046
  1080|1.366|1.334|1.302|1.273|1.244|1.217|1.191|1.166|1.142|1.120|1.097|1.076|1.056
  1090|1.379|1.346|1.314|1.284|1.256|1.228|1.202|1.176|1.153|1.130|1.107|1.086|1.065
  ====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====

      TABLE IV.——FOR REDUCING OBSERVATIONS OF THE BAROMETER TO SEA
                          LEVEL.——_Continued._

  ==========+=======================================================================
  Height, in|          Temperature of external air——degrees Fahrenheit.
  feet.     |
      +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
      | -20°| -10°|  0° |  10°|  20°|  30°|  40°|  50°|  60°|  70°|  80°|  90°| 100°
  ----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
  1100|1.391|1.358|1.326|1.296|1.267|1.239|1.213|1.187|1.163|1.140|1.117|1.096|1.075
  1110|1.403|1.370|1.338|1.307|1.278|1.250|1.224|1.198|1.173|1.150|1.127|1.106|1.085
  1120|1.416|1.382|1.350|1.319|1.289|1.261|1.235|1.208|1.184|1.160|1.137|1.115|1.094
  1130|1.428|1.394|1.361|1.330|1.301|1.272|1.245|1.219|1.194|1.170|1.147|1.125|1.104
  1140|1.440|1.406|1.373|1.342|1.312|1.283|1.256|1.229|1.204|1.180|1.157|1.135|1.113
      |     |     |     |     |     |     |     |     |     |     |     |     |
  1150|1.453|1.418|1.385|1.353|1.323|1.294|1.267|1.240|1.215|1.191|1.167|1.145|1.123
  1160|1.465|1.430|1.397|1.365|1.334|1.305|1.278|1.251|1.225|1.201|1.177|1.154|1.133
  1170|1.477|1.442|1.409|1.376|1.345|1.315|1.289|1.261|1.235|1.211|1.187|1.164|1.142
  1180|1.489|1.454|1.420|1.388|1.357|1.327|1.299|1.272|1.245|1.221|1.197|1.174|1.152
  1190|1.502|1.466|1.432|1.399|1.368|1.338|1.310|1.282|1.256|1.231|1.207|1.183|1.161
      |     |     |     |     |     |     |     |     |     |     |     |     |
  1200|1.514|1.478|1.444|1.411|1.379|1.349|1.321|1.293|1.266|1.241|1.217|1.193|1.171
  1210|1.526|1.490|1.456|1.422|1.390|1.360|1.332|1.303|1.276|1.251|1.227|1.203|1.180
  1220|1.539|1.502|1.467|1.434|1.401|1.371|1.342|1.314|1.288|1.261|1.237|1.212|1.190
  1230|1.551|1.514|1.479|1.445|1.413|1.382|1.353|1.324|1.297|1.271|1.247|1.222|1.199
  1240|1.563|1.526|1.491|1.457|1.424|1.393|1.364|1.335|1.307|1.281|1.257|1.232|1.209
      |     |     |     |     |     |     |     |     |     |     |     |     |
  1250|1.576|1.538|1.502|1.468|1.435|1.404|1.374|1.345|1.317|1.291|1.266|1.242|1.218
  1260|1.588|1.550|1.514|1.479|1.446|1.415|1.385|1.356|1.328|1.302|1.276|1.251|1.228
  1270|1.600|1.562|1.526|1.491|1.457|1.426|1.396|1.366|1.338|1.312|1.286|1.261|1.237
  1280|1.612|1.574|1.538|1.502|1.469|1.437|1.407|1.377|1.348|1.322|1.296|1.271|1.247
  1290|1.625|1.586|1.549|1.514|1.480|1.448|1.417|1.387|1.359|1.332|1.306|1.280|1.256
      |     |     |     |     |     |     |     |     |     |     |     |     |
  1300|1.637|1.598|1.561|1.525|1.491|1.459|1.428|1.398|1.369|1.342|1.316|1.290|1.266
  1310|1.649|1.610|1.573|1.536|1.502|1.470|1.439|1.408|1.379|1.352|1.326|1.300|1.275
  1320|1.661|1.622|1.584|1.548|1.513|1.481|1.449|1.419|1.390|1.362|1.336|1.309|1.285
  1330|1.674|1.634|1.596|1.559|1.525|1.492|1.460|1.429|1.400|1.372|1.346|1.319|1.294
  1340|1.686|1.640|1.608|1.571|1.536|1.503|1.471|1.440|1.410|1.382|1.356|1.329|1.304
      |     |     |     |     |     |     |     |     |     |     |     |     |
  1350|1.698|1.658|1.620|1.582|1.547|1.514|1.482|1.450|1.420|1.393|1.366|1.339|1.313
  1360|1.710|1.669|1.631|1.593|1.558|1.524|1.492|1.461|1.431|1.403|1.375|1.348|1.323
  1870|1.722|1.681|1.643|1.605|1.569|1.535|1.503|1.471|1.441|1.413|1.385|1.358|1.332
  1380|1.735|1.693|1.655|1.616|1.581|1.546|1.514|1.482|1.451|1.423|1.395|1.368|1.342
  1390|1.747|1.705|1.666|1.628|1.592|1.557|1.524|1.492|1.462|1.433|1.405|1.377|1.351
      |     |     |     |     |     |     |     |     |     |     |     |     |
  1400|1.759|1.717|1.678|1.639|1.603|1.568|1.535|1.503|1.472|1.443|1.415|1.387|1.361
  1410|1.771|1.729|1.690|1.650|1.614|1.579|1.546|1.513|1.482|1.453|1.425|1.397|1.370
  1420|1.783|1.741|1.701|1.662|1.625|1.590|1.556|1.524|1.492|1.463|1.435|1.406|1.380
  1430|1.796|1.753|1.713|1.673|1.636|1.601|1.567|1.534|1.503|1.473|1.444|1.416|1.389
  1440|1.808|1.765|1.724|1.685|1.047|1.612|1.577|1.545|1.513|1.483|1.454|1.426|1.399
      |     |     |     |     |     |     |     |     |     |     |     |     |
  1450|1.820|1.777|1.736|1.696|1.658|1.623|1.588|1.555|1.523|1.493|1.464|1.436|1.408
  1460|1.832|1.788|1.748|1.707|1.670|1.633|1.599|1.565|1.533|1.503|1.474|1.445|1.418
  1470|1.844|1.800|1.759|1.719|1.081|1.644|1.609|1.576|1.543|1.513|1.484|1.455|1.427
  1480|1.857|1.812|1.771|1.730|1.692|1.655|1.620|1.586|1.554|1.523|1.493|1.465|1.437
  1490|1.869|1.824|1.782|1.742|1.703|1.666|1.630|1.597|1.564|1.533|1.503|1.474|1.446
      |     |     |     |     |     |     |     |     |     |     |     |     |
  1500|1.881|1.836|1.794|1.753|1.714|1.677|1.641|1.607|1.574|1.543|1.513|1.484|1.456
  ====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====



                              APPENDIX A.

                        SUGGESTIONS TO TEACHERS.


It is the object of this book to lead the student to the independent
discovery of the most important facts in our ordinary weather conditions,
and of the interrelations of the different weather elements. This
practical study having taught something as to the real nature of
atmospheric phenomena by actual observation, rapid and substantial
progress may be made in the knowledge of the distribution and of the
explanation of similar phenomena in other parts of the world, as derived
through a study of the text-books. By means of this combination of the two
kinds of study, the inductive and the didactic, the advantages of both may
be preserved, and the slow progress of the first method and the unsound
progress of the second may be avoided. This book is not a text-book, and
it therefore does not attempt to give explanations of various phenomena
discovered by the class. Explanations will, of course, be called for by
the scholars, in increasing number as the work progresses, and the larger
relations of the study become apparent. It is best, if possible, to leave
the more complicated matters (such as the cause of the deflection of the
wind from the gradient, of cyclones and anticyclones, etc.) until the
subjects can be taken up in detail and fully explained, for instance in
the later years of the high school course. It is not advisable to raise
such complicated questions in the grammar school work if they can be
avoided. The teacher who has a fairly good knowledge of one comprehensive
modern text-book of meteorology, such as Davis’s _Elementary Meteorology_,
will find himself sufficiently well equipped to answer the questions put
by the class.

The value of the work outlined in this little book can be much increased
if the larger applications of the lessons here learned are strongly
emphasized. Suggestions along this line have been made in fine print
throughout the text, but the examples given may be further extended to the
great advantage of the student. Careful attention ought to be given to the
formulating and writing out of the generalizations reached by the class,
for in these written summaries the results are preserved in compact form.


                               CHAPTER I.

The work outlined in this chapter is adapted to the lower grades in the
grammar school. It is assumed that the pupils have already had some
preliminary training in the simplest non-instrumental weather
observations, such as can readily be made during the primary school years.
For the convenience of teachers who may desire it, a brief outline of work
suited to the primary school grades is here given. It is desirable that
even older scholars be given some such training as this before they take
up the exercises of Chapter II.

The central idea in this elementary work is to train the children in
intelligent weather observation, so that they may come to appreciate what
our typical weather changes are; that they may recognize the types as they
recur, and may see how each example differs from, or accords with, those
that have preceded it. We are all so directly affected by the weather
conditions prevailing at any time that even the youngest children are
forced, unconsciously to be sure, to take some notice of these changes.
The work of the teacher is, therefore, simply to direct attention to what
is already seen.

When the children come to school on some snowy winter day, with a
northeast wind, chilling and damp, attention may be called to the need of
overshoes and overcoats, to the piling up of the snow in deep drifts at
certain places near the school or in the town, while in other places the
ground is left bare; to the ease with which snowballs may be made, and to
other facts which will very readily suggest themselves. A day or two after
such a storm, when the sun is shining bright in a cloudless sky, when
there is no wind and the air is dry, cold, and crisp, the contrasts
between these two weather types should be brought out. Instead of snow we
now have sunshine: instead of a damp, chilling northeaster we now have a
calm and the air is dry; snowballs cannot easily be made in the early
morning because the snow is frozen hard and is too dry, but towards noon,
if the temperature rise high enough, there may be thawing on the tops or
sides of the snowdrifts, and there the snow becomes soft enough for
snowballing. Another weather type, often noted during our winter in the
central and eastern United States, and strongly contrasted with both of
the preceding conditions, is that which brings us a warm, damp, southerly
wind, frequently accompanied by heavy rains. As these damp winds blow over
snow-covered surfaces they become foggy and the ground is said to “smoke”;
the heavy rain rapidly melts the snow; slush and mud make bad walking;
rivers and brooks rise rapidly, perhaps overflowing their banks; low-lying
places become filled with standing water. These and other features should
all be brought out by the teacher, not by telling the class of them
directly, but by judicious questioning, and they should be contrasted with
the conditions which may immediately follow, when the storm has cleared
off, and when the low temperatures brought by a cold wave, with its dry
northwest wind, have resulted in freezing lakes, rivers, and brooks, and
when skating and sliding may be indulged in. Early summer weather
conditions, with their characteristic warm spells, cumulus clouds,
thunderstorms, and (near the coast) sea breezes, furnish another long list
of typical changes that should be just as carefully noted and described as
the more striking winter characteristics. Autumn types add further to the
list, which might be extended almost indefinitely.

One whole year of the grammar school course may well be given to the
observations suggested in Chapter I, provided that there is no need of
hastening on to the more advanced work. The advantage of extending the
course over a whole school year is great, because such extension gives
opportunity for becoming familiar with late summer, autumn, winter,
spring, and early summer weather types, and this is far better than
attempting to crowd all the work into one short season. The interest of a
class can easily be kept up throughout a school year by means of a
progressive system of observations. It is best to vary the observations
from time to time, and to arrange them so that, beginning with the more
simple, they shall gradually become more complete and more advanced as
the year goes on. Thus, starting with temperature observations alone,
these may be continued for one or two weeks before they are supplemented
by records of wind direction and velocity. After some practice in the
observation of these two weather elements (say during one month), data as
to the state of the sky may be added. Cloud observations themselves may
well be graded during successive weeks, so that, beginning with the
simplest notes concerning amounts of cloudiness, the pupils shall
gradually advance to the point of observing, and perhaps even of
sketching, the common cloud forms and their changes. Thus an important
step will have been taken towards appreciating the need of a standard
cloud classification, which may be given later.

The addition of records of precipitation completes the list of simple
non-instrumental weather observations, and these records, as well as the
cloud records, can easily be graded, so that, during successive weeks,
every week’s work shall be different from that of every other week. In
this progression from the simpler to the more complicated observations
lies the secret of making the work attractive. Nothing will sooner check
interest in the study than the necessity of making exactly the same
observations day after day and week after week throughout the year. A
graded course of non-instrumental observation, such as is suggested, gives
a very practical general knowledge of our common weather types and
changes, and of the relations of one weather element to another. The
questions asked under the different headings in this chapter are designed
to awaken the interest of the scholars, and to call their attention to the
more important points of diurnal, cyclonic, and seasonal changes in
weather elements. The teacher will readily think of other questions which
may be suggested for the consideration of the class.

Although the non-instrumental records are of little value for future
reference, as compared with the instrumental observations, they should
nevertheless be systematically preserved by the class in their record
books. After discussion of the daily observations made by the different
scholars, or by one of their number, the records may be written upon one
of the blackboards reserved for this purpose. At the close of the day, or
the next morning, the blackboard notes should be entered in a record book
kept in the schoolroom. The teacher may guide in the discussion of the
observations; may suggest points overlooked by the scholars; may draw
comparisons between the weather conditions of other weeks and of other
days. This talking over of the observations is most important, as it never
fails to bring out much of interest.


                              CHAPTER II.

This work may usually be begun in the early years of the grammar school
course, as soon as the non-instrumental observations have been
satisfactorily completed. The scheme of progressive observations already
suggested may be followed to advantage in the instrumental work as well as
in the non-instrumental. It is often a good plan to have a different
scholar assigned to the task of taking the observations every day, or it
may be more advisable to divide the work, making one responsible for the
temperature observations, another for the precipitation, etc. It is well
to have the daily instrumental weather records written upon the
blackboard in the schoolroom, as already suggested in the case of the
non-instrumental observations. At the end of each day the blackboard data
should be entered in a permanent record book by some one of the scholars,
and some ingenuity can be exercised in devising the best scheme for
keeping this record. The record book should be carefully preserved in the
schoolroom, where it may be referred to by the scholars of future years
when any unusually severe storm, or a spell of excessively hot or dry
weather, or a remarkable cold wave occurs, in order that comparison with
past occurrences of a similar kind may be made. It is well to have the
record book of large size, and to have each day’s record entered across
two full pages. On the left-hand page the temperature, pressure, rainfall,
wind direction and velocity, etc., may be entered, each observation in its
proper column, the number of columns being increased according to the
increasing number of observations. The right-hand page may be left for
“Remarks.” These “Remarks” should include notes of any meteorological
phenomena which did not find a place in the columns reserved for the
regular observations, _e.g._, occurrence of hail, or frozen rain; damage
by lightning, winds, or floods; freezing up of rivers or brooks;
interruption of railroad or street-car traffic by snow, etc., and, in
general, explanatory comments on the weather conditions. Instructive
lessons may be taught as to the relation of the local weather conditions
which prevail in the vicinity of the school, and those of other portions
of the country, by comments on newspaper despatches concerning gales along
the coast or on the lakes, and resulting damage to shipping; of snow
blockades and stalled trains; of severe thunderstorms and tornadoes; of
hot waves and sunstrokes, or of cold waves and the destruction of crops or
fruits by the frost. The scholars should be encouraged to bring into the
class any comments on such phenomena as may be of interest in the work.
Such of these newspaper clippings as are of the most value may be pasted
in the space reserved for the “Remarks,” where they may be referred to by
succeeding classes; and in this space also may be pasted at the end of
each week the barograph and thermograph sheets, if these instruments are
in use at the school.


                              CHAPTER III.

These observations may usually be profitably undertaken in the later
grammar and in the high school years. The instruments described, while all
desirable, are by no means all necessary, and no teacher should postpone
the establishment of a course in observational meteorology for the reason
that a complete set of first-class instruments cannot be secured at the
start.

If the school is provided with a psychrometer, there will be no need of
the ordinary thermometer, because the psychrometer gives the true air
temperature. It is well, however, to have both stationary wet and dry-bulb
thermometers, in the shelter, for ordinary school use, and also a sling
psychrometer for use in the meteorological _field work_ which forms an
important part of the more advanced instrumental work in meteorology. The
sling psychrometer may, of course, be used simply as an ordinary sling
thermometer.

The simple form of mercurial barometer, without vernier and without
attached thermometer, described in Chapter II, will be found the best
barometer for general school use. The standard barometer, described in
this chapter, is too expensive and too complicated to come into extended
use in our schools. Full instructions concerning the care, the reading,
and the corrections of the standard mercurial barometer are published by
the Weather Bureau, and to these instructions teachers who have such an
instrument are referred. (See Appendix B.)

The form of table given at the end of this chapter is intended merely as a
suggestion, and not as a rigid scheme to be adopted in every school. In
using the instruments here described, practice with the maximum and
minimum thermometers (in addition to the simpler work of Chapter II) may
be given before any attempt is made to have the class use the
psychrometer. And in using the psychrometer one week may well be given to
the determination of the dew-point alone, before the wet and dry-bulb
readings are employed to determine the relative humidity. Absolute
humidity, which is not referred to in this chapter, may, if the teacher
deem it advisable, be added as another weather element for study. A
refinement in the notes on the state of the sky is suggested, viz., that
cloudiness should be recorded in tenths of the sky covered by clouds. This
is an advance over the earlier, less accurate cloud observations, and is
in line with such a progressive scheme as has been recommended. This book
is not intended to present a rigid scheme of observational work in
meteorology, alike for all schools, but rather to make suggestions for the
guidance of teachers in laying out such a course as may fit their own
cases.

Under the heading _Summary of Observations_ only a few of the most
important climatic elements have been noted. The list may easily be
extended by the addition of such data as the following: For temperature,
mean diurnal range; mean diurnal variability (the mean of the differences
between the successive daily means). For humidity, monthly mean absolute
humidity. For precipitation, the maximum daily precipitation; the number
of rainy and snowy days in every month, the number of clear, fair, and
cloudy days in every month; the mean frequency of rainfall in every month
(number of rainy days divided by the total number of days); the number of
days with thunderstorms, etc.

It is important that the monthly summaries should be discussed in the
class, and that the scholars should give verbal statements as to the
numerical results which they have obtained. In this way the work will have
a living interest, which the mere compilation of summaries does not
possess.


                              CHAPTER IV.

The first thing for any teacher to do who intends to establish a course in
meteorology is to secure a supply of daily weather maps. Arrangements
should be made to have them mailed regularly from the nearest
map-publishing station of the Bureau. It is important that the Saturday
morning map, which is usually not sent to schools, should be included in
the set, as the break of two days (Saturday and Sunday) in every week
seriously interferes with the value of the work that may be done on
consecutive maps. The maps should be securely fastened up in the
schoolroom or in the hall. It is advisable to keep at least two maps thus
on view all the time, in order that the scholars may be able to study the
changes from day to day by comparing the last two or three maps with one
another. As soon as they are removed from the wall, the maps should be
carefully filed away for future reference. They may be conveniently kept
in stiff brown paper folders, each month’s maps being enclosed in a
separate folder, with the name of the month and the year written on the
outside. It is a good plan to keep with the file of maps any newspaper
clippings referring to notable meteorological phenomena associated with
the conditions shown on the maps. Thus, newspaper accounts of the damage
done by a hurricane along the Atlantic Coast; of the blockades caused by a
heavy snowstorm in the Northwest; of tornadoes in the Mississippi Valley;
of hot waves and sunstrokes in our larger cities, will serve to enliven
the study of the maps, and will also help, in later references to them, to
recall interesting points that might otherwise escape the memory. It is
true that newspapers are prone to exaggerate, and that they are lamentably
inaccurate in their use of meteorological terms; but nevertheless they
may often be profitably used in such general studies as these. Besides the
complete weather map, the school will need a supply of blank weather maps,
as used by the Weather Bureau. These may usually be secured from the
nearest map-publishing station at cost price. In the case of the
preparation of illustrations for permanent class use, as suggested later,
it is advisable to employ the blank maps used as the base of the
Washington daily weather maps, and to be obtained, at cost price, on
application to the Chief of the Weather Bureau, Washington, D. C. These
are larger in size (16-1/2 by 23-3/4 inches) than the station maps, and
the paper is of a better quality. Colored illustrations with these
Washington blank maps as the base furnish an economical, simple, and
effective means of teaching elementary meteorology.


                               CHAPTER V.

In this chapter a series of six consecutive weather maps is taken as the
basis of the work. The study of the weather elements on such a series of
maps gives a far clearer understanding of the distribution of these
elements and of their relation one with another, than if a far larger
number of single maps are studied which do not follow one another in
regular sequence. Teachers should add to the map-drawing exercises by
giving their classes the data from other sets of maps selected from the
school files. Summer maps as well as winter ones should be used, in order
that too much emphasis may not be laid on winter conditions. The search
for the various cities in which Weather Bureau stations are established,
involved in the work of entering the data on the blank maps, furnishes
excellent practice in geography. This exercise may be varied, and practice
in the location of the different States may be given, if the teacher reads
out to the class the temperatures in different parts of the various
States, as, _e.g._, central Arizona, 34°; southwestern Tennessee, 30°;
northern Nevada, 38°, etc. As the State names are not given on the Weather
Bureau maps, such a method as this will give a very desirable familiarity
with the relative positions of the States of the Union. If the school
possesses a blackboard outline map of the United States, it may be a good
plan, if the class is small, to have one of the pupils enter the
temperatures and draw the isotherms on the board before the class, and to
let the others correct him if they see that he is going wrong. As to
irregularities in the isotherms, which may cause trouble, the officials of
the Weather Bureau vary somewhat among themselves in dealing with such
cases, and no definite rules can be laid down to fit all occasions. It is
best to select maps with few irregularities at first, and in time
experience will show how the exceptional cases may be treated.

By the scheme of coloring isothermal maps, suggested in this chapter, a
valuable series of permanent illustrations of noteworthy weather types for
class use can readily be prepared at very slight cost. For the purposes of
these colored illustrations it is best to use the large scale Washington
blank weather maps, as suggested in the preceding chapter, and to have
each map mounted on heavy cardboard after it has been colored. By means of
this mounting the maps are prevented from tearing, and can be kept smooth
and in good condition.

The scheme of coloring may be varied to suit the fancy of the scholars,
for the preparation of these permanent illustrations may well be intrusted
to those of the class who are especially interested in the work, and who
are skillful in their use of the paint brush. The work is really very
simple and needs only ordinary care. As soon as the drawing of isotherms
and the coloring of isothermal charts have been sufficiently studied, the
teacher should hang up the daily weather map each day (or a blank map with
the isotherms for the day drawn on it), and should call attention to the
temperature changes from day to day. In this way the facts of actual
temperature changes experienced by the class will be associated with the
larger temperature changes shown on the weather maps.

The sections on temperature gradients may be postponed, if the teacher
deems it advisable, until other matters of a simpler nature are passed.
The idea of rates of change is not an easy one for students to grasp, and
it is far better to postpone the consideration of this subject than to
involve the class in any confusion at this stage. It is a mistake,
however, to omit these sections altogether, as a clear conception of the
principle of rates is a valuable part of a student’s mental equipment. It
is a good plan, in the exercises on lines of temperature decrease, to
have maps prepared with _faint_ isotherms and _heavy_ lines of temperature
decrease, in order to emphasize the idea of change of temperature, in
contrast to the idea of constancy of temperature expressed by the
isotherms.


                              CHAPTER VI.

In the work on the wind charts it is essential to proceed very slowly, in
order that the best results may be obtained by the pupils. Some of the
aberrant wind courses, which complicate the discovery of the cyclonic and
anticyclonic spirals, may well be omitted in the case of the younger
classes, and considerable assistance in facilitating these discoveries may
be given by suggestions as to adding intermediate dotted wind arrows, in
sympathy with the observed wind directions. The anticyclonic systems are
always much more difficult to discover than the cyclonic, and care should
be taken to assist the class in this matter as much as seems necessary.

The questions in the text are merely suggestive, and are by no means as
numerous as it would be well to make them in the class. The discovery of
the spirals will probably be made by degrees. The concise formulation of
the facts discovered will furnish excellent basis for exercises in
writing.

It is interesting to note that the discussion as to whether the winds blow
circularly around, or radially in towards, centers of low pressure, which
usually comes up in every class in meteorology at some stage in the study,
was carried on in a very animated way about the middle of the present
century in this country. Two noted American meteorologists, Redfield and
Espy, and their respective followers, took opposite sides in this
controversy, Redfield maintaining at the start that the winds moved in
circles, and Espy maintaining that they followed radial courses. The truth
lay between the two.


                              CHAPTER VII.

The study of atmospheric pressure is not easy, because the pupils cannot
perceive the changes in pressure from day to day by their unaided senses.
Especially difficult does this study become if the class has not already
had some practice in making barometer readings. When this observational
work has not preceded the consideration of the isobaric lines of the daily
weather maps, the teacher should introduce the subject of pressure very
carefully. The experiment with the Torricellian tube will show the class
something of the reality of atmospheric pressure, and the variations in
this pressure from day to day can readily be made apparent by means of a
few barometer readings, if time cannot be spared for a regular and
continued series of barometer observations. A word may be said as to the
correction of barometric readings for local influences, in order to make
these readings comparable, if this matter has not been previously met
with, but care should be taken not to confuse the younger pupils too much
with explanations at this stage of the work. It may be well to omit this
point unless it is brought up by some pupil. The questions asked in the
text are merely suggestive. They may be added to and varied at the
discretion of the teacher.

Lines of pressure-decrease should be drawn on all isobaric charts studied
in the class, as they are highly instructive. When the isobars are near
together, these lines of pressure-decrease may be drawn heavier, to
indicate a steeper gradient. The convergence of these lines towards
regions of low pressure, and their divergence from regions of high
pressure, seen on every map on which these gradients are drawn, emphasizes
an important lesson. Before measuring rates of pressure-decrease by means
of a scale, considerable practice should be given in the study, by means
of the eye, of the rapidity or slowness of decrease of pressure, as shown
by the heavier or lighter lines of pressure-decrease. When the broad facts
of differing rates are comprehended, then the actual measurement of these
rates is a comparatively easy matter. In any case, however, an
appreciation of these rates of change is not always readily gained, even
by older scholars in the high school. It is, therefore, of prime
importance to proceed very slowly indeed at this point, and to have every
step fully understood before another step is taken.

The instructions in the text for measuring rates of pressure-decrease are,
that these rates shall be recorded as so many hundredths of an inch of
change of pressure in one latitude-degree. This is done for the sake of
simplicity. If this rate is expressed in hundredths of an inch of pressure
in a quarter of a latitude-degree of distance, the numerical value is the
same as if expressed in millimeters of pressure per latitude-degree of
distance.


                             CHAPTER VIII.

The fact of the prevalence of different kinds of weather over the country
at the same time is of great importance. It should be strongly emphasized
by the teacher in the course of the discussion of the maps of weather
distribution. Additional exercises of the same sort may be given to
advantage, by letting the class plot and study the weather signs taken
from any current weather map. Instructive lessons may be taught by talking
over, in the class, the different ways in which people all over the
country are affected by the character of the weather that happens to
prevail where they are.


                           CHAPTERS IX-XVIII.

The correlation exercises will, as a whole, teach few entirely new facts
to the brighter scholars who have faithfully completed the preceding work
in observations and in the construction and study of the daily weather
maps. These exercises do, however, lead to detailed examination and to the
careful working out of the relations which may have been previously
noticed in a general way only. They give the repeated illustration which
is necessary in order to impress firmly on the mind the lesson that the
weather map has to teach.

It is a good plan to let different scholars work out the problems for
different months. The results reached in each case should be discussed in
the class, and thus each member may have the double advantage of working
out his own problem, and of profiting by the work done by his fellows.
Throughout these exercises care should be taken to have weather maps of
all months studied. The exercise on the correlation of the velocity of the
wind with the pressure cannot be undertaken unless the work on
temperature and pressure gradients (Chapters V and VII) has been
completed.


                            CHAPTERS XX-XXV.

It is not expected that any one scholar can accomplish all that is here
outlined. Examples may be selected from the list, as opportunity offers,
so that each scholar shall become familiar with several problems.

Few of the problems suggested call for continuous routine observation at
fixed hours. They require, on the other hand, an intelligent examination
of ordinary weather phenomena, with special reference to discovering their
explanation. In most of the problems a small number of observations will
suffice. Under the supervision of the teacher, different problems may be
assigned to the several members of a class; or several scholars may work
on different parts of the same problem, exchanging records in order to
save time. All the scholars should have a general knowledge of the results
which have been obtained from the observations made by the other members
of their class. The teacher will use his discretion in arranging the order
of the problems, and in selecting those that are best suited to the season
in which the work is done, to the locality in which the school is
situated, and to the facilities and apparatus at command. Although the
variety of accessible problems is less in city schools than in country
schools, much may be done in the city as well as in the country.

The opportunities for carrying out such observational work vary so much in
different schools that it is impossible to give specific instructions,
which shall be available in all cases. Some general suggestions are
therefore given, which the teacher may supplement by more detailed
instructions framed to fit the particular circumstances of each case.

A review of the headings of the different problems shows that a very
general correlation exists among them, whereby the subjects of every
heading are associated with those of nearly every other. In other words,
every weather element is treated as a function of several other elements.
It follows from this that the variety of work here outlined is more
apparent than real, and that many problems which appear from their wording
to be entirely new are in large part rearrangements of problems previously
encountered.



                              APPENDIX B.

             THE EQUIPMENT OF A METEOROLOGICAL LABORATORY.


                           _A._ INSTRUMENTS.

_Exposed Thermometer_ (United States Weather Bureau pattern), with brass
support, $2.75.

_Maximum and Minimum Thermometers_ (United States Weather Bureau pattern),
mounted together on one board, $6.25.

_Wet- and Dry-Bulb Thermometers_ (United States Weather Bureau pattern),
mounted on one board, complete with water cup, $6.50.

_Sling Psychrometer_ (designed by Professor C. F. Marvin, of the United
States Weather Bureau), consisting of two exposed mercurial thermometers,
mounted on an aluminum back, and provided with polished, turned hard-wood
handle and brass trimmings, $5.00.

_Sling Psychrometer_, consisting of two cylindrical bulb thermometers,
mounted one a little above the other upon a light brass frame, with a
perforated guard to protect the bulbs while swinging, but which can be
raised (by sliding upon the frame) for the purpose of moistening the linen
covering of the wet bulb. Much less liable to be broken than the Weather
Bureau pattern, $5.00.

_Rain Gauge_ (United States Weather Bureau standard), 8 inches in
diameter, complete, with measuring stick, $5.25.

_Rain Gauge_, 3 inches in diameter, with overflow and measuring stick,
$1.25.

_Wind Vane_ (United States Weather Bureau pattern), $10.00.

_Anemometer_ (United States Weather Bureau pattern), with indicator,
aluminum cups, and electrical attachment, $25.00.

The same, with painted brass cups, $23.00.

_Anemometer Register_ (United States Weather Bureau pattern), with pen and
ink attachment, $35.00.

The same, with pencil attachment (old style), $24.00.

_Aneroid Barometer_ (for meteorological work), $14.00-$16.00.

    NOTE.——Much cheaper aneroids can be purchased, and may be used
    to some advantage in the simpler observations in schools.

_Mercurial Barometer_ (Standard United States Weather Bureau pattern),
complete with attached thermometer, vernier, etc., $30.00-$33.00.

    NOTE.——The above instruments, as used by the United States
    Weather Bureau, are made by H. J. Green, 1191 Bedford Avenue,
    Brooklyn, N. Y. The prices are those given in Green’s latest
    catalogue.

_Mercurial Barometer._ New improved form, especially designed for school
use. Mounted on mahogany back. Scale engraved on aluminum. Divisions of
scale on metric and English systems. No vernier, $5.75.

    (L. E. Knott Apparatus Co., 14 Ashburton Place, Boston, Mass.)

_Thermograph_ (designed by Dr. Daniel Draper, of New York). Consists of a
bimetallic thermometer in a case which carries a disk, with a chart upon
its axle instead of hands like the ordinary clock. A pen (resting on the
face of the disk) registers the fluctuations of temperature as the chart
is carried around. Sizes, 14 × 20 inches, $30.00; 10 × 14 inches, $15.00.
This instrument may be purchased of H. J. Green.

_Thermograph._ Self-recording thermometer (as adopted by the United States
Weather Bureau), made by Richard Frères, of Paris. Records continuously on
a sheet of paper wound around a revolving drum, which is driven by
clock-work inside. Standard size (without duty), $30.00.

_Barograph._ Self-recording barometer (as adopted by the United States
Weather Bureau), made by Richard Frères, of Paris. Similar in general
arrangement to the thermograph. Standard size (without duty), $27.60.

These last two instruments can be procured through Glaenzer Frères &
Rheinboldt, 26 & 28 Washington Place, New York City.

_Instrument Shelter_ (standard United States Weather Bureau pattern) will
hold a set of maximum and minimum thermometers, psychrometer, and a
thermograph. May be set up on top of posts driven into the ground, or may
be attached to a wall, $18.00.

_Barometer Box_, for the standard mercurial barometer. Made of mahogany,
with glass panels on front and sides; lock and key, and with fittings
complete, $8.50.

These may be purchased of H. J. Green.


                            _B._ TEXT-BOOKS.

_The Story of the Earth’s Atmosphere._ DOUGLAS ARCHIBALD. New York, D.
Appleton & Co., 1898. 18mo, pp. 194. 40 cents.

To be recommended to the general reader who wishes to gain some knowledge
of meteorology quickly. Not a text-book. Contains a chapter on “Flight in
the Atmosphere.”

_Elementary Meteorology._ WILLIAM MORRIS DAVIS. Boston, Ginn & Co., 1898.
8vo, pp. 355. $2.50.

The most complete of the modern text-books, and the best adapted for use
in the systematic teaching of meteorology. The modern views are presented
clearly and without the use of mathematics. Portions of it are somewhat
too advanced for school study, but teachers will find it invaluable as a
reference book in directing the laboratory work, and in answering the
questions of school classes.

_A Popular Treatise on the Winds._ WILLIAM FERREL. New York, John Wiley &
Sons, 1890. 8vo, pp. 505. $3.40.

This can hardly be regarded as a _popular_ treatise. It embodies, in
condensed and chiefly non-mathematical form, the results of Ferrel’s
researches during his long and profound study of the general circulation
and phenomena of the atmosphere. Teachers who advance far into meteorology
will find this book indispensable. It is not at all suited for general
class-room use.

_American Weather._ A. W. GREELY. New York, Dodd, Mead & Co., 1888. 8vo,
pp. 286. Out of print, but secondhand copies are probably obtainable.

Deals, as the title implies, especially with the weather phenomena of the
United States. Contains brief accounts of individual hot and cold waves,
hurricanes, blizzards and tornadoes, and gives specific data concerning
maxima and minima of temperature, precipitation, etc., in the United
States.

_Meteorology: Practical and Applied._ JOHN WILLIAM MOORE. London, F. J.
Rebman, 1894. 8vo, pp. 445. 8 shillings.

A readable book. Considerable space is given to instrumental meteorology.
Contains chapters on the climate of the British Isles and on the relations
of weather and disease in the British Isles. Especially adapted for the
use of English readers.

_Elementary Meteorology._ ROBERT H. SCOTT. International Scientific
Series. London, Kegan Paul, Trench & Co., 1885; Boston, A. A. Waterman &
Co., 1889. 8vo, pp. 410. 6 shillings.

The standard text-book in Great Britain. The author is secretary to the
Meteorological Council of the Royal Society. Fairly complete, but now
somewhat out of date in some portions. It is a useful book in a
meteorological library, but does not treat the subject in a way very
helpful to the teacher.

_Meteorology._ THOMAS RUSSELL. New York, The Macmillan Company, 1895. 8vo,
pp. 277.

Brief and incomplete as a text-book of meteorology, but containing a very
comprehensive account, fully illustrated, of rivers and floods in the
United States, and their prediction.

_Elementary Meteorology._ FRANK WALDO. New York, American Book Company,
1896. 8vo, pp. 373. 90 cents.

A compact summary. Useful to teachers as a handy reference book.

_Modern Meteorology._ FRANK WALDO. New York, Charles Scribner’s Sons,
1893. 8vo, pp. 460. $1.25.

Very complete account of meteorological apparatus and methods, and
admirable summary of recent German studies of the thermodynamics and
general motions of the atmosphere.


              _C._ INSTRUCTIONS IN THE USE OF INSTRUMENTS.

_Instructions for Voluntary Observers._ 1899. 8vo, pp. 23. Brief
instructions for taking and recording observations of temperature and
precipitation with ordinary and maximum and minimum thermometers and with
the rain gauge.

_Barometers and the Measurement of Atmospheric Pressure._ C. F. MARVIN.
1894. 8vo, pp. 74. A pamphlet of information respecting the theory and
construction of barometers in general, with summary of instructions for
the care and use of the standard Weather Bureau instruments.

_Instructions for Obtaining and Tabulating Records from Recording
Instruments._ 1898. 8vo, pp. 31. Contains directions concerning the care
and use of the Richard thermograph and barograph.

    NOTE.——These pamphlets are prepared under the direction of
    Professor Willis L. Moore, Chief of the United States Weather
    Bureau, and are published, under authority of the Secretary of
    Agriculture, by the Weather Bureau. They will be found the best
    guides in making observations, the care of instruments, etc.


                          _D._ JOURNALS, ETC.

_Monthly Weather Review._ Prepared under the direction of Willis L. Moore,
Chief of Weather Bureau, Professor Cleveland Abbe, Editor. United States
Department of Agriculture, Weather Bureau, Washington, D. C. 10 cents a
copy.

An invaluable publication for teachers and students alike. Contains
complete meteorological summaries for each month; accounts of all notable
storms, cold and hot waves, etc.; and a large number of articles on a wide
range of meteorological subjects. The charts show the tracks of areas of
high and low pressure which crossed the United States during the month,
the total precipitation, sea-level pressure, temperature and surface
winds, percentage of sunshine, etc., for the month. Other charts are also
frequently added.

_The Journal of School Geography._ Professor Richard E. Dodge, Teachers
College, Columbia University, New York City, Editor. Publication Office,
41 No. Queen Street, Lancaster, Pa. Ten numbers a year. $1.00 per annum.

A monthly journal devoted to the interests of the common school teacher of
geography. Contains numerous articles and notes on meteorological and
climatological subjects.

_Science._ Edited by Professor J. McK. Cattell, Columbia University, New
York City, New York, The Macmillan Company. Weekly. $5.00 per annum.

Devoted to the advancement of all sciences. Contains brief _Current Notes
on Meteorology_, which summarize the more important meteorological
publications.

_Monthly Bulletins of the Climate and Crop Service of the Weather Bureau._

These _Bulletins_ are issued every month at the central office of the
Weather Bureau in each State, under the direction of the Section Director
of the Climate and Crop Service in that State. They contain meteorological
data for the month, and frequently notes of interest. The annual summaries
are especially valuable.


                              _E._ CHARTS.

_Daily Weather Maps._ These are published at the central office of the
Weather Bureau in Washington, and at eighty-four other stations of the
Bureau throughout the United States. It is best to have the daily maps
sent from the nearest map-publishing station, and not from Washington, as
the delay in the latter case is often so great that much of the immediate
value of the maps is lost.

_Climate and Crop Bulletin of the United States Weather Bureau._
Washington, D. C. Monthly.

Chart showing, by means of small maps, the actual precipitation,
departures from normal precipitation, departures from normal temperature,
and maximum and minimum temperatures. Also a printed summary of the
weather and of the crop conditions in the different sections of the United
States. Issued on the first of each month.

_Snow and Ice Chart of the United States Weather Bureau._ Washington, D.
C. Weekly during the winter season.

Based on data from regular Weather Bureau stations, supplemented by
reports from selected voluntary observers. Shows, by shading, the area
covered with snow at 8 P.M. each Tuesday during the winter, and by lines,
the depth of snow in inches. Explanatory tables and text accompany the
chart.

_Storm Bulletin of the United States Weather Bureau._ Washington, D. C.
Issued at irregular intervals.

Charts, with text, illustrating the history of individual notable storms.

_Pilot Chart of the North Atlantic and North Pacific Oceans._ Hydrographic
Office, Bureau of Equipment, Department of the Navy, Washington, D. C.
Monthly. Price 10 cents a copy.

Shows calms and prevailing winds, ocean currents, regions of fog and
equatorial rains, the positions of icebergs and wrecks, steamship and
sailing routes, storm tracks, magnetic variation, etc. Also gives isobars
and isotherms and a forecast for the month succeeding the date of
publication, and a review of the weather over the oceans for the preceding
month. Supplementary charts are occasionally issued.

_Rainfall and Snow of the United States as compiled to the End of 1891,
with Annual, Seasonal, Monthly, and other Charts._ MARK W. HARRINGTON.
United States Department of Agriculture. Weather Bureau, Bulletin C,
Washington, D. C. 1894. Atlas, 18 × 24 inches. Charts 23. Text, 4-80 pp.

Contains twenty-three charts as follows: Monthly rainfall, seasonal
rainfall, annual rainfall, monthly snowfall, monthly maxima of rainfall,
rainy seasons, details of rainfall, details of occurrence of
thunderstorms. Well adapted to serve as illustrations for use in the
class-room. The text is explanatory, and is published separately in quarto
form.

_Rainfall of the United States, with Annual, Seasonal, and other Charts._
ALFRED J. HENRY. United States Department of Agriculture, Weather Bureau,
Bulletin D, Washington, D. C. 1897. 9-1/4 × 11-1/2 inches. Pp. 58. Charts
10. Plates III.

A more recent publication than the preceding one, the averages having been
compiled to the end of 1896. The charts are smaller than most of those in
Bulletin C, and therefore not so well adapted for class-room illustration.
The chart of mean annual precipitation is the latest and best published.
The rainfall of the crop-growing season receives separate treatment, and
is illustrated by means of two charts. The discussion in the text is
excellent.


                      _F._ METEOROLOGICAL TABLES.

_Smithsonian Meteorological Tables._ Smithsonian Miscellaneous
Collections, 844. Washington, D. C. 1893. 8vo. Pp. 262.

A very complete set of tables.

_Handbook of Meteorological Tables._ H. A. HAZEN (of the United States
Weather Bureau). Washington, D. C. 1888. 8vo. Pp. 127. $1.50.

Contains forty-seven tables, comprising all that are needed by the working
meteorologist. Includes tables for Fahrenheit and Centigrade conversions,
for barometric hypsometry and reduction to sea level, for the
psychrometer, etc.

_Tables for Obtaining the Temperature of the Dew-Point, Relative Humidity,
etc._ United States Department of Agriculture, Weather Bureau, Washington,
D. C. 1897. 8vo. Pp. 29.

These are the tables now in use by the Weather Bureau.


                          _G._ ILLUSTRATIONS.

_Classification of Clouds for the Weather Observers of the Hydrographic
Office._ Hydrographic Office, Bureau of Navigation, Department of the
Navy, Washington, D. C. 1897. Sheet of twelve colored views. Price 40
cents. In book form, with descriptive text, $1.00.

An excellent set of cloud views, classified according to the
_International Nomenclature_. The text describes the various cloud forms
and shows their value as weather prognostics. An attractive addition to
the furnishings of a schoolroom.

_Selected List of Cloud Photographs and Lantern Slides._

Consists of twenty-eight photographs, and the same number of lantern
slides, of the typical cloud forms, selected by the present writer from
the collection in the Physical Geography Laboratory of Harvard University.
The photographs (20 cents each, mounted) and slides (40 cents each) may be
purchased of E. E. Howell, 612 17th Street, N. W., Washington, D. C. A
description of these views was published in the _American Meteorological
Journal_ for July, 1894 (Boston, Mass., Ginn & Company).

_Photographs._ Photographs of miscellaneous meteorological phenomena, such
as snow and ice storms, damage by storm-waves or high winds, wind-blown
trees, lightning, etc., may often be purchased of local dealers. They add
to the attractiveness of a schoolroom and furnish excellent illustrations
in teaching.

                             _H._ GENERAL.

The following _Bulletins_ of the Weather Bureau may be found useful as
reference books:

No. 1. _Notes on the Climate and Meteorology of Death Valley, California._
MARK W. HARRINGTON. 8vo. 1892. Pp. 50.

No. 8. _Report on the Climatology of the Cotton Plant._ P. H. MELL. 8vo.
1893. Pp. 68.

No. 10. _The Climate of Chicago._ H. A. HAZEN. 8vo. 1893. Pp. 137.

No. 11. _Report of the International Meteorological Congress held at
Chicago, III., Aug. 21-24, 1893._ 8vo. Pt. I, 1894, pp. 206. Pt. II, 1895,
pp. 583. Pt. III, 1896, pp. 772. Pt. IV, not yet issued.

No. 15. _Protection from Lightning._ ALEXANDER MCADIE. 8vo. 1895. Pp. 26.

No. 17. _The Work of the Weather Bureau in Connection with the Rivers of
the United States._ WILLIS L. MOORE. 8vo. 1896. Pp. 106.

No. 19. _Report on the Relative Humidity of Southern New England and Other
Localities._ A. J. HENRY. 8vo. 1896. Pp. 23.

No. 20. _Storms, Storm Tracks and Weather Forecasting._ FRANK H. BIGELOW.
8vo. 1897. Pp. 87.

No. 21. _Climate, of Cuba._ Also, _A Note on the Weather of Manila._ W. F.
R. PHILLIPS. 8vo. 1898. Pp. 23.

No. 23. _Frost: When to expect it and how to lessen the Injury therefrom._
W. H. HAMMON. 8vo. 1899. Pp. 37.

No. 25. _Weather Forecasting: Some Facts Historical, Practical, and
Theoretical._ WILLIS L. MOORE. 8vo. 1899. Pp. 16.

No. 26. _Lightning and the Electricity of the Air._ In two parts. A. G.
MCADIE and A. J. HENRY. 8vo. 1899. Pp. 74.

The following miscellaneous publications of the Weather Bureau may also
prove of value.

_Injury from Frost and Methods of Protection._ W. H. HAMMON. 8vo. 1896.
Pp. 12.

_Some Climatic Features of the Arid Regions._ WILLIS L. MOORE. 8vo. 1896.
Pp. 19.

_Investigation of the Cyclonic Circulation and the Translatory Movement of
the West Indian Hurricanes._ The late REV. BENITO VIÑES, S. J. 8vo. 1898.
Pp. 34.

Requests for weather maps, _Bulletins_, and other publications of the
Weather Bureau should be sent to the Chief of the Weather Bureau,
Washington, D. C. All requests are dealt with on their merits, and in
cases where it is deemed that effective use will be made of the
publications they are usually sent free of charge.



                                 INDEX.


          A.

  Anemometer, 38-41.

  Aneroid barometer, 23, 24.

  Anticyclones, 76.
    ———— and pressure changes, 141.
    ———— and temperature, 104-106.
    ———— and weather, 109-112.
    ———— and wind circulation, 98-100.
    ———— form and dimensions of, 96-98.
    ———— progression of, 111, 112.
    ———— tracks of, 111, 112.


          B.

  Backing winds, 118.

  Barograph, 36, 37.
    ———— records, 37, 38.

  Barometer, aneroid, 23, 24.
    ———— corrections, 33, 34.
    ———— mercurial, 19-23, 32, 33.
    ———— reduction to freezing, 143, 144, 166, 167.

  Barometer reduction to sea level, 144, 145, 168-170.

  Buran, 75, 76.

  Buys-Ballot’s Law, 99.


          C.

  Cherrapunjee, rainfall at, 138.

  Clouds and upper air currents, 136, 137.

  Clouds as weather prognostics, 137.
    ———— forms and changes of, 136.
    ———— movements of, 41-43, 46, 95, 136, 137.

  Clouds, observations of, 5, 9, 41-43, 45, 46.

  Cold wave, 62, 75, 76, 103, 106.

  Cold-wave forecasts, 63.

  Correlations of weather elements, 91-113.

  Cyclones, 76.
    ———— and pressure changes, 141.
    ———— and temperature, 104-106.
    ———— and weather, 109-111, 113, 114.
    ———— and wind circulation, 98-100.
    ———— form and dimensions of, 96-98.
    ———— progression of, 111, 112.
    ———— tracks of, 111, 113.
    ———— tropical, 106.
    ———— velocity of, 112, 113.


          D.

  Dew, 134, 136.
    ———— point, 30.
    ———— ———— tables, 142, 143, 146-155.

  Diurnal variation of temperature, 125-127.

  Diurnal variation of wind velocity, 130.

  Doldrums, 29.


          E.

  Equipment of a meteorological laboratory, 186.

  Evaporation, 29.


          F.

  Fahrenheit, 12, 13.

  Ferrel’s Law, 93.

  Forecasts, cold-wave, 63.
    ———— weather, 49, 114-124.

  Franklin, Benjamin, 114, 115.

  Frost, 135.
    ———— warnings, 135.


          G.

  Galileo, 12, 19.

  Gradients, pressure, 82-85.
    ———— temperature, 64-70.
    ———— vertical temperature, 129.


          H.

  Humidity, 29, 132-134.
    ———— diurnal variation of, 133.
    ———— relative, 30, 32, 45, 133.
    ———— relative, and wind direction, 133.
    ———— tables, 143, 156-165.

  Hurricane, 100.


          I.

  Inversions of temperature, 129.

  Isobaric charts, 76-82, 85, 95, 96.

  Isobars, 77-82, 121.

  Isothermal charts, 63, 68-70.

  Isotherms, 55-57, 121.


          L.

  Land and sea breezes, 3, 131, 132.

  Loomis, 97, 113.


          M.

  Meteorological tables, 146-170.

  Mistral, 75.

  Monsoons, 100.

  Mountain and valley winds, 131.


          N.

  Nephoscope, 41-43.


          O.

  Observational meteorology, problems in, 125-141.

  Observations, advanced instrumental, 26-46.

  Observations, cloud, 5, 9, 41-43.
    ———— elementary instrumental, 11-26.

  Observations, non-instrumental, 1-10.

  Observations, rainfall, 9, 10, 26.
    ———— state of sky, 5, 9.
    ———— temperature, 1-3, 25.
    ———— wind, 3-5, 8, 9, 25.


          P.

  Pampero, 76, 104.

  Pascal, 21.

  Precipitation, 7, 9, 10, 138, 139.

  Pressure and wind direction, 91-93.
    ———— and wind velocity, 93-96.
    ———— atmospheric, 19-21.
    ———— charts, 76-83.
    ———— cyclonic variation of, 140, 141.
    ———— decrease with altitude, 139, 140.
    ———— diurnal variation of, 140, 141.
    ———— gradient, 82-85.

  Prevailing westerly winds, 93, 95, 96, 112, 113.

  Psychrometer, 28.
    ———— sling, 31.

  Purga, 75.


          R.

  Rain, see Precipitation.
    ———— heavy, 138, 139.
    ———— gauge, 15-17.

  Rainfall records, 17, 18, 26.


          S.

  Sensible temperatures, 29, 30.

  Sirocco, 103, 104, 133, 134.

  Sling psychrometer, 31.

  Smudges, 135.

  State of sky, 5, 9, 45.

  Suggestions to teachers, 171.


          T.

  Temperature charts, 58-60.
    ———— distribution, 51-63.
    ———— diurnal range, 125-127.
    ———— forecasts, 116.
    ———— gradient, 64-70.

  Temperature gradient, vertical, 129.
    ———— inversions, 129.
    ———— maximum and minimum, 45.
    ———— mean, 43, 45.
    ———— observations, 1-3, 8-10, 25, 43, 45.
    ———— range, 45, 125-127.
    ———— sensible, 29, 30.
    ———— vertical distribution of, 128, 129.

  Thermograph, 34-36.
    ———— records, 35, 36.

  Thermometer, 12.
    ———— attached, 33.
    ———— maximum and minimum, 26-28.
    ———— shelter, 13, 14.
    ———— wet- and dry-bulb, 28, 30, 31.

  Torricelli, 19, 20.

  Trade winds, 93.


          V.

  Veering winds, 118.

  Vernier, 33.

  Vertical temperature gradient, 129.


          W.

  Water vapor, 28.

  Weather, 85-90.
    ———— and wind direction, 106-109.
    ———— changes, sequence of, 113, 114.
    ———— charts, 85-90.

  Weather forecasts, 49, 114-124.
    ———— map data, 90.
    ———— maps, 47-51.
    ———— prognostics, 137.
    ———— signs, 85.
    ———— temperate zone, 88-90.
    ———— torrid zone, 88-90.

  Wind charts, 70-75.
    ———— direction and pressure, 91-93.
    ———— ———— and relative humidity, 133.
    ———— ———— and temperature, 101-104.
    ———— ———— and weather, 106-109.
    ———— ———— forecasts of, 118.
    ———— observations, 3-5, 25, 40, 41.
    ———— rose, 103, 108, 109.
    ———— vane, 15.
    ———— velocity, 3, 40, 41, 45.
    ———— ———— and pressure, 93-96.
    ———— ———— diurnal variation of, 130.
    ———— ———— forecasts of, 118.
    ———— ———— scale, 3.

  Winds around cyclones and anticyclones, 98-100.

  Winds, mountain and valley, 131, 132.
    ———— prevailing westerly, 93, 95-96, 112, 113.

  Winds, trade, 93.

  Woeikof, 69.

       *       *       *       *       *



                        TRANSCRIBER’S AMENDMENTS


Transcriber’s Note:

Blank pages have been deleted.

Some illustrations have been moved.

The footnotes have been coalesced and moved so as to follow the
referencing paragraph.

The publisher’s inadvertent omissions of important punctuation have been
corrected.

Some wide tables have been re-formatted to narrower equivalents with some
words replaced with commonly known abbreviations and possibly a key.

The table on page 44 has been flipped around a diagonal axis so that the
original column headings are now row headings.

The following list indicates any additional changes made. The page number
represents that of the original publication and applies in this etext
except for footnotes and illustrations since they may have been moved.

  Key: {}[]:

    Page          Change

     6  Than {}[the] day before yesterday?
    62  reduction of temperature{ to}[ ]from 16° to 36°.
    75  general relation between winds and cold {wave}[waves]
   159  | 47 | 44 | 42  | {29}[39] | 36  | 34  | 31  | 53
   162  {103}[108]  |  97 |  93 |  90 |  87 |  83 |  80 |
   170  

       *       *       *       *       *





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