CHAPTER XIX.——WEATHER FORECASTING _Practical Exercises in Elementary Meteorology

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

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.

+---------+---------+--------------+------------------------------------+

+---------+---------+--------------+----------------------------------- +

|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.

+--------+-----------+--------------+------------+----------+--------------------------+

+--------+-----------+--------------+------------+----------+--------------------------+

| Oct. 3 | 7.30 A.M. | Cool | NE. | Moderate | Temperature falling |

| | | | | | |

| “ “ | 11 A.M. | “ “ | “ “ | Brisk | Temperature the |

| | | | | | |

| “ “ | 3 P.M. | “ “ | “ “ | High | Wind velocity still |

+--------+-----------+--------------+------------+----------+--------------------------+

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.

+-------+----------+--------------+----------------------+---------+----------------------+

+-------+----------+--------------+-----------+----------+---------+----------------------+

|Dec. 18| 9 A.M. | Very cold | NW. | Brisk | Clear |Very cold all night. |

| | | | | | |up. |

| | | | | | | |

| “ “ | 5 P.M. | “ “ | “ | “ | “ |Same conditions. |

| | | | | | | |

| “ 19| 8.30 A.M.| A little | “ | Moderate | Fair |Wind less violent. |

+-------+----------+--------------+-----------+----------+---------+----------------------+

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.

+-------+---------+------------+--------------------+--------+-------------------+------------------+

| | | +----------+---------+ STATE +-------+-----+-----+ |

| | | |DIRECTION.|VELOCITY.| SKY. | OF |KIND.|AM’T.| |

+-------+---------+------------+----------+---------+--------+-------+-----+-----+------------------+

| | | | | | | | | | |

| “ “ | 12 M. | “ | “ | “ |Overcast| | “ | “ | “ |

| | | | | | | | | | |

| “ “ | 4 P.M. | “ | “ |Moderate |Overcast| | | |Stopped raining |

| | | | | | | | | | |

| “ 22| 8 A.M. | Cool | NW. | Brisk | Clear | | | |Cleared off during|

+-------+---------+------------+----------+---------+--------+-------+-----+-----+------------------+

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°.

CHAPTER XIX.——WEATHER FORECASTING 114