Isotherms on weather maps are drawn for _Practical Exercises in Elementary Meteorology

They are drawn in smooth curves and not in angular sections. Two isotherms

cannot cross one another, for if they did you would have two temperatures,

differing by 10°, at the point of crossing, which is obviously impossible.

Complete the chart for this day by drawing the remaining isotherms,

_i.e._, those for 50°, 20°, 10°, 0°, -10°, -20°, and -30°, bearing in mind

what has been said in regard to the determination of the positions of

isotherms when the _exact_ temperature you are seeking is not given on the

map.

The dotted lines in Fig. 18 show the positions of the isotherms when

drawn. Notice how clearly the temperature distribution now stands out, and

how simple the description of that distribution has become. Observe that

the isotherms, although more or less irregular, show a good deal of

uniformity in their general courses, and this uniformity is a great

assistance in drawing them. Study the distribution of temperature on this

map, and the positions of the isotherms, very carefully.

Construct isothermal charts for the remaining days of the series. Use a

new blank map for each day, and take the temperature observations from

the table in Chapter VIII. Proceed as in the case of the first day. Draw

the isotherms for every even 10° of temperature, taking care to study the

course of each line before you begin to draw the line. The charts when

completed form a series in which the temperature distribution over the

United States is shown at successive intervals of 24 hours.

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

In order to bring out the temperature distribution on the maps more

clearly, color (with colored pencils or water colors) all that portion of

each map which lies within the -20° isotherm a dark blue; that portion

which is between the 0° isotherm and the -20° isotherm a somewhat lighter

shade of blue, and those districts which are between 0° and +30° a still

lighter blue. The portion of the map above 30° and below 40° may be left

uncolored, while the districts having temperatures over 40° may be colored

red. In the map for the third day the district which has temperatures

below -50° should be colored darker blue than any shade used on the other

maps, or black, in order to emphasize the extremely low temperatures

there found. Figs. 19-24, on which the isotherms are shown, also

illustrate the appearance of these maps when the different temperature

areas are colored, as has been suggested.

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

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

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

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

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

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

Study the maps individually at first. Describe the temperature

distribution on each map. Ask yourself the following questions in each

case: Where is it coldest? Where warmest? What is the lowest temperature

on the map? What is the highest? At what stations were these readings

made?

Then compare the successive maps and answer these questions: What changes

have taken place in the intervening 24 hours? In what districts has the

temperature risen? What is the greatest rise that has occurred? Where? In

what districts has the temperature fallen? What was the greatest fall in

temperature and where did it occur? Has the temperature remained nearly

stationary in any districts? In which? You will find it a help in

answering such questions to make out a table of all the stations, and to

indicate in columns, after the names of the stations, the number of

degrees of rise or fall in temperature at each place during the 24-hour

interval between the successive maps. When the temperature is higher at

any station than it was on the preceding day, note this by writing a plus

sign (+) before the number of degrees of rise in temperature. When the

temperature has fallen, put a minus sign (-) before the number of degrees

of fall. Thus, New Orleans, La., had a temperature of 48° on the first

day. On the second it had 33°. Therefore the change at New Orleans was

-15° in the 24 hours. At Key West, Fla., the change was +11° in the same

time.

Write a brief account of the temperature distribution on each day of the

series, and of the changes which took place between that day and the one

preceding, naming the districts and States over which the most marked

falls and rises in temperature occurred, with some indication of the

amount of these changes. Note especially the changes in position, and the

extent, of the districts with temperatures below -20°; between 0° and

-20°, and between 30° and 0°. Write out a clear, concise statement of the

temperature distribution and changes shown on the whole set of six maps.

=Cold Waves.=——The series of charts for these six days furnishes an

excellent illustration of a severe cold wave.

A _cold wave_, as the term is now used by the Weather Bureau, means,

during December, January, and February, a fall in temperature of from 20°

to 16° in 24 hours, with a resulting reduction of temperature to between

0° and 32°, and, during the months from March to November inclusive, a

fall of from 20° to 16° in 24 hours, with a reduction of temperature from

16° to 36°. During December, January, and February a _cold wave_ means

the following falls and reductions of temperature. Over the Northwestern

States, from western Wisconsin to Montana, including Wyoming, Nebraska,

and western Iowa, and over northeastern New York and northern New

Hampshire, northern Vermont and northern Maine, a fall of 20° or more to

zero or below; over southern New England and adjoining districts, the

Lake region, the central valleys and west to Colorado, including northern

New Mexico and northwestern Texas, a fall of 20° or more to 10° or below;

over southern New Jersey, Delaware, eastern Maryland, Virginia, western

North Carolina, northwestern South Carolina, northern Georgia, northern

Alabama, northern Mississippi, Tennessee, southern Kentucky, Arkansas,

Oklahoma, and southern New Mexico, a fall of 20° or more to 20° or below;

over eastern North Carolina, central South Carolina, central Georgia,

central Alabama, central Mississippi, central and northern Louisiana and

central and interior Texas, a fall of 18° or more to 25° or below; along

the Gulf coasts of Texas, Louisiana, Mississippi, and Alabama, over all

of Florida, and over the coasts of Georgia and South Carolina, a fall of

16° or more to 32° or below. From March to November inclusive a _cold

wave_ means falls of temperature of the same amounts over the same

districts, with resulting temperatures of 16°, 24°, 28°, 32°, and 36°

respectively.

Notice that the region from which the greatest cold came in this cold wave

is Canada. In that northern country, with its short days and little

sunshine, and its long, cold nights, everything is favorable to the

production of very low temperatures.

Cold waves occur only in winter. In the summer cool spells, with similar

characteristics, may be called _cool waves_.

=Cold-Wave Forecasts.=——A severe cold wave in winter does much damage to

fruit and crops growing out of doors in our Southern States, and to

perishable food products in cars, on the way from the South to supply the

great cities of the North. Therefore it is important that warnings should

be issued giving early information of the coming cold, so that farmers and

fruit growers and shippers may take every precaution to protect their

crops and produce. Our Weather Bureau takes special pains to study the

movements of cold waves and to make forecasts of them, and so well are the

warnings distributed over the country that the fruit growers and the

transportation companies, and the dealers in farm produce, are able every

winter to save thousands of dollars’ worth of fruit and vegetables which

would otherwise be lost. Cold-wave warnings are heeded by many persons

besides those who are directly interested in fruits and farm products. The

ranchmen in the West, with thousands of cattle under their charge; the

trainmen in charge of cattle trains; the engineers of large buildings,

such as hotels, stores, and office buildings, who must have their fires

hotter in cold weather,——these and many more watch, and are governed by,

the cold-wave forecasts of our Weather Bureau.

=Mean Annual and Mean Monthly Isothermal Charts.=——We have thus far

considered isothermal charts for the United States only, based on the

temperature observations made at a single moment of time. It is, of

course, possible to draw isothermal charts, the data for which are not

the temperatures at a given moment, but are the mean or average

temperatures for a month or a year. Such charts have been constructed for

other countries besides our own, as well as for the whole world. An

isothermal chart based on the mean annual temperatures is known as a

_mean annual isothermal chart_. These charts show at once the average

distribution of temperature for the month or for the year, just as the

ones we have drawn show the distribution of temperature over the United

States at a single moment.

_B._ =Direction and Rate of Temperature Decrease. Temperature

Gradient.=——Take your isothermal map for the first day and imagine

yourself at Kansas City, Mo. In what direction must you go from Kansas

City in order to enter most rapidly into colder weather? In what direction

must you go from Kansas City in order to enter most rapidly into warmer

weather? Take the case of Salt Lake City. In what direction must you go

from that station in order to enter most rapidly into colder weather? Into

warmer weather? What are the corresponding directions in the case of

Spokane, Wash.? Of Bismarck, N. Dak.? Of Buffalo, N. Y.? Of Montreal,

Que.? Of Portland, Me.? Of Sacramento, Cal.?

Draw a line from Kansas City to the nearest point at which there is a

temperature 10° lower than at Kansas City. Evidently this point is on the

isotherm of 0°, and will be found if a line be drawn from Kansas City

towards, and at right angles to, the isotherm of 0°. Continue the line

beyond the 0° isotherm in the direction of still lower temperatures,

_i.e._, to the isotherms of -10°, -20°, and -30°. Beyond the isotherm of

-30° the line must stop. Draw similar lines from Seattle, Wash.; Salt Lake

City, Utah; Denver, Col.; St. Paul, Minn.; Cleveland, O.; and New York, N.

Y. Prolong these lines all across the map, so that they will extend from

the regions of highest temperature to those of the lowest. A number of

intermediate lines may also be added. Note that the various directions

followed by these lines are square to, or at right angles to, the

successive isotherms, and that although the lines all run from higher to

lower temperatures, they do not all trend in the same direction. These

lines may be called _lines of decrease of temperature_. Fig. 25 shows a

few of these lines of decrease of temperature drawn for the first day.

Draw similar lines on the other isothermal charts, for the same stations.

Are the directions of temperature decrease the same on these charts as on

the chart for the first day, for Kansas City, Seattle, Salt Lake City,

Denver, St. Paul, Cleveland, New York? Draw lines of decrease of

temperature from the following additional stations: Key West, Fla.; New

Orleans, La.; Charleston, S. C.; El Paso, Tex.; San Diego, Cal.; Hatteras,

N. C.

Compare the directions of these lines on the different days. How do they

change from one day to the next?

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

Next select some line of decrease of temperature on the map for the first

day which begins in Texas, and follow it northward. Where, along this

line, is the decrease of temperature most rapid? Evidently this must be

where the isotherms are closest together, because every isotherm that is

crossed means a change of temperature of 10°, and the more isotherms there

are in a given distance, the more rapidly the temperature is changing.

Where the isotherms are closest together, a given decrease of temperature

is passed over in the least distance, or, conversely, a greater decrease

of temperature is experienced in a given distance. Study this question of

rapidity or slowness of temperature decrease on the whole series of

charts. On which of the charts, and where, do you find the most rapid

decrease? The slowest decrease? Is there any regularity in these _rates_

of temperature decrease either on one map or in the whole series of maps?

The term _temperature gradient_ is used by meteorologists to describe the

_direction_ and _rate of temperature decrease_ which we have been

studying.

If we are to compare these rates of temperature change, we must have some

definite scale of measurement. Thus, for example, in speaking of the wind

velocity we say the velocity of the wind is so many miles per hour; in

describing the grade of a railroad we say it is so many feet in a mile. In

dealing with these temperature changes, we adopt a similar scheme. We say:

The rate of temperature decrease is so many degrees Fahrenheit in a

distance of one latitude degree (about 70 miles). In order to make our

measurements, we use a scale of _latitude degrees_, just as, in

calculating railroad grades, we must have a way to measure the miles of

track in which the ascent or descent of the roadbed is so many feet. Take

a strip of paper 6 inches long, with a straight edge, and lay this edge

north and south at the middle of the weather map, along a longitudinal or

meridian line. Mark off on the strip of paper the points where any two

latitude lines cross the meridian line. These latitude lines are five

(latitude) degrees apart. Therefore divide the space between them on your

paper into five divisions, and each of these will measure just one

latitude degree. Continue making divisions of the same size until you have

ten altogether on the strip of paper. Select, on any weather map, some

station lying between two isotherms at which you wish to measure the rate

of temperature decrease. Take, for instance, Buffalo, N. Y., on the first

day. What you want to find is this: What is the _rate of temperature

decrease_, or the _temperature gradient_, at Buffalo? Lay your paper scale

of latitude degrees through Buffalo, from the isotherm of 10° to the

isotherm of 0°, and as nearly as possible at right angles to the

isotherms.[3] Count the number of latitude degrees on your scale between

the isotherms of 10° and 0°, on a line running through Buffalo. There are,

roughly, about two degrees of latitude in this distance. That is, in the

district in which Buffalo lies, the temperature is changing _at the rate_

of 10° Fahrenheit (between isotherms 10° and 0°) in two latitude degrees.

As our standard of measurement is the amount of change of temperature in

one latitude degree, we divide the 10 (the number of degrees of

temperature) by the 2 (the number of degrees of latitude), which gives us

5 as the rate of decrease of temperature per latitude degree at Buffalo,

N. Y., at 7 A.M., on the first day of the series. The temperature gradient

at Buffalo is therefore 5. The rule may be stated as follows: Select the

station for which you wish to know the rate of temperature decrease or

temperature gradient. Lay a scale of latitude degrees through the station,

and as nearly as possible at right angles to the adjacent isotherms. If

the station is exactly on an isotherm then measure the distance _from_ the

station to the nearest isotherm indicating a temperature 10° lower. The

scale must, however, be laid perpendicularly to the isotherm, as before.

Divide the number of degrees of difference of temperature between the

isotherms (always 10°) by the distance (in latitude degrees) between the

isotherms, and the quotient is the _rate of temperature decrease per

latitude degree_. Or, to formulate the operation:

_R = T / D_,

in which _R_ = rate; _T_ = temperature difference between isotherms

(always 10°), and _D_ = distance between isotherms in latitude degrees.

Thus, a distance of 10 latitude degrees gives a rate of 1; a distance of 5

gives a rate of 2; a distance of 2 gives a rate of 5; a distance of 4

gives a rate of 2.5, etc.

[Footnote 3: Unless the isotherms are exactly parallel, the scale cannot

be at right angles to both of them. It should, however, be placed as

nearly as possible in that position.]

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

_A._ For a considerable number of stations in different parts of the same

map, as for each of the six days of the series.

And, using the school file of weather maps,

_B._ For one station during a winter month and during a summer month,

measuring the rate on each map throughout the month and obtaining an

average rate for the month.

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

during the same months.

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

Northwest during a winter month.

_E._ For a station in the central United States, and one on the Pacific

Coast, the Gulf Coast, and the Atlantic Coast, respectively, during

different months of the winter and summer.

The determination of the rates of temperature decrease under these

different conditions over the United States prepares us for an

appreciation of the larger facts, of a similar kind, to be found on the

mean annual and mean monthly isothermal charts of various countries, and

also of the whole world.

=Temperature Gradients on Isothermal Charts of the Globe.=——The mean

annual isothermal charts of the globe (see page 63) bring out some very

marked contrasts in rates of temperature decrease. Thus, along the eastern

side of the North American continent the isotherms are crowded close

together, while on the western coast of Europe they are spread far apart.

Between southern Florida and Maine there is the same change in mean annual

temperature as is found between the Atlantic coast of the Sahara and

central England. The latter is a considerably longer distance, and this

means that the decrease of temperature is much slower on the European

Atlantic coast than on the North American Atlantic coast. In fact, the

rate of temperature decrease with latitude in the latter case is the most

rapid anywhere in the world, in the same distance. These great

contrasts in temperature which occur within short distances along the

eastern coast of North America have had great influence upon the

development of this region, as has been pointed out by Woeikof, an eminent

Russian meteorologist. The products of the tropics and of the Arctic are

here brought very near together; and at the same time intercommunication

between these two regions of widely differing climates is very easy.

Labrador is climatically an Arctic land, and man is there forced to seek

his food chiefly in the sea, for nature supplies him with little on shore,

while southern Florida is quite tropical in its temperature conditions and

in the abundance of its vegetation. Between the Pacific coasts of Asia and

of North America there is a similar but less pronounced contrast, the

isotherms being crowded together on the eastern coast of China and

Siberia, and being spread apart as they cross the Pacific Ocean and reach

our Pacific Coast.

In general, we naturally expect to find that the temperature decreases as

one goes poleward from the equator; from lower latitudes, where the sun is

always high in the heavens, to higher latitudes, where it is near the

horizon, and its warming effect is less. But there are some curious

exceptions to this general rule. The lowest temperatures on the January

isothermal chart (-60°) are found in northeastern Siberia, and not, so far

as our observations go, near the North Pole. If you find yourself at this

“cold pole,” as it is called, in Siberia in January, you can reach higher

temperatures by traveling north, south, east, or west. In other words,

here is a case of _increase_ of temperature in a _northerly_ direction, as

well as east, south, and west. Again, there is a district of high

temperature (90°) over southern Asia in July, from which you can travel

south towards the equator and yet reach lower temperatures.

In our winter months the contrasts of temperature in the United States

are, as a rule, violent, there being great differences between the cold of

the Northwest and the mild air of Florida and the Gulf States. In the

summer, on the other hand, the distribution of temperature is relatively

equable, the isotherms being, as a rule, far apart. In summer, therefore,

we approach the conditions characteristic of the Torrid Zone. These are

uniformly high temperatures over large areas. The same thing, on a larger

scale, is seen over the whole Northern Hemisphere. During our winter

months the isotherms are a good deal closer together than they are during

the summer, or, in more technical language, the temperature

gradient between the equator and the North Pole is steeper in winter than

in summer.

CHAPTER VI.

WINDS.

The observational work already done, whether non-instrumental or

instrumental, has shown that there is a close relation between the

_direction of the wind_ at any station and the _temperature_ at that

station. Our second step in weather-map drawing is concerned with the

winds on the same series of maps which we have thus far been studying from

the point of view of temperature alone.

In the second column of the table in Chapter VIII are given the wind

directions and the wind velocities (in miles per hour) recorded at the

Weather Bureau stations at 7 A.M., on the first day of the series. Enter

on a blank weather map, at each station for which a wind observation is

given in the table, a small arrow flying _with_ the wind, _i.e._, pointing

in the direction _towards_ which the wind is blowing. Make the lengths of

the wind arrows roughly proportionate to the velocity of the wind, the

winds of higher velocities being distinguished by longer arrows, and those

of lower velocities by shorter arrows. The letters _Lt._ (= light) in the

table denote wind velocities of 5 miles, or less, per hour.

When you have finished drawing these arrows, you will have before you a

picture of the wind directions and velocities all over the United States

at the time of the morning observation on this day. (See solid arrows in

Fig. 26.)

The wind arrows on your map show the wind directions at only a few

scattered points as compared with the vast extent of the United States. We

must remember that the whole lower portion of the atmosphere is moving,

and not merely the winds at these scattered stations. It will help you to

get a clearer picture of this actual movement of the atmosphere as a

whole, if you draw some additional wind arrows between the stations of

observation, but in sympathy with the observed wind directions given in

the table and already entered on your map. These new arrows may be drawn

in broken lines, and may be curved to accord in direction with the

surrounding wind arrows. Heavier or longer lines may be used to indicate

faster winds. (See broken arrows, Fig. 26.)

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

It is clear that the general winds must move in broad sweeping paths,

changing their directions gradually, rather than in narrow belts, with

sudden changes in direction. Therefore long curving arrows give a better

picture of the actual drift of the atmospheric currents than do short,

straight, disconnected arrows.

Study the winds on this chart with care. Describe the conditions of wind

distribution in a general way. Can you discover any apparent relation

between the different wind directions in any part of the map? Is there

any system whatever in the winds? Write out a brief and concise

description of the results of the study of this map.

Enter on five other blank maps the wind directions given in the table in

Chapter VIII for the other five days of the series, making, as before, the

lengths of the arrows roughly proportionate to the velocity of the wind,

and adding extra broken arrows as suggested. (See Figs. 27-31.)

_A._ Study the whole series of six maps. Describe the wind conditions on

each map by itself, noting carefully any system in the wind circulation

that you may discover. Examine the wind velocities also. Are there any

districts in which the velocities are especially high? Have these

velocities any relation to whatever wind systems you may have discovered?

If so, include in your description of these systems some consideration of

the wind _velocities_ as well as of the wind _directions_.

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

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

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

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

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

_B._ Compare each map of the series with the map preceding it. Note what

changes in direction and velocity have taken place at individual

stations. Group these changes as far as possible by the districts over

which similar changes have occurred. Compare the wind systems on each map

with those on the map for the preceding day. Has there been any

alteration in the position or relation of these systems? Write for each

day an account of the conditions on that map, and of the changes that have

taken place in the preceding 24-hour interval.

_C._ Write out a short connected account of the wind conditions and

changes illustrated on the whole set of six maps.

In the last chapter we studied the progression of the cold wave of low

temperatures in an easterly direction across the United States. Notice now

the relation of the winds on the successive maps of our series to the

movement of the cold wave. Place your wind charts and isothermal charts

for the six days side by side, and study them together. The temperature

distribution on the second day differs from that on the first. What are

the chief differences? Examine the wind charts for these two days. Do you

detect any differences in the wind directions or systems on these days? Do

these differences help to explain some of the changes in temperature?

Compare the temperature distribution on the second day with that on the

third. What are the most marked changes in the distribution? What changes

in the winds on the corresponding wind maps seem to offer an explanation

of these variations?

Proceed similarly with each map of the series. Formulate, in writing, the

general relation between winds and cold waves, discovered through your

study of these charts.

=Cold Waves in Other Countries.=——Cold waves in the United States come, as

has been seen, from the northwest, that being the region of greatest

winter cold. In Europe, cold waves come from the northeast. This is

because northwest of Europe there is a large body of warm water supplied

by the Gulf Stream drift, and therefore this is a source of warmth and not

of cold. The cold region of Europe is to the northeast, over Russia and

Siberia.

Cold waves have different names in different countries. In southern France

the cold wind from the north and northeast is known as the _mistral_,

derived from the Latin word _magister_, meaning _master_, on account of

its strength and violence. In Russia the name _buran_ or _purga_ is given

to the cold wave when it blows along with it the fine dry snow

from the surface of the ground. This _buran_ is apt to cause the loss of

many lives, both of men and cattle. In the Argentine Republic the coolest

wind is from the southwest. It is known as a _pampero_, from the Spanish

_pampa_, a _plain_.

=Cyclones and Anticyclones.=——A system of winds blowing towards a common

center (such as is well shown over the Gulf States on the weather map for

the second day, and over the middle Atlantic coast on the third day) is

called by meteorologists a _cyclone_. The name was first suggested by

Piddington early in this century. It is derived from the Greek word for

_circle_, and hence it embodies the idea of a circular or spiral movement

of the winds. A system of _outflowing_ winds, such as that over the

northwestern United States shown on the maps for the first five days, and

over the western Gulf States on the sixth day is called an _anticyclone_.

This name was proposed by Galton in 1863, and means the opposite of

_cyclone_.

CHAPTER VII.

PRESSURE.

_A._ =Lines of Equal Pressure: Isobars.=——One of the most important

weather elements is the _pressure_ of the atmosphere. This has already

been briefly discussed in the sections on the mercurial barometer (Chapter

II). It was there learned that atmospheric pressure is measured by the

number of inches of mercury which the weight of the air will hold up in

the glass tube of the barometer. Our sensation of heat or cold gives us

some general idea as to the air temperature. We can tell the wind

direction when we know the points of the compass, and can roughly estimate

its velocity. No instrumental aid is necessary to enable us to decide

whether a day is clear, fair or cloudy, or whether it is raining or

snowing. Unlike the temperature, the wind, or the weather, the pressure

cannot be determined by our own senses without instrumental aid. The next

weather element that we shall study is pressure.

Proceed as in the case of the thermometer readings. Enter upon a blank

map the barometer readings for the different stations given in the third

column of the table in Chapter VIII. When this has been done you have

before you the actual pressure distribution over the United States at 7

A.M., on the first day of the series. Describe the distribution of

pressure in general terms. Where is the pressure highest? Where lowest?

What are the highest and the lowest readings of the barometer noted on the

map? What is the difference (in inches and hundredths) between these

readings?

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

Draw lines of equal pressure, following the same principles as were

adopted in the case of the isotherms. The latter were drawn for every even

10° of temperature. The former are to be drawn for every even .10 inch of

pressure. Every station which has a barometer reading of an even .10 inch

will be passed through by some line of equal pressure. Philadelphia, Pa.,

with 29.90 must be passed through by the 29.90 line; Wilmington, N. C.,

with 30.00, must have the 30.00 line passing through it, etc. Chicago,

with 30.17 inches, must lie between the lines of 30.10 and 30.20 inches,

and nearer the latter than the former. Denver, Col., with 30.35 inches,

must lie midway between the 30.30 and 30.40 lines (Fig. 32).

Lines of equal pressure are called _isobars_, a word derived from two

Greek words meaning _equal pressure_.

Describe the distribution of pressure as shown by the arrangement of the

isobars. Note the differences in form between the isotherms and the

isobars. The words _high_ and _low_ are printed on weather maps to mark

the regions where the pressure is highest and lowest.

Draw isobars for the other days, using the barometer readings given in the

table in Chapter VIII. Figs. 33-38 show the arrangement of the isobars on

these days.

The pressure charts may be colored, as indicated by the shading in these

figures, in order to bring out more clearly the distribution of pressure,

according to the same general scheme as that adopted in the temperature

charts. Color _brown_ all parts of your six isobaric charts over which the

pressures are below 29.50 inches; color _red_ all parts with pressure

above 30.00 inches. Use a _faint shade of brown_ for pressures between

29.50 inches and 29.00 inches, and a _darker shade_ for pressures below

29.00 inches. In the case of pressures over 30.00 inches, use a _pale red_

for pressures between 30.00 and 30.50 inches, and a _darker shade of red_

for pressures above 30.50 inches. By means of these colors the pressure

distribution will stand out very clearly. The scheme of color and shading

may, of course, be varied to suit the individual fancy.

Study the isobaric chart of each day of the series by itself at first.

Describe the pressure distribution on each chart.

Then compare the successive charts. Note what changes have taken place in

the interval between each chart and the one preceding; where the pressures

have risen; where they have fallen, and where they have remained

stationary. Write a brief account of the facts of pressure change

illustrated on the whole series of six charts.

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

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

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

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

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

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

Compare the charts of temperature and of pressure, first individually,

then collectively. What relations do you discover between temperature

distribution and pressure distribution on the isothermal and the isobaric

charts for the same day? What relations can you make out between the

changes in temperature and pressure distribution on successive days? On

the whole series of maps? Write out the results of your study concisely

and clearly.

Compare the wind charts and the pressure charts for the six days. Is there

any relation between the direction and velocity of the winds and the

pressure? Observe carefully the changes in the winds from day to day on

these charts, and the changes in pressure distribution. Formulate and

write out a brief general statement of all the relations that you have

discovered.

=Mean Annual and Mean Monthly Isobaric Charts.=——We have thus far been

studying isobaric charts based on barometer readings made at a single

moment of time. Just as there are mean annual and mean monthly isothermal

charts, based on the mean annual and mean monthly temperatures, so there

are mean annual and mean monthly isobaric charts for the different

countries and for the whole world, based on the mean annual and mean

monthly pressures. The mean annual and mean monthly isobaric charts of

the world show the presence of great oval areas of low and high pressure

covering a whole continent, or a whole ocean, and keeping about the same

position for months at a time. Thus, on the isobaric chart showing the

mean pressure over the world in January, there are seen immense areas of

high pressure (anticyclones) over the two great continental masses of the

Northern Hemisphere. These anticyclonic areas, although vastly greater in

extent than the small ones seen on the weather maps of the United States,

have the same system of spirally _outflowing_ winds. Over the

northeastern portion of the North Pacific and the North Atlantic, in

January, are seen immense areas of low pressure (cyclones) with spirally

_inflowing_ winds. In July the northern continents are covered by

cyclonic areas, and the central portion of the northern oceans by

anticyclonic areas.

_B._ =Direction and Rate of Pressure Decrease: Pressure Gradient.=——In

Chapter V we studied the direction and rate of temperature decrease, or

temperature gradient. We saw that the direction of this decrease varies in

different parts of the map, and that the rate, which depends upon the

closeness of the isotherms, also varies. An understanding of temperature

gradients makes it easy to study the directions and rates of pressure

decrease, or _pressure gradients_, as they are commonly called. Examine

the series of isobaric charts to see how the lines of pressure decrease

run. Draw lines of pressure decrease for the six isobaric charts, as you

have already done on the isothermal charts. When the isobars are near

together, the lines of pressure decrease may be drawn heavier, to indicate

a more rapid rate of decrease of pressure. Fig. 39 shows lines of pressure

decrease for the first day. Note how the arrangement and direction of

these lines change from one map to the next. Compare these lines with the

lines of temperature decrease.

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

Next study the _rate_ of pressure decrease. This rate depends upon the

closeness of the isobars, just as the rate of temperature decrease depends

upon the closeness of the isotherms. Examine the rates of pressure

decrease upon the series of isobaric charts. On which charts do you find

the most rapid rate? Where? On which the slowest? Where? Do you discover

any relation between rate of pressure decrease and the pressure itself?

What relation?

When expressed numerically, the barometric gradient is understood to mean

the number of hundredths of an inch of change of pressure in one latitude

degree. Prepare a scale of latitude degrees, and measure rates of pressure

decrease, just as you have already done in the case of temperature. In

this case, instead of dividing the difference in temperature between the

isotherms (10° = _T_) by the distance between the isobars (_D_), we

substitute for 10° of temperature .10 inch of pressure (_P_). Otherwise

the operation is precisely the same as described in Chapter V. The rule

may be stated as follows: Select the station for which you wish to know

the rate of pressure decrease or the barometric gradient. Lay your scale

through the station, and as nearly as possible at right angles to the

adjacent isobars. If the station is exactly on an isobar, then measure the

distance _from_ the station to the nearest isobar indicating a lower

pressure. The scale must, however, be laid perpendicularly to the isobars,

as before. Divide the number of hundredths of an inch of pressure

difference between the isobars (always .10 inch) by the number expressing

the distance (in latitude degrees) between the isobars; the quotient is

the rate of pressure decrease per latitude degree. Or, to formulate the

operation,

_R_ = _P_ / _D_,

in which _R_ = rate; _P_ = pressure difference between isobars (always .10

inch), and _D_ = distance between the isobars in latitude degrees.

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

_A._ For a number of stations in different parts of the same map, as,

_e.g._, Boston, New York, Washington, Charleston, New Orleans, St. Louis,

St. Paul, Denver, and on the same day.

_B._ For one station during a winter month and during a summer month,

measuring the rate on each map throughout the month, and obtaining an

average rate for the month.

Have these gradients at the different stations any relation to the

proximity of low or high pressure? To the velocity of the wind?

=Pressure Gradients on Isobaric Charts of the Globe.=——The change from

low pressure to high pressure or _vice versa_ with the seasons, already

noted as being clearly shown on the isobaric charts of the globe,

evidently means that the directions of pressure decrease must also change

from season to season. The rates of pressure decrease likewise do not

remain the same all over the world throughout the year. If we examine

isobaric charts for January and July, we shall find that these gradients

are stronger or steeper over the Northern Hemisphere in the former month

than in the latter.

CHAPTER VIII.

WEATHER.

Hitherto nothing has been said about the _weather_ itself, as shown on the

series of maps we have been studying. By weather, in this connection, we

mean the state of the sky, whether it is clear, fair, or cloudy, or

whether it is raining or snowing at the time of the observation. While it

makes not the slightest difference to our feelings whether the _pressure_

is high or low, the _character of the weather_ is of great importance.

The character of the weather on each of the days whose temperature, wind,

and pressure conditions we have been studying is noted in the table in

this chapter. The symbols used by the Weather Bureau to indicate the

different kinds of weather on the daily weather maps are as follows: [full

moon] clear; [quarter moon] fair, or partly cloudy; [new moon] cloudy;

[circle-R] rain; [circle-S] snow.

Enter on a blank map, at each station, the sign which indicates the

weather conditions at that station at 7 A.M., on the first day, as given

in the table. When you have completed this, you have before you on the map

a bird’s-eye view of the weather which prevailed over the United States at

the moment of time at which the observations were taken. Describe in

general terms the distribution of weather here shown, naming the districts

or States over which similar conditions prevail. Following out the general

scheme adopted in the case of the temperature and the pressure

distribution, separate, by means of a line drawn on your map, the

districts over which the weather is prevailingly cloudy from those over

which the weather is partly cloudy or clear. In drawing this line,

scattering observations which do not harmonize with the prevailing

conditions around them may be disregarded, as the object is simply to

emphasize the _general_ characteristics. Enclose also, by means of another

line, the general area over which it was snowing at the time of

observation, and shade or color the latter region differently from the

cloudy one. Study the weather distribution shown on your chart. What

general relation do you discover between the kinds of weather and the

temperature, winds, and pressure?

Proceed similarly with the weather on the five remaining days, as noted in

the table. Enter the weather symbols for each day on a separate blank map,

enclosing and shading or coloring the areas of cloud and of snow as above

suggested. In Figs. 40-45 the cloudy areas are indicated by single-line

shading, and the snowy areas by double-line shading.

Now study carefully each weather chart with its corresponding temperature,

wind, and pressure charts. Note whatever relations you can discover among

the various meteorological elements on each day. Then compare the weather

conditions on the successive maps. What changes do you note? How are these

changes related to the changes of temperature; of wind; of pressure?

Write a summary of the results derived from your study of these four sets

of charts.

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

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

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

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

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

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

=The Weather of Temperate and Torrid Zones.=——The facts of the presence of

clear weather in one region while snow is falling in another, and of the

variability of our weather from day to day in different parts of the

United States, are emphasized by these charts of weather conditions. This

changeableness of weather is a marked characteristic of the greater

portion of the Temperate Zones, especially in winter. The weather maps for

successive days do not, as a rule, show a repetition of the same

conditions over extended regions. In the Torrid Zone it is different. Over

the greater part of that zone the regularity of the weather conditions is

such that, day after day, for weeks and months, the same features are

repeated. There monotony, here variety, is the dominant characteristic of

the weather.

PART IV.——THE CORRELATIONS OF THE WEATHER

ELEMENTS AND WEATHER FORECASTING.

CHAPTER IX.

CORRELATION OF THE DIRECTION OF THE WIND AND THE

PRESSURE.

The study of the series of weather maps in Chapters V-VIII has made it

clear that some fairly definite relation exists between the general flow

of the winds and the distribution of pressure. We now wish to obtain some

more definite result as to the relation of the direction of the wind and

the pressure. In doing this it is convenient to refer the wind direction

to the _barometric_ or _pressure gradient_ at the station at which the

observation is made. The barometric gradient, it will be remembered, is

the line along which there is the most rapid change of pressure, and lies

at right angles to the isobars (Chapter VII).

[Illustration: FIG. 46.]

Take a small piece of tracing paper, about 3 inches square, and draw upon

it a diagram similar to the one here shown. Select the station (between

two isobars on any weather map) at which you intend to make your

observation. Place the center of the tracing paper diagram over the

station, with the dotted line along the barometric gradient, the minus end

of the line being towards the area of low pressure. Observe into which of

the four sectors (marked _right_, _left_, _with_, _against_) the wind

arrow at the station points. Keep a record of the observation. Repeat the

observation at least 100 times, using different stations, on the same map

or on different maps. Tabulate your results according to the following

scheme, noting in the first column the date of the map, in the second,

third, fourth, and fifth columns the number of winds found blowing _with_,

to the _right_ or _left_ of, and _against_, the gradient.

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

AND THE PRESSURE.

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

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

| | | | | |

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

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

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

At the bottom of each column write down the number of cases in that

column, and then determine the percentages which these cases are of the

total number of observations. This is done by dividing the number of cases

in each column by the sum-total of all the observations. When you have

obtained the percentage of each kind of wind direction, you have a

numerical result.

A graphical presentation of the results may be made by laying off radii

corresponding in position to those which divide the sectors in Fig. 46,

and whose lengths are proportionate to the percentages of the different

wind directions in the table. Thus, for a percentage of 20, the radii may

be made 1 inch long, for 40%, 2 inches, etc. When completed, the relative

sizes of the sectors will show the relative frequencies of winds blowing

in the four different directions with reference to the gradient, as is

indicated in Fig. 47.

=The Deflection of the Wind from the Gradient: Ferrel’s Law.=——The law of

the deflection of the wind prevailingly to the right of the gradient is

known as _Ferrel’s Law_, after William Ferrel, a noted American

meteorologist, who died in 1891. The operation of this law has already

been seen in the spiral circulation of the winds around the cyclone and

the anticyclone, as shown on the maps of our series. In the case of the

cyclone the gradient is directed inward towards the center; in the case

of the anticyclone the gradient is directed outward from the center. In

both cases the right-handed deflection results in a spiral whirl, inward

in the cyclone, outward in the anticyclone. The operation of this law is

further seen in the case of the _Northeast Trade Winds_. These winds blow

from about Lat. 30° N. towards the equator, with wonderful regularity,

especially over the oceans. Instead of following the gradient and blowing

as north winds, these trades turn to the right of the gradient and become

_northeast_ winds, whence their name. From about Lat. 30° N. towards the

North Pole there is another great flow of winds over the earth’s surface.

These winds do not flow due north, as south winds. They turn to the

right, as do the trades, and become southwest or west-southwest winds,

being known as the _Prevailing Westerlies_. Ferrel’s Law thus operates in

the larger case of the general circulation of the earth’s atmosphere, as

well as in the smaller case of the local winds on our weather maps.

[Illustration: FIG. 47.]

CHAPTER X.

CORRELATION OF THE VELOCITY OF THE WIND AND

THE PRESSURE.

Prepare a scale of latitude degrees, as explained in Chapter V. Select

some station on the weather map at which there is a wind arrow, and at

which you wish to study the relation of wind velocity and pressure. Find

the rate of pressure change per degree as explained in Chapter VII. Note

also the velocity, in miles per hour, of the wind at the station. Repeat

the operation 100 or more times, selecting stations in different parts of

the United States. It is well, however, to include in one investigation

either interior stations alone (_i.e._, more than 100 miles from the

coast) or coast stations alone, as the wind velocities are often

considerably affected by proximity to the ocean. And, if coast stations

are selected, either onshore or offshore winds should alone be included in

one exercise. The investigation may, therefore, be carried out so as to

embrace the following different sets of operations:——

_A._ Interior stations.

_B._ Coast stations with onshore winds.

_C._ Coast stations with offshore winds.

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

TABLE II.——CORRELATION OF WIND VELOCITY AND BAROMETRIC

GRADIENT.

For interior (or coast) stations, with onshore (or offshore) winds, in the

United States during the month (or months) of

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

|Change per Latitude | ∞-20 | 20-10 | 10-5 | 5- | 3-1/2- | 2-1/2 | etc. |

|Degree | | | | 3-1/2 | 2-1/2 | -2 | |

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

|Isobars in Latitude | 0-1/2 | 1/2-1 | 1-2 | 2-3 | 3-4 | 4-5 | etc. |

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

| | | | | | | | |

| | | | | | | | |

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

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

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

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

The wind velocity for each station is to be entered in the column at whose

top is the rate of pressure change found for that station. Thus, if for

any station the rate of pressure change is 3-1/2 (_i.e._, .03 inch in one

latitude degree), and the wind velocity at that station is 17 miles an

hour, enter the 17 in the fourth and fifth columns of the table. When you

find that the rate of pressure change for any station falls into two

columns of the table, as, _e.g._, 10, or 5, or 3-1/3, then enter the

corresponding wind velocity in both those columns.

In the space marked _Sums_ write the sum-total of all the wind velocities

in each column. The _Cases_ are the number of separate observations you

have in each column. The _Means_ denote the average or mean wind

velocities found in each column, and are obtained by dividing the sums by

the cases.

Study the results of your table carefully. Deduce from your own results a

general rule for wind velocities as related to barometric gradients.

=The dependence of wind velocities on the pressure gradient= is a fact of

great importance in meteorology. The ship captain at sea knows that a

rapid fall of his barometer means a rapid rate of pressure change, and

foretells high winds. He therefore makes his preparations accordingly, by

shortening sail and by making everything fast. The isobaric charts of the

globe for January and July show that the pressure gradients are stronger

(_i.e_., the rate of pressure change is more rapid) over the Northern

Hemisphere in January than in July. This fact would lead us to expect

that the velocities of the general winds over the Northern Hemisphere

should be higher in winter than in summer, and so they are. Observations

of the movements of clouds made at Blue Hill Observatory, Hyde Park,

Mass., show that the whole atmosphere, up to the highest cloud level,

moves almost twice as fast in winter as in summer. In the higher

latitudes of the Southern Hemisphere, where the barometric gradients are

prevailingly much stronger than in the Northern, the wind velocities are

also prevailingly higher than they are north of the equator. The

prevailing westerly winds of the Southern Hemisphere, south of latitude

of 30° S., blow with high velocities nearly all the time, especially

during the winter months (June, July, August). These winds are so strong

from the westward that vessels trying to round Cape Horn from the east

often occupy weeks beating against head gales, which continually blow

them back on their course.

CHAPTER XI.

FORM AND DIMENSIONS OF CYCLONES AND ANTICYCLONES.

_A._ =Cyclones.=——Provide yourself with a sheet of tracing paper about

half as large as the daily weather map. Draw a straight line across the

middle of it; mark a dot at the center of the line, the letter _N_ at one

end, and the letter _S_ at the other. Place the tracing paper over a

weather map on which there is a fairly well enclosed center of low

pressure (_low_), having the dot at the center of the _low_, and the line

parallel to the nearest meridian, the end marked _N_ being towards the top

of the map. When thus placed, the paper is said to be _oriented_. Trace

off the isobars which are nearest the center. In most cases the 29.80-inch

isobar furnishes a good limit, out to which the isobars may be traced.

Continue this process, using different weather maps, until the lines on

the tracing paper begin to become too confused for fairly easy seeing.

Probably 15 or 20 separate areas of low pressure may be traced on to the

paper. It is important to have all parts of the cyclonic areas represented

on your tracing. If most of the isobars you have traced are on the

southern side of cyclones central over the Lakes or lower St. Lawrence, so

that the isobars on the northern sides are incomplete, select for your

further tracings weather maps on which the cyclonic centers are in the

central or southern portions of the United States, and therefore have

their northern isobars fully drawn.

When your tracing is finished you have a _composite portrait_ of the

isobars around several areas of low pressure. Now study the results

carefully. Draw a heavy pencil or an ink line on the tracing paper, in

such a way as to enclose the average area outlined by the isobars. This

average area will naturally be of smaller dimensions than the outer

isobars on the tracing paper, and of larger dimensions than the inner

isobars, and its form will follow the general trend indicated by the

majority of the isobars, without reproducing any exceptional shapes.

Write out a careful description of the average _form_, _dimensions_

[measured by a scale of miles or of latitude degrees (70 miles = 1 degree

about)] and _gradients_ of these areas of low pressure, noting any

tendency to elongate in a particular direction; any portions of the

composite where the gradients are especially strong, weak, etc.

_B._ =Anticyclones.=——This investigation is carried out in precisely the

same manner as the preceding one, except that anticyclones (_highs_) are

now studied instead of cyclones. The isobars may be traced off as far away

from the center as the 30.20-inch line in most cases. When, however, the

pressure at the center is exceptionally high, it will not be necessary to

trace off lower isobars than those for 30.30, or 30.40, or sometimes 30.50

inches.

=Loomis’s Results as to Form and Dimensions of Cyclones and

Anticyclones.=——One of the leading American meteorologists, Loomis, who

was for many years a professor in Yale University, made an extended study

of the form and dimensions of areas of low and high pressure as they

appear on our daily weather maps. In the cases of areas of low pressure

which he examined, the average form of the areas was elliptical, the

longer diameter being nearly twice as long as the shorter (to be exact

the ratio was 1.94 : 1). The average direction of the longer diameter he

found to be about NE. (N. 36° E.), and the length of the longer diameter

often 1600 miles. In the case of areas of high pressure, Loomis also

found an elliptical form predominating; the longer diameter being about

twice as long as the shorter (ratio 1.91 : 1), and the direction of trend

about NE. (N. 44° E.). These characteristics hold, in general, for the

cyclonic and anticyclonic areas of Europe also. The cyclones of the

tropics differ considerably from those of temperate latitudes in being

nearly circular in form.

CHAPTER XII.

CORRELATION OF CYCLONES AND ANTICYCLONES WITH THEIR

WIND CIRCULATION.

_A._ =Cyclones.=——Something as to the control of pressure over the

circulation of the wind has been seen in the preliminary exercises on the

daily weather maps. We now proceed to investigate this correlation further

by means of the composite portrait method. This method is a device to

bring out more clearly the general systems of the winds by throwing

together on to one sheet a large number of wind arrows in their proper

position with reference to the controlling center of low pressure. In this

way we have many more observations to help us in our investigation than if

we used only those which are given on one weather map, and the circulation

can be much more clearly made out.

Provide yourself with a sheet of tracing paper, prepared as described in

Chapter XI. Place the paper over an area of low pressure on some weather

map, with the dot at the center of the _low_, and having the paper

properly oriented, as already explained. Trace off all the wind arrows

around the center of low pressure, making the lengths of these arrows

roughly proportionate (by eye) to the velocity of the wind, according to

some scale previously determined upon. Include on your tracing all the

wind arrows reported at stations whose lines of pressure-decrease converge

towards the low pressure center. Repeat this operation, using other

centers of low pressure on other maps, until the number of arrows on the

tracing paper is so great that the composite begins to become confused. Be

careful always to center and orient your tracing paper properly. Select

the weather maps from which you take your wind arrows so that the

composite shall properly represent winds in all parts of the cyclonic

area.

Deduce a general rule for the circulation and velocity of the wind in a

cyclonic area, as shown on your tracing, and write it out.

_B._ =Anticyclones.=——This exercise is done in precisely the same way as

the preceding one, except that anticyclones and their winds are studied

instead of cyclones.

Deduce a general rule for the circulation and velocity of the wind in an

anticyclonic area, as shown on your tracing, and write it out.

=The control of the wind circulation by areas of low and high pressure= is

one of the most important laws in meteorology. Buys-Ballot, a Dutch

meteorologist, first called attention to the importance of this law in

Europe, and it has ever since been known by his name. Buys-Ballot’s Law is

generally stated as follows: _Stand with your back to the wind, and the

barometer will be lower on your left hand than on your right._[4] This

statement, as will be seen, covers both cyclonic and anticyclonic systems.

The circulations shown on your tracings hold everywhere in the Northern

Hemisphere, not only around the areas of low and high pressure seen on the

United States weather maps, but around those which are found in Europe and

Asia, and over the oceans as well. Mention has already been made, in the

chapter on isobars (VII), of the occurrence of immense cyclonic and

anticyclonic areas, covering the greater portion of a continent or an

ocean, and lasting for months at a time. These great cyclones and

anticyclones have the same systems of winds around them that the smaller

areas, with similar characteristics, have on our weather maps. A further

extension of what has just been learned will show that if in any region

there comes a change from low pressure to high pressure, or _vice versa_,

the system of winds in that region will also change. Many such changes of

pressures and winds actually occur in different parts of the world, and

are of great importance in controlling the climate. The best-known and the

most-marked of all these changes occurs in the case of India.

During the winter, an anticyclonic area of high pressure is central over

the continent of Asia. The winds blow out from it on all sides, thus

causing general northeasterly winds over the greater portion of India.

These winds are prevailingly dry and clear, and the weather during the

time they blow is fine. India then has its dry season. As the summer comes

on, the pressure over Asia changes, becoming low; a cyclonic area replaces

the winter anticyclone, and inflowing winds take the place of the

outflowing ones of the winter. The summer winds cross India from a general

southwesterly direction, come from over the ocean, and are moist and

rainy. India then has its rainy season. These seasonal winds are known as

_Monsoons_, a name derived from the Arabic and meaning _seasonal_.

[Footnote 4: In the Northern Hemisphere.]

[Illustration: FIG. 48.]

The accompanying figure (Fig. 48) is taken from the _Pilot Chart of the

North Atlantic Ocean_, published by the Hydrographic Office of the United

States Navy for the use of seamen. It shows the wind circulation around

the center of a cyclone which is moving northward along the Atlantic Coast

of the United States. The long arrow indicates the path of movement; the

shorter arrows indicate the directions of the winds. By means of such a

diagram as this a captain is able to calculate, with a considerable degree

of accuracy, the position of the center of the cyclone, and can often

avoid the violent winds near that center by sailing away from it, or by

“lying to,” as it is called, and waiting until the center passes by him at

a safe distance. These cyclones which come up the eastern coast of the

United States at certain seasons are usually violent, and often do

considerable damage to shipping. The Weather Bureau gives all the warning

possible of the coming of these _hurricanes_, as they are called, by

displaying _hurricane signals_ along the coast, and by issuing telegraphic

warnings to newspapers. In this way ship captains, knowing of the approach

of gales dangerous to navigation, may keep their vessels in port until all

danger is past. Millions of dollars’ worth of property and hundreds of

lives have thus been saved.

CHAPTER XIII.

CORRELATION OF THE DIRECTION OF THE WIND AND THE

TEMPERATURE.

It is evident, from even the most general observation of the weather

elements, that the temperature experienced at any place is very largely

dependent upon the direction of the wind. Thus, for instance, in the

United States, a wind from some northerly point is likely to bring a lower

temperature than a southerly wind. To investigate this matter more

closely, and to discover how the winds at any station during any month are

related to the temperatures noted at that station, we proceed as

follows:——

Select the Weather Bureau station at which you wish to study these

conditions. Note the direction of the wind and the temperature at that

station on the first day of any month. Prepare a table similar to the

following one.

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

AND THE TEMPERATURE.

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

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

| DIRECTIONS | N. | NE. | E. | SE. | S. | SW. | W. | NW. | |

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

| | | | | | | | | | |

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

| | | | | | | | | | |

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

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

| | | | | | | | | | |

+ +----+-----+----+-----+----+-----+----+-----+ |

| | | | | | | | | | |

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

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

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

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

Enter the temperature at 8 A.M. on the first day of the month in a column

of the table under the proper wind direction. Thus, if the wind is NE.,

and the temperature 42°, enter 42 in the second column of the table.

Repeat the observation for the same station, and for all the other days of

the month, recording the temperatures in each case in their appropriate

columns in the table. Omit all cases in which the wind is _light_, because

winds of low velocities are apt to be considerably affected by local

influences. When the observations for the whole month have been entered in

the table, add up all the temperatures in each column (_sums_). Find the

mean temperature (_means_) observed with each wind direction by dividing

the sums by the number of observations in each column (_cases_). Add all

the sums together; divide by the total number of cases, and the result

will be the mean temperature[5] for the month at the station. The general

effect of the different wind directions upon the temperature is shown by a

comparison of the means derived from each column with the mean for the

month.

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

true mean temperature.]

[Illustration: FIG. 49.]

A graphic representation of the results of this investigation will help to

emphasize the lesson. Draw, as in the accompanying figure (Fig. 49), eight

lines from a central point, each line to represent one of the eight wind

directions. About the central point describe a circle, the length of whose

radius shall correspond to the mean temperature of the month, measured on

some convenient scale. Thus, if the mean temperature of the month is 55°

and a scale of half an inch is taken to correspond to 10° of temperature,

the radius of the circle must be five and a half times half an inch, or

2-3/4 inches. Next lay off on the eight wind lines the mean temperatures

corresponding to the eight different wind directions, using the same scale

(1/2 in. = 10°) as in the previous case. Join the points thus laid off by a

heavy line, as shown in Fig. 49. The figure, when completed, gives at a

glance a general idea of the control exercised by the winds over the

temperatures at the station selected. Where the heavy line crosses a wind

line _inside_ the circle it shows that the average temperature

accompanying the corresponding wind direction is below the mean. When the

heavy line crosses any wind line _outside_ the circle, it shows that the

average temperature accompanying the corresponding wind direction is above

the mean. Such a figure is known as a _wind rose_.

=The cold wave and the sirocco= are two winds which exercise marked

controls over the temperature at stations in the central and eastern

United States. The _cold wave_ has already been described in Chapter V.

It is a characteristic feature of our winter weather. It blows down from

our Northwestern States or from the Canadian Northwest, on the western

side of a cyclone. It usually causes sudden and marked falls in

temperature, sometimes amounting to as much as 50° in 24 hours. The

_sirocco_ is a southerly or southwesterly wind. It also blows into a

cyclone, but on its southern or southeastern side. Coming from warmer

latitudes, and from over warm ocean waters, the sirocco is usually a warm

wind, in marked contrast to the cold wave. In winter, in the Mississippi

Valley and on the Atlantic Coast, the sirocco is usually accompanied by

warm, damp, cloudy, and snowy or rainy weather. The high temperatures

accompanying it (they may be as high as 50° or 60° even in midwinter) are

very disagreeable. Our warm houses and our winter clothing become

oppressive and we long for the bright, crisp, cold weather brought by the

_cold wave_. In summer when a sirocco blows we have our hottest spells.

Then sunstrokes and prostrations by the heat are most common, and our

highest temperatures are recorded. The word _sirocco_ (from

_Syriacus_=Syrian) was first used as the name of a warm southerly wind in

Italy. The cause and the characteristics of the Italian sirocco and of

the American sirocco are similar, and the name may therefore be applied

to our wind as well as to the Italian one. In the Southern Hemisphere, at

Buenos Ayres, Argentine Republic, there is a similar contrast between two

different winds. The _pampero_ is similar in many respects to our cold

wave. It is a dry, cool, and refreshing wind, blowing over the vast level

stretches of the Argentine pampas from the southwest. The _norte_ is a

warm, damp, depressing northerly wind corresponding to our sirocco.

CHAPTER XIV.

CORRELATION OF CYCLONES AND ANTICYCLONES AND THEIR

TEMPERATURES.

_A._ =Cyclones.=——It follows from the two preceding exercises that some

fairly definite distribution of temperature, depending upon the wind

direction, should exist around areas of low and high pressure. Try to

predict, on the basis of the results obtained in Chapters XII and XIII,

what this relation of temperatures and cyclones and anticyclones is. Then

work out the relation independently of your prediction, by studying actual

cases obtained from the weather maps, as follows:——

[Illustration: FIG. 50.]

Prepare a sheet of tracing paper as shown in Fig. 50. The diameter of the

circle should be sufficiently large to include within the circle the

average area covered by a cyclone on the weather maps. Place the tracing

paper, properly divided in accordance with the figure, over a well-defined

area of low pressure on a weather map, centering and orienting it

carefully. Take the temperature at the station which lies nearest the

center of the figure as the standard. Notice the temperatures at all the

other stations which fall within the limits of the circle, and mark down

at the proper places on the tracing paper, the + or - departures of these

local temperatures from the standard temperature. Thus, if the standard is

37°, and a station has a temperature of 46°, enter +9° at the proper place

on your tracing paper. Again, if a certain station has 24°, enter -13° at

the proper place on the paper. Continue this process until your paper has

all of its divisions well filled. It is best to select all the maps used

in this investigation from the same month, for in that case the data are

more comparable than if different months are taken. When a sufficient

number of examples has been obtained, find the average departure (+ or -)

of the temperatures in each division of the tracing from the central

standard temperature. Express these averages graphically by means of a

_wind rose_, as in the last exercise.

=Another Method.=——The above correlation may be investigated by means of

another method, as follows:——

Prepare a piece of tracing paper by drawing an N. and S. line upon it, and

placing a dot at the center of the line. Lay the paper over an area of low

pressure on any weather map, centering and orienting it properly, as in

the previous exercises. Trace off the isotherms which are near the center

of low pressure. Repeat this process with several maps, selecting

different ones from those used in the first part of this exercise.

Formulate a rule for the observed distribution of temperature, and

determine the reasons for this distribution. Note carefully any effects of

the cyclone upon the temperature gradient.

_B._ =Anticyclones.=——The correlation of anticyclones with their

temperatures is studied in precisely the same way as the preceding

correlation. Both methods suggested in the case of cyclones should be used

in the case of anticyclones. When your results have been obtained,

formulate a general rule for the observed distribution of temperature in

anticyclones, and determine the reasons for this distribution.

Find from your composites the average temperature of cyclones and of

anticyclones, and compare these averages.

=The unsymmetrical distribution of temperature around cyclones=, which is

made clear by the foregoing exercises, is very characteristic of these

storms in our latitudes, and especially in the eastern United States.

That this has an important effect upon weather changes is evident, and

will be further noted in the chapter on _Weather Forecasting_. The

cyclones which begin over the oceans near the equator at certain seasons,

and thence travel to higher latitudes,——_tropical cyclones_, so

called,——differ markedly from our cyclones in respect to the distribution

of temperature around them. The temperatures on all sides of tropical

cyclones are usually remarkably uniform, the isotherms coinciding fairly

closely with the isobars. The reason for this is to be found in the

remarkable uniformity of the temperature and humidity conditions over the

surrounding ocean surface, from which the inflowing winds come. In the

case of our own cyclones, in the eastern United States, the warm southerly

wind, or sirocco, in front of the center has very different

characteristics from those of the cold northwesterly wind, or _cold wave_,

in the rear, as has become evident through the preceding exercise. These

winds, therefore, naturally show their effects in the distribution of the

temperatures in different parts of the cyclonic area.

CHAPTER XV.

CORRELATION OF THE DIRECTION OF THE WIND AND THE

WEATHER.

Select a file of daily weather maps for some month. Commencing with the

first map in the set, observe the weather and the direction of the wind at

a considerable number of stations in the same general region (as, _e.g._,

the Lake region, the lower Mississippi Valley, the Pacific Coast, etc.).

Enter each case in a table, similar to Table IV below, by making a check

in the column under the appropriate wind direction and on a line with the

appropriate type of weather.

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

AND THE WEATHER.

At ________ during the Month of ________.

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

| | N. | NE. | E. | SE. | S. | SW. | W. | NW. |TOTALS|PERCENTAGES|

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

|Clear | | | | | | | | | D | J |

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

|Fair | | | | | | | | | F | K |

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

|Cloudy | | | | | | | | | G | L |

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

|Rain and Snow| | | | | | | | | H | M |

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

|Totals | | | | | | | | | A | |

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

|Percentages | B | C |etc.| | | | | | | |

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

Count every observation of _rain_ or _snow also as cloudy_, for it usually

rains or snows only when the sky is cloudy. Continue your observations on

all the maps for the month you have chosen. Then count up the whole number

of cases of _clear_ weather you have found with north winds, and write

down this sum in the first space, in the column reserved for N. winds. Do

the same with _fair_ and _cloudy_ weather. Add up and enter at the bottom

of the column in the space marked _Totals_ the whole number of

observations of _clear_, _fair_, and _cloudy_ weather you have observed

with N. winds. Then find what percentage of the weather with N. winds was

_clear_, and enter this percentage next to the sum of _clear_ weather

observations, in the first division in the column headed N. Do the same

for _fair_, _cloudy_, and _rainy_ or _snowy_ weather, deriving the

percentages of rain or snow from the total of _clear_ and _fair_ and

_cloudy_. Repeat this process of summarizing in every column. Your

results will then show the percentages of the different kinds of weather

noted with the different wind directions.

The lower division of the table and the last two columns on the right are

to be used for a general summary of the whole investigation. By adding

together all the totals of _clear_, _fair_, and _cloudy_ weather observed

with all the eight wind directions you obtain the whole number of

observations you have made. Enter this in the space marked A, at the right

of the table. From this grand total and the total number of observations

in each column you may find (in percentages) the relative frequencies of

the different wind directions. These should be entered under the totals at

the bottom of each column, in the spaces marked _Percentages_ (spaces B,

C, etc.). The total number of observations of _clear_, _fair_, _cloud_,

and _rain_ or _snow_, noted with all the wind directions, are to be

entered in spaces D, F, G, and H, at the right of the table. From these

totals, and from the grand total in space A, we can determine the relative

frequency (in percentages) of each kind of weather during the month. These

results should be entered in spaces J, K, L, and M.

Study these results carefully. Formulate them in a brief written

statement. Express graphically the following:——

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

the month.

_B._ The percentages of the different kinds of weather noted during the

whole month for all wind directions.

[Illustration: FIG. 51.]

A wind rose, indicating the percentages of frequency of different winds

during a month, or a year, or several years, may be constructed as shown

in Fig. 51.

A certain convenient scale is adopted as representing a frequency of 10%,

and a circle is drawn with this unit as a radius. A second circle, with a

radius twice as long, represents a frequency of 20%, and a third circle,

with a radius three times as long, represents a frequency of 30%.

Additional circles may be added if necessary. Distances corresponding to

the different percentages of frequency of the eight wind directions are

then laid off along the eight radii of the circles, and the points thus

fixed are joined by a line.

The results asked for under question _B_ may be plotted as a weather rose

on a diagram similar to that above figured. In this case the percentages

of frequency of the different varieties of weather (_clear_, _fair_,

_cloudy_, _stormy_) may be indicated on the same figure by using different

kinds of lines. Thus, a _solid_ line may be employed to represent _clear_

weather; a _broken_ line for _fair_; a _broken and dotted_ line for

_cloudy_; and a _dotted_ line for _stormy_ weather.

CHAPTER XVI.

CORRELATION OF CYCLONES AND ANTICYCLONES AND THE

WEATHER.

_A._ =Cyclones.=——Prepare a piece of tracing paper as shown in Fig. 52,

making the diameter of the outer circle about 1000 miles[6] and of the

inner circle 500 miles. Place this diagram over a cyclone on any weather

map, centering and orienting it carefully. Trace off the weather signs

(indicating _clear_, _fair_, _cloudy_, _rain_ or _snow_) around the

cyclonic center from the map on to the tracing paper, taking only

observations which are not more than halfway from the cyclonic center to

the neighboring anticyclonic center. Repeat this process with successive

weather maps until the diagram is well filled in all its different

divisions.

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

_A._ Draw a line on the tracing paper enclosing the average area of

_cloud_ (including _rain and snow_), and a second line enclosing the

average area of _precipitation_ (_rain_ or _snow_).

_B._ Determine the percentages of _clear_, _fair_, _cloudy_, and _stormy_

observations for each division of the tracing paper, _i.e._, (_a_) for the

eight sectors of the large circle; (_b_) for the whole of the small

circle; and (_c_) for the portion of the diagram between the circumference

of the inner circle and the circumference of the outer circle.

[Illustration: FIG. 52.]

_C._ Write out in general terms a description of the weather distribution

in cyclones as illustrated by your own investigation.

_B._ =Anticyclones.=——This exercise is done in the same way as the

preceding one, except that anticyclones are substituted for cyclones.

=The association of fair weather with anticyclones and of stormy weather

with cyclones= is one of the most important lessons learned from a study

of the weather maps. The great areas of high and low pressure control our

weather. On land, where daily weather maps are so easily accessible, a

glance at the map serves in most cases to give a fairly accurate idea of

the position and extent of cyclones and anticyclones, and hence also of

the distribution of weather. At sea, on the other hand, the navigator has

no daily weather maps to refer to, and his knowledge of the weather

conditions which he may expect must be gained from his own observations

alone. Of these local observations, the pressure readings are by far the

most important. A falling barometer usually means the approach of a

cyclone, with wind, or storm, or both. A rising barometer, on the other

hand, is usually an indication of the fine weather associated with an

anticyclone. The unsymmetrical distribution of weather, characteristic of

our cyclones in the United States, and also of most cyclones in the

Temperate Zone, is associated with their unsymmetrical form, and the

unsymmetrical distribution of their temperature already studied. Tropical

cyclones have a wonderfully uniform distribution of weather on all sides

of their centers, just as they have a symmetrical form and an even

temperature distribution all around them.

CHAPTER XVII.

PROGRESSION OF CYCLONES AND ANTICYCLONES.

So far no definite study has been made of the changes in the positions of

cyclones and anticyclones. If these areas of stormy and fair weather

always occupied the same geographic positions, the different portions of

the country would always have the same kinds of weather. A knowledge of

the movements of the areas of low and high pressure makes weather

forecasting possible.

_A._ =Cyclones.=——Select a set of daily weather maps for a month. Turn to

the first map of the series. Note the position of the center of low

pressure, and indicate this position on a blank weather map of the United

States by marking down a small circle at the proper place. If there are

two or more areas of low pressure on the map, indicate the position of

each one of them in a similar way. Turn to the second map of the series,

and again enter on the blank map the position of the center of low

pressure. Connect the two positions of each center by a line. This line

may be called the _track_ of the low pressure center. Continue this

process through the whole set of maps, connecting all the new positions

with the last positions of their respective centers. Mark each position

with the appropriate date in small, neat figures. When completed your map

will show at a glance the tracks followed by all the cyclones which

traveled across the United States during the month you selected. Study

these tracks carefully. Notice whether there is any prevailing direction

in which the cyclones move, and whether they show any preference for

particular paths across the country. Can you frame a general rule for the

prevailing direction and path of movement? Are there any cases which do

not accord with the rule? If so, describe them. In what position, with

reference to the cyclonic tracks during the month you are studying, is the

region in which you are now living?

Next determine the _velocities_ with which these cyclones moved. Prepare a

scale of latitude degrees, as described in Chapter V, or of miles, as

given at the bottom of the weather map. Measure the distances, in miles,

between the successive positions of all the cyclonic centers. Divide these

distances by 24 in order to obtain the velocity in _miles per hour_. What

is the highest velocity per hour with which any cyclone moved during the

month? What is the lowest? What is the mean, or average, velocity?

Study the tracks and velocities of cyclones in a similar way during

several other months. Compare the positions of the tracks, and the

velocities of progression, in summer and winter.

_B._ =Anticyclones.=——Study the tracks and velocities of anticyclones in

precisely the same manner. Compare the results derived from your

investigations in the two cases.

=Cyclonic Tracks and the Prevailing Westerly Winds.=——The correspondence

in the direction of movement of most of our cyclones and of the

prevailing westerly winds of the Northern Hemisphere (see Chapter IX)

will readily be noted. Our weather maps show us the atmospheric

conditions over the United States alone, and we can therefore trace the

progression of our cyclones over but a limited area of the Northern

Hemisphere. An examination of the daily weather maps of the North

Atlantic and North Pacific Oceans, which are based on observations made

on board ships, and of the weather maps of other countries, shows us that

these atmospheric disturbances which appear on our maps may often be

traced for long distances across oceans and lands, and that they in

reality form a great procession across the northern portion of this

hemisphere, and towards and around the North Pole.

=The average velocity of cyclones in the United States= was carefully

determined by Loomis. His observations show that the mean hourly velocity

of cyclones for the entire year is 28.4 miles, the maximum (34.2 miles)

coming in February, and the minimum (22.6 miles) in August. Over the North

Atlantic Ocean the hourly velocity is 18 miles; in Europe, 16.7 miles. The

greater velocity of cyclonic movement in the United States in winter

recalls what was said at the close of Chapter X concerning the steeper

barometric gradient and the more rapid movement of the whole atmosphere

over the Northern Hemisphere in winter.

CHAPTER XVIII.

SEQUENCE OF LOCAL WEATHER CHANGES.

The next, and last, step in our study of the correlation of the various

weather elements concerns the sequence of weather changes at a station

before, during, and after the passage of a cyclone and of an anticyclone.

_A._ =Cyclones.=——I. Select some station which the weather maps show to

have been directly on the track of a well-developed cyclone, _i.e._, to

have been passed over by the center of the cyclone. Note the weather

conditions at this station, before, during, and after the passage of the

storm. Tabulate your observations according to the following scheme:——

TABLE V.

Weather Changes at ________________________ during ______________.

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

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

| | | | | | | |

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

In the last column of the table enter the direction and the distance of

the cyclonic center from the station, at each observation.

II. Select a station which was north of the track of a cyclone, and

tabulate (in a separate table) the weather conditions at that station

before, during, and after the passage of the center.

III. Do the same for a station which was south of the track of a cyclone.

Repeat these observations for several stations.

_B._ =Anticyclones.=——Make a similar series of observations for the

passage of an anticyclone centrally over, north and south of, several

stations.

Study the sequence of the weather changes shown in the various tables.

Deduce a general rule for these changes and write it out.

CHAPTER XIX.

WEATHER FORECASTING.

In a letter dated at Philadelphia, July 16, 1747, Benjamin Franklin wrote

to his friend Jared Eliot as follows: “We have frequently along the North

American coast storms from the northeast which blow violently sometimes

three or four days. Of these I have had a very singular opinion for some

years, viz.: that, though the course of the wind is from northeast to

southwest, yet the course of the storm is from southwest to northeast; the

air is in violent motion in Virginia before it moves in Connecticut, and

in Connecticut before it moves at Cape Sable, etc. My reason for this

opinion (if the like have not occurred to you) I will give in my next.”

In a second letter to the same correspondent, dated Philadelphia, Feb. 13,

1749-50, Franklin states his reasons as follows: “You desire to know my

thoughts about the northeast storms beginning to leeward. Some years

since, there was an eclipse of the moon at nine o’clock in the evening,

which I intended to observe; but before night a storm blew up at

northeast, and continued violent all night and all the next day; the sky

thick-clouded, dark, and rainy, so that neither moon nor stars could be

seen. The storm did great damage all along the coast, for we had accounts

of it in the newspapers from Boston, Newport, New York, Maryland, and

Virginia; but what surprised us was to find in the Boston newspapers an

account of the observation of that eclipse made there; for I thought as

the storm came from the northeast it must have begun sooner at Boston than

with us, and consequently have prevented such an observation. I wrote to

my brother about it, and he informed me that the eclipse was over there an

hour before the storm began. Since which time I have made inquiries from

time to time of travelers, and observed the accounts in the newspapers

from New England, New York, Maryland, Virginia, and South Carolina; and I

find it to be a constant fact that northeast storms begin to leeward, and

are often more violent there than to windward” (Sparks’s _Life of

Franklin_, VI, 79, 105, 106).

The fact that our northeast storms come from the southwest, which was

first noticed by Benjamin Franklin some years before he put the suggestion

just quoted in writing, was one of the great contributions to meteorology

made by Americans. Modern weather forecasting essentially depends upon the

general eastward movement of cyclones and anticyclones, with their

accompanying weather conditions.

The daily weather map shows us the actual condition of the weather all

over the United States at 8 A.M., “Eastern Standard Time.” The positions

of cyclones and of anticyclones; of areas of clear, fair, cloudy or stormy

weather, and of regions of high or low temperatures, are plainly seen at a

glance. These areas of fair and foul weather, with their accompanying

systems of spiralling winds, move across country in a general easterly

direction. Knowing something of their direction and rate of movement, we

can determine, with greater or less accuracy, their probable positions in

12, 24, 36, or 48 hours. The prediction or foretelling of the weather

which may be expected to prevail at any station or in any district is

_weather forecasting_.

Weather forecasts are usually made on our daily weather maps for 24 hours

in advance. It is by no means an easy thing to make accurate weather

forecasts. Careful study and much practice are required of the forecasters

of the Weather Bureau before they are permitted to make the official

forecasts which are printed on the daily maps and in the newspapers.

A simple extension and application of the principles learned through the

preceding exercises make it possible for us to forecast coming weather

changes in a general way. These suggestions are, however, not at all to be

considered as a complete discussion of this complicated problem.

Weather forecasts include the probable changes in _temperature_, _wind

direction_ and _velocity_, and _weather_. Pressure is not included. Begin

your practice in weather forecasting by considering only the changes that

may be expected at your own point of observation, and at first confine

yourself to predicting temperature changes alone.

=Temperature.=——Provide yourself with a blank weather map. Draw an

isotherm east and west across the map, through your station. Draw a few

other isotherms all the way across the map, parallel with the first one,

and so arranged that they will be equal distances apart, the most

northerly one running through northern Maine and the Northwestern States,

and the most southerly one through southern Florida and Texas. Recalling

what you have already discovered concerning the eastward movement of our

weather conditions, what forecast will you make as to the coming

temperatures at your station? Add some additional east and west isotherms,

so that there will be twice as many on your map as before. What effect

will this change in the temperature distribution on your map have upon

the temperature forecast you make for your station? Formulate a general

rule as to temperature forecasts under the conditions of isothermal

arrangement here suggested.

On a second blank weather map draw an isotherm through your station

inclined from northwest to southeast. Draw a few other isotherms parallel

to the first, and each one representing a temperature 10° higher than that

indicated by the adjacent isotherm on the east. Make a general forecast of

the temperature conditions that may be expected at your station, as to

_kind of change_, if any; _amount of change_, and _rapidity of change_. Of

the isotherms just drawn, erase every second one; still, however, letting

those that are left represent differences of temperature of 10°. What

forecast will you now make as to temperature? How does this forecast

compare with that just made?

Now draw twice as many isotherms on your map as you had in the first

place, still letting these lines represent differences of temperature of

10° in each case. Make a forecast of the kind, amount, and rapidity of

temperature change at your station under the conditions represented on

this map. How does this forecast compare with the two just made? Formulate

a general rule governing temperature forecasts in cases of isothermal

arrangement such as those here considered.

Take another blank map. Draw through your station an isotherm inclined

from northeast to southwest. Draw other isotherms parallel to this, west

of your station, letting each successive isotherm represent a temperature

10° lower than that indicated by the adjacent isotherm on the east. Make a

temperature forecast for your station under these conditions. Diminish and

increase the number of isotherms on your map, as suggested in the

preceding example, making temperature forecasts in each case, and

comparing the three sets of forecasts. Formulate a general rule for

temperature forecasts made under these systems of isotherms.

Make temperature forecasts from the daily weather maps for your own

station, using the knowledge that you have already gained as to the

progression of cyclones and anticyclones (Chapter XVII), and as to the

temperature distribution in these areas (Chapter XIV), to help you in this

work. Study each day’s map carefully before you decide on what you will

say. Then write out your own forecast, and afterwards compare your

forecast with that made by the Weather Bureau. Note also, by reference to

your own instrumental observations, whether the succeeding temperature

conditions are such as you predicted.

=Wind Direction.=——The weather maps already studied taught us that our

winds habitually move in spirals. The composite picture of the wind

circulation around cyclones and anticyclones (Chapter XII) further

emphasized this important fact. Evidently this law of the systematic

circulation of the winds around centers of low and high pressure may be

utilized in making forecasts of wind direction.

Applying the knowledge already gained concerning cyclonic and anticyclonic

wind circulations, ask yourself what winds a station should have which is

within the range of the cyclonic wind system, and is in the following

positions with reference to the center: _northeast_, _north_, _northwest_,

_east_, _at the center_, _west_, _southeast_, _south_, _southwest_. Ask

yourself precisely the same questions with reference to a station within

an anticyclonic wind system. Write out a general rule for the kinds of

wind changes which may be expected to take place under these different

conditions.

When a station is south of the track of a passing cyclone its winds are

said to _veer_, and the change in the direction of its winds is called

_veering_. A station north of the track of a passing cyclone has a change

of direction in its winds which is known as _backing_, the winds

themselves being said to _back_.

=Wind Velocity.=——What general relation between wind velocities and areas

of low and high pressure did you discover in your study of the weather

maps? What was the result of your work on the correlation of the velocity

of the wind and the barometric gradient in Chapter X? And what general

statement as to the relation between the velocity of the wind and its

distance from a cyclonic or anticyclonic center may be made as the result

of your work in Chapter XII, on the correlation of cyclones and

anticyclones with their wind circulation? These results must be borne in

mind in making predictions of coming changes in wind velocities. Forecasts

of wind velocities are made in general terms only,——_light_, _moderate_,

_fresh_, _brisk_, _high_, _gale_, _hurricane_,——and are not given in miles

per hour.

Make forecasts of wind direction and velocity from the daily weather maps

for your own station. Continue these for a week or two, keeping record of

the verification or non-verification of each of your forecasts. Then make

daily forecasts of temperature and of wind direction and velocity

together. Write out your own forecast for each day before you compare it

with the official forecast, and if the two differ, keep note of which one

seemed to you to be the most accurate.

=Weather.=——What general relation between kind of weather and cyclones and

anticyclones was illustrated on the six maps of our series? What is the

average distribution of the different kinds of weather around cyclones and

anticyclones, as shown by your composites? (Chapter XVI.) What changes in

weather will ordinarily be experienced at a station as a cyclone

approaches, passes over, and moves off? What conditions will prevail in an

anticyclone?

Isotherms on weather maps are drawn for every even 10° of temperature.