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Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

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given ofT has a temperature no higher than that of the
boiling water.

All this heat is absorbed in the work of chanGfimi; the
water to steam, and it is called the latent heat of steam. It
has not been lost, however ; it is merely stored-up energy.
It is retained just so long as the water remains in the form
of vapor ; it is given out the moment the vapor is con-
densed, or changes to a liqviid.

This property of water is one of the greatest importance,
for, as will be shown, it is a chief factor in the atmospheric
disturbances called storms. The amount of heat thus ren-
dered latent is very great. For every pound of water con-
verted to steam, as much heat is required as would raise
nearly half a ton of water one degree F.

Dew. — Dew is the moisture that gathers on the ground
after sundown. Both the air and the ground lose a part
of their heat. The latter cools more rapidly, however,
and finally the layer of air next the ground is chilled be-
low the dew-point. When this occurs, the excess of vapor
in the form of minute drops gathers on the grass and on
other objects near the ground. The moi.sture that gathers
on the outside of a glass of iced water is an exani])l(\

Dew does not always form at night, and for this there
are sev(;ral reasons. A stifi' breeze may keep the air
thoroughly mi\(!d, and thereby prevent an}' part of it
from being cliilled to the dew-point. The air may contain



234 PHYSICAL GEOGRAPHY

so little vapor that a fall of fifteen or twenty degrees does
not bring the temperature to the dew-point.^ A cloudy
sky, especially if the clouds hang low, prevents the radia-
tion of heat, and the formation of dew.^

The amount of moisture in the air varies much. In
tropical regions, especially those near the sea, the amount
is proporti(^nately very great. Sometimes it is so near
the point of saturation that the air becomes hazy. In
such regions dew forms copiously. In temperate latitudes
the amount is much less than in tropical regions.

In the California and Sound valleys, where there are no
summer rains, the fall of dew in early summer is excessive,
and to a great extent the grain crop is dependent upon it.
The same phenomenon occurs in most mountain valleys.

If the temperature of the dew-point be lower than 0°
(32° F.), the moisture may pass immediately into the crys-
talline form, frosf. Sometimes the minute frost crystals
form in the air, but usually they accumulate on the grass,
the leaves, and other objects near the ground. Sometimes
the frost is simply frozen dew. Except at considerable
altitudes frost does not occur in tropical regions. In tem-
perate latitudes it may occur at any time between late fall
and spring. Late spring frosts are apt to occur after fruit-
trees have budded, and they are therefore commonly known
as killing frosfs. The cold Avave that follows a spring storm
is very apt to lower the temperature to the freezing-point,
and if the air be moist, a killing frost commonly occurs.
Fortunately its occurrence usually can be predicted.

Clouds.— When the temperature falls so low that a part
of the vapor is condensed, the latter does not at first
gather into large drops ; on the contrary, the drops are so
minute that they float in the air. This floating mist of the
air is called fog or cloud, according as it is at the surface
of the earth or high in the air.



THE MOISTURE OF THE ATMOSPHERE 235



Nearly always the air is filled with dust-motes and other
floating matter, and much of the condensing vapor gathers
on these. Not only do the dust-motes form a lodgement
for the condensing vapor, but they cool more rapidly than
the air, and thereby quicken the process of condensation.
Floating matter in the air thus becomes an active agent in
cloud formation. The cooling of the air below the dew-
point, however, is the essential, and this may occur in
several ways. Thus, when a mass of air is pushed upward,
not only is it chilled by going into a cooler position, but
it is also cooled by its own expansion. It is prol)able
that the greater amount of cloud is formed in this manner.
Thus, in equatorial regions,
where there is a constant
up-draught of warm, moist
air, there is a perpetual
cloud-belt.

The intrusion of warm
winds into cold regions, or
of cold winds into warm re-
gions, is also a common
cause of fog and cloud. If
the intruding wind is at the surface of the earth fog
results ; if at a considerable elevation cloud is formed.
The fogs and cloud banks so common off the coast of New-
foundland are formed in this way.

Whenever a warm sea- wind blows against a high moi;n-
tain-slope, a part of the air is driven up the slope, and,
some of its moisture being condensed, cloud is formed.
Almost always high mountain-crests near the ocean are
shrouded in clouds, and not infrequently a cloud banner
streams from the leeward side of a high peak.^

Clouds usually take characteristic forms, and these are
governed mainly by the presence or absence of wind, or by




CIRRO-STRATUS CLOUDS



236



PHYSICAL GEOGRAPHY



tlioir heiglit. Cirrus clouds are light and feathery in ap-
pearance and commonly white in color. These clouds take
various forms. When they are flaky or fleecy they are the
" mackerel" clouds heralded by sailors as forecasters of fine
weather; but cirrus " streamers" are frequently found as
an advance indication of an approaching cyclone. Often
the patches of cirrus cloud are ranged in parallel strips ;
and occasionally they radiate like the spokes of a wheel.




CUMULUS CLOUDS



Commonly their altitude is between five and ten miles.
On account of their great height it is obvious that they
consist of minute ice crystals. Cirri may form above
another cloud, the two being apparently related, but they
never form under other clouds.

Cumulus clouds are the day clouds of summer weather.
They appear like great, rounded domes resting on a hori
zontal base. A gently warmed current of air rises until,
being chilled both by expansion and great altitude, con-
densation begins. The process continues until a dense



THE MOISTURE OF THE ATMOSPHERE 237



mass of cloud is formed. This form is the almost miiver-
sal cloud of tropical regious. It is abundant in warm
temperate climates, but rare in cold latitudes. It does not
form at night nor in cold weather, for the simple reason
that the up-draught of warm air is too feeble, and there is
not enough vapor present to form clouds of sensible dimen-
sions. Ordinarily, cumulus clouds have no especial sig-
nificance as weather forecasters. They indicate nothing
more than the presence of moisture, and, as a rule, their
size shows whether there is considerable vapor or oidy
a little. If, however, a mass of cloud loses its flat base,
becoming ragged or festooned at the lower side, it usually
portends high winds and local showers.

Stratus clouds are so called because they are flat layers
of nearly uniform thickness. Normally they are the low-
est of all clouds, and probably contain the greatest amount
of foreign matter. These
clouds are commonly ob-
served at morning and even-
ing, and stillness of air is
essential to their formation.

The Nimbus is the shape-
less rain-cloud that hovers
near the surface of the earth.
The ujjper part consists of
light fog or mist ; the lower,

of falling drops. Usually it seems to form in clear air, and
it gathers when the temperature reaches the dew point.

Clouds are moved hither and thither by the wind, but
the matter composing the cloud is usually in motion even
when the air is still. A casual inspection of any summer
cloud shows that it is constantly moving within itself.
Practicallv. cloud is ilonting nioistuti', l)iit in realitvthc




STRATUS CLOUDS



minute drops are always slowly fallin



The



drojilet falls



338 PHYSICAL GEOGRAPHY

luitil it reaches a region of greater warmth ; then it is
changed to vapor, and the latter at once ascends until it is
again condensed — the process being constantly repeated.

Rain. — The difference between rain and cloud consists
very largely in the size of drops, but there is also a differ-
ence in their physical condition. The drops of cloud
matter, or " water dust," are minute, and practically they
float in the air; those of rain are each many thousand
times as large, and fall quickly to the ground. The causes
that operate to produce fog and cloud, however, also pro-
duce rain — namely, the cooling of Avater vapor below the
dew-point.

The vapor precipitated as rain may pass through the
cloud stage, it is true ; but the latter is one of short dura-
tion, and, as a rule, when condensation begins, it proceeds
very rapidly. Kain is rarely associated with fair-weather
clouds, and, excepting local showers, is not derived from
them. In almost every instance general rains are derived
from warm ocean winds that, blowing inland, are chilled.

In general, more rain falls in tropical regions than
elsewhere : does the map confirm this statement ? The
equatorial cloud-ring is also a rain-belt, and under it pre-
cipitation is almost continuous. The amount of rain fall-
ing in the torrid zone is sufficient to cover it to a depth
probably of more than one hundred inches. In the tem-
perate zone it is a little more than one-third, and in polar
regions about one-eighth as much.

Rainfall is not uniform for all places in the same lati-
tude." On slopes that face ocean winds it is greatest,
while in regions shut off from the sea by high ranges it is
little or nothing. For example, on the southern slope of
the Himalayas the precipitation varies from two hundred
to six hundred inches ; on the north side it is less than
ten. On the western slope of the Sierra Nevada and Cascade



240 PHYSICAL GEOGEAPHY

Ranges it is ten times as great as on the eastern. Explain
why the difierence exists.

As a rule, precipitation is greatest at the coast and de-
creases toward the interior.'' On the Atlantic coast of
the United States it is nowhere less than forty inches ;
west of the one hundredth meridian it is less than fifteen.
On the north(^'n shores of South America it is over one
hundred inches ; a few hundred miles inland it is about
one-quarter as much. In the uplands of the eastern slope
of the Andes it again increases ; why ?

Not only does the amount of rainfall vary in different
localities, for the reasons noted, but there is also much
difference in the time of its distribution. In some local-
ities it comes in the form of occasional showers ; in others
long periods of rain and drought alternate at given inter-
vals " — that is, the rainfall is periodical and seasonal.

An examination of the wdnd chart, p. 221, will help to
explain this fact. The slopes of the continents that face
ocean winds, as a rule, have periodical rains. Thus, the
western coast of North America faces the Prevailing West-
erlies of the Pacific Ocean. In summer these winds are
blowing into a region that is warmer, and therefore but
little rain falls. In winter, on the other hand, the temper-
ature of the land is much lower, and therefore rain may
be of daily occurrence.

On the Mexican coast, where, on account of low latitude,
the climate is almost always mild, but little rain falls.
Along the coast of the United States it varies from ten or
twelve inches at San Diego to sixty or seventy at Puget
Sound; while at Sitka, Alaska, it is about one hundred
inches. How will the difference in latitude explain this ?
In what part of the Pacific Coast of South America are
the conditions similar ? On the Atlantic coast of Europe
the conditions are much the same ; most of the precipita-



THE MOISTUEE OF THE ATMOSPHERE 241

tiou occurs duriug the winter mouths, but on account of
high latitude a considerable rain falls in summer.

In tropical regions, where the winds have an easterly
origin, the easterly slopes receive the heaviest fall of rain.
In these regions, however, the rainfall follows the passage
of the equatorial cloud-belt back and forth. This belt is
comparatively narrow — scarcely five hundred miles in
breadth. Dming the spring months of the Northern
Hemisphere it moves northward with the sun, deluging
the land over which it passes with almost continuous rain.
After reaching its northern limit it tm-ns southward, re-
passing over the same belt. In the American continent
the cloud-belt does not pass far south of the equator ; in
Africa it reaches much farther south.

A moment's study will show that at each tropic, or limit of
the cloud-belt, there will be one rainy and one dry season,
while at intervening latitudes there may be two. Which
of these conditions applies to Cuba? to the Central Amer-
ican states ? to the Caribbean coast of South America ?

Regions swept by monsoons usually have periodical
rains also. The reason is obvious : during one part of the
year the winds blow from the land ; the remaining time
from the sea. The rains of the Indian coast of Asia are
an excellent example. During the winter months of the
northern hemisphere the prevailing winds are land winds;
but with the bursting of the April monsoon the season of
heavy rain begins and the parched land is quickly covered
with verdure.

A large part of the land surface of the earth is watered,
not by seasonal and peri«^dical rains, but by the pre-
cipitation that comes with the irregular juovements of the
atmosphere known as stonns. These regions as a rule are
eithfa- far inland, or else high mountain ranges shut them
oil' from the reach of ocean winds.



242 PHYSICAL GEOGRAPHY

That part of the ITnited States east of the Rocky
Mountains is an exampk;. The ranges of the great high-
hmds precipitate practically all the moisture brought from
the Pacific, and therefore there are no periodical rains.
Moisture gathers from the Gulf and also from the ocean,
but for the greater part it is not precipitated until the
cyclonic movement, which constitutes the storm, takes
place. These disturbances occur so frequently, and there
are so many of them, that almost every part of the region
receives a plentiful supply of moistiu'e. Similar con-
ditions exist in parts of Eurasia and Africa.

Effects of Altitude. — As a rule, more rain falls at sea-
level than at higher altitudes : very little falls above the
height of ten or twelve thousand feet. On mountain-
slopes, however, the greatest precipitation takes place be-
low three thousand and five thousand feet. The reason is
two-fold. In moderately warm regions rain clouds com-
monly do not reach much above this altitude ; moreover
at this height the ground may be cold enough to condense
moisture when it is too warm to do so at a loAver level.
This fact is often observed in desert regions.

Rainless Regions.— There are two principal causes
for the existence of rainless regions. There may be a
barrier of high mountains that shut off rain -bearing winds;
or, vapor may pass into a warmer region where it cannot
be condensed. The Basin Ptegion of the western high-
lands, the basin north of the Himalaya Mountains, and the
Andine desert, are examples showing the effects of moun-
tain barriers. The mountains reach higher than the rain
winds. The two African deserts and much of the Mexican
coast show the effects of hot inland regions. The ocean
winds that penetrate these regions are warmed and not
cooled, and therefore they are relatively drier.

Snow.— When the condensing vapor freezes before it



THE MOISTUEE OF THE ATMOSPHERE 243

can gather iuto drops, snow results. It is evident, more-
over, that snoAv cannot form unless the temperature is as
low as 0° (32° F.). If condensation takes place very
slowly in still air, the frozen droplets aggregate into beau-
tiful crystalline forms,^- but if condensation is rapid, each
flake is a tangle of broken crystals.

Inasmuch as snow depends on a low temperature, it is
evident that the distribution is governed both by latitude
and altitude. In polar regions snow covers the groimd
the greater part of the year, and at a little distance from
the sea it never melts. In equatorial regions the line of
perpetual snow is about sixteen thousand feet above sea-
level ; in temperate latitudes it varies from seven thousand
to twelve thousand feet.

Hail. — Hail consists of pellets of ice, formed in the air,
and a shower of them constitutes a hailstorm. Usually a
hailstorm consists of alternate shells of snow and crvstal-
line ice.'^ In some instances sharp, dog-toothed crj-stals
of ice project from the outer surface. Hailstones var}'
in size from tin}' pellets to masses an inch in diameter.
Larger stones occur, but they are formed by the cohesion
of small ones. Hailstorms are more frequent in warm
weather than in cold. For reasons unknown certain local-
ities are especially subject to them. They ver^ frequently
accompany thunderstorms.

QUESTIONS AND EXERCISES— Find the annual rainfall of the
neighborhood in which you live by striking an average of the yearly
precipitation for at least ten years. (The statistics may be learned front
the nearest Weather Station.)

Make a record of the early and late frosts for the year. What fruit
crops are injured by killing frosts in the neighborhood in which you
live ?

Learn, from the nearest Weather Station, the months in which the
greatest amount of rain or snow falls ; the least

What crops or plants of commercial value would suffer or peiish



244 PHYSICAL GEOGRAPHY

if the rainfall in the State in which you live were decreased one-
third ?

Note the character and kinds of cloud visible during several days ;
at what time were stratus clouds visible ?

Explain how smoke may gradually gather cloud matter. Why is
this most apt to take place toward evening ?

The receiver of a rain gauge is a cylindrical cup four inches in diame-
ter. For convenience of measurement the water caught is poured into
a glass tube one inch in diameter : a depth of one inch of rain in the
receiver will make how many inches in the tube ?

Explain how a crust forms on the surface of snow.

At a convenient opportunity, catch flakes of snow on a piece of black
cloth ; examine them with a magnifying-glass and make drawings of
their shapes. (Observe the conditions noted on p. 243.)

COLLATERAL READING AND REFERENCE

Tyndall. — Forms of Water.

U. S. Wkather Bureau. — Monthly Weather Review. Mid-
summer and midwinter issues of any year.

Greely.— American Weather— pp. 77-81, 134-162.
Waldo. — Elementary Meteorology— pp. 142-165.



NOTES

' The expressions "air absorbs water in the form of a vapor "
and " warm air can hold more water vapor than cold air " are so
popular that ordinarily they pass for scientific truths. They are
certainly convenient, but a moment's reflection shows them to
be inexact.

^The phenomenon popularly known as "the sun drawing
water " is due to the passage of rays of light through rifts in the
clouds. The passage of the rays is marked by minute dust-moats,
which reflect and scatter some of the light.

' At times it may be noticed that wet clothing exposed all day
to the air refuses to dry. The reason is that the air is already
saturated, and because of this no further evaporation can take
place.

Sometimes dew forms copiously with but a slight fall of tem-
perature, while perhaps on a following night, none may appear,



THE MOISTURE OF THE ATMOSPHERE 245

though the temperature is much lower. An inspection of the
table on p. 380, will explain how this may occur. If there were
seven grains of water vapor in each cubic foot of air, a fall of
temperature from (iS^ (F.) to Qi^ would be attended with dew ;
but if only three grains were present, the thermometer might
sink as low as 40° without any sign of dew.

^A cloth screen within four or five feet of the ground will have
the same effect.

* These cloud banners were noticed in the Alps by Professor
Tyndall, and were first described by him. They may be often
seen streaming from the summit of Tacoma, Washington, and
the alleged smoke from the
crater of Mount Hood, Oregon,
is nothing but a similar phe-
nomenon.

' This indication has had a
recognized place in weather-
lore for two thousand years.
It is mentioned in Virgil :

Tenuia . . lanse per ccelum val-

lera ferri, ...

and it is found among Teutonic peoples, as well ; hence the
popular saying —

Mackerel sky, twelve hours dry.

*Not infrequently a column of smoke, from a factory chimney
or a steamer's smoke-stack, becomes the nucleus of a stratus
cloud. The smoke ascends until buoyancy and gravity balance
each other, and then settles in the form of a thin, flat layer.
Each particle becomes a surface of condensation, and the cloud
matter continues to gather until it is swept away by the wind,
or the conditions are dianged.

'The heaviest annual fall i.<; probably at Cherrapunji, India,
where the average is about 500 inches. In August, 1841, the total
fall for the month was 264 inches, and in 1801 the yearly fall
reached the enormous amount of 110.") inches — about 2S> inches a
day ! On June 14, 1870, 40.0 indu-s fell in twenty-f<»ur hours.
In the three days ending February, 1893, an aggregate of 35.8




246 PHYSICAL GEOGRAPHY

inches fell at liri.sbaiie, Australia. In the United States 21.4
ini'hes I'ell at Alexanth-ia, Louisiana, in one day, and at Triadel-
pliia, West Virginia, 6.9 inches fell in fifty-five minutes. All
these instances, however, are very unusual. Commonly, not
more than two inches fall in a day.

'" The greater the distance from the coast the more abnormal
is the character of the rainfall. In the Basin Region of the west-
ern United States, the rain is restricted to showers of short du-
ration, and these often take the form of cloud-hiirsts. There is a
sudd(>n darkening of the sky, a terrific downpour of water-
perhaps thi-ee or four inches in fifteen minutes— and then the sun
is again licking up the water from the almost hissing rock waste.
The specific cause of cloud-bursts is not known.

" In regions visited by i)eriodical rains, not infrequently the
air is so loaded with dust, at the end of the dry season, that the
first rain is discolored and even muddy. The yellow and golden
rain, once a great mystery, is commonly due to the pollen of
pine. Examined under a microscope the character of this pollen
is such as to leave no doubt as to its origin. Showers of frogs,
fishes, and angleworms (!) have been reported, but not an in-
stance has been substantiated. It is not impossible that a water-
spout might whirl a school of fishes into the air, and then over
the land, but no tornado known has been so selective as to con-
fine itself exclusively to frogs and angleworms. The latter sim-
ply emerge from their hiding-places at the onset of the shower.
Among other abnormal showers are the rains from cloudless skies.
Instances are common, especially in mountainous localities. The
precipitation in such cases is very slight and the showers rarely
cover more than a square mile or two. The sky is cloudless
merely because there are not enough drops in the air at any
moment noticeably to interrupt the light.

'- With one or two exceptions all the illustrations of snow
crystals are copies of drawings made in the arctic regions by
Captain Scoresby. A few drawings have been made by Professoi
Tyndall, and recently excellent photographs have been obtained ;
these show that ice-crystals and snow-fiakes are not so regular
nor so complicated in structure as those observed by Scoresby.
In order to obtain good specimens of crystals, they must be
gathered on a perfectly still day when the temperature is several
degrees below the freezing-point. It is best to catch them on a



THE MOISTURE OF THE ATMOSPHERE 247

piece of black cloth, and if they are to be examined under a mi-
croscope the glass slide on which the flake rests should be covered
with the same material. The crystalline forms observed in sun-
shine are materially different from those found in cloudy weather,
'^ The peculiar structure of hail-pellets has led to the theory
that the stone has been whirled alternately into warm and cold
layers of air; this is onlj' a supposition, and concerning their
formation nothing certain is known. As a theory, however, it is
not unreasonable. Ordinarily, hail-storms are of only a few
minutes' duration, and the amount falling is a small fraction of
an inch in depth. In 1888, at Moradabad, India, hail fell to a
depth of several inches, and in one district two hundred and
thirty-five people were killed. In June, 1879, a storm swept over
central New York and Massachusetts, during which stones seven


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Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 16 of 25)