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Jacques W. (Jacques Wardlaw) Redway.

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therefore, is Avarnied and pushed upwards by the inflow of
colder air. This process results in two great movements,

namely — a sur-

face flow toward
the equator, and
upper currents
from the equato-
rial toiuard po-
lar regions.

But the colder
air comes from
the regions where
the speed of the
earth's rotation
is comparatively
slow, and enters
latitudes where
it is much great-
er ; and not be-
ing able to acquire this speed at once it lags behind, pro-
ducing a current to the westward. The rising current
moves into a region in which the speed of rotation is not
so great, and therefore moves eastward, as well as toward
the north.

Winds are usually named according to the direction
from which they come. But in the two great movements
described the easterly and the westerly components are
much more noticeable than the polar and equatorial move-




GF.NERAL MOVEMENTS OF THE ATMOSPHERE.



218



PHYSICAL GEOGRAPHY



ments. For this reason it is customary to recognize three
great belts of winds — a belt of equatorial or easterly, be-
tween two zones of extra-tropical or westerly winds. These
general movements are very strongly marked in the oceans,
but they are greatly modihed by the continents ; in inland
mountainous regions they might escape notice except
through long-continued observations ; in the great lowland
plains they are more regular.

Trade Winds. — The surface winds that flow into trop-
ical regions to take the place of the warm air that is




January.



PREVAILING WINDS OF THE ATLANTIC.



July.



pushed upward, form the well-known Trade Winds. What
is their direction in the northern half of the belt ? in the
southern half ? Toward the centre of the belt they are
practically strong, steady easterly winds.

The zone of Trade Winds is about fifty degrees in width.
Its position is not stationary ; it swings back and forth,
north and south, as the seasons change. In the Atlantic
Ocean the shifting of the belt is not far from eight or ten



THE ATMOSPHERE AND ITS PROPERTIES 219

degrees ; in the Pacific it is slightly greater. The belt
reaches its northern limit in early autumn ; its southern
limit in early spring. The winds are regular and constant
the year round, and their velocity is not far from twelve or
fifteen miles an hour.

Formerly, when most of the ocean commerce depended
on sailing vessels, these winds were of great importance —
hence their name. A vessel entering the Trade Wind
belt could rely on steady winds with but little interruption
from cyclones.

Along the line where the northerly and the southerly
components of the Trade Winds meet, there is a narrow
belt which is characterized by an absence of steady winds.
This belt is the updraught of heated air and is called the
Epiatorial Calms, or Doldrums. This calm belt is scarcely
more than two or three hundred miles in breadth. Some-
times vessels were becalmed several weeks in crossing it.
The wind comes only in fits and puffs, or with an occa-
sional thunderstorm of great violence.

Prevailing Westerlies. — The air that flow-s from equa-
torial regions as an upper current,^ in temperate latitudes
sinks to the surface and becomes a belt of westerly winds,
now generally called the Prevailing Westerlies ;^ what is
their direction in the Northern Hemisphere ? in the
Southern Hemisphere ? Like the Trade Winds both belts
move northward and soutliAvard with the changes of the
seasons.

In the Northern Hemisphere the Prevailing Westerlies
are neither so strong nor so steady as the Trade Winds,
and in higher latitudes they often give way to winds of
northerly origin. On the coast of the Gulf of Mexico the
Prevailing Westerlies, in the summer season, are reinforced
by Trade Winds which are deflected by the highlands of
Mexico. The resulting winds sweep up the Mississippi



220 PHYSICAL GEOGRAPHY

Yalloy find thence turn across the Atlantic, carrying with
them a great deal of the moisture that supplies the East-
ern TTnited States with rain.

In the Southern Hemisphere, the Prevailing Westerlies
are best known as the Roaring Forties. They cover a
very broad stretch of sea, and they furnish an excellent
illustration of the theoretical movement of the constant
winds. When the trade route between Europe and the
East Indies lay around the Cape of Good Hope, the Koar-
ing Forties were a very important factor, as the sailing
master could depend on a twenty or thirty knot breeze the
year round. It was then a common practice for vessels
bound for Australia or New Zealand to continue the route
eastward and return by way of Cape Horn. Trace this
route on a globe.

The descent of the upper currents to the sui'face, which
is the origin of the Prevailing Westerlies, is marked by
calm belts — the Calms of Cancer, and the Calms of Capri-
corn. Like the zones of constant wdnds the calm belts
also shift north and south with the season. They are in-
terrupted by the continents and are scarcely to be noticed
Avithin a hundred miles of their shores. The Calms of
Cancer are the well-known " Horse Latitudes." ^ The Calms
of Capricorn are the wider and more continuous of the two
calm belts.

Monsoons. — Along many coasts having a southerly or
a southwesterly exposure the summer winds have a direc-
tion nearly opposite those of the winter season ; that is,
about half the year they blow from the sea ; the remaining
half toioard the sea. These winds are called monsoons.''
Two causes operate to give these winds their peculiar
character — in some instances singly ; in others together.

In the first place, any great body of land is apt to be-
come much warmer than the sea in summer and colder in



222 PHYSICAL GEOGRAPHY

Aviuter. As a result, during summer there is an updraught
of warm air pushed upward by the inflow of sea air. In
winter the conditions are reversed ; cold air flows from the
land to the sea. In other instances, a region may be so
situated that it is in the southeast Trade Winds at one
part of the j^ear, and in the northeast part the remainder.
The monsoons of the Mexican coast are probably due
to this cause.

The most remarkable monsoons, however, are those of
the Indian coast.^ From April to October the southerly
half of the belt of Trade Winds reaches far inland, pouring
a deluge of rain upon the land. During the rest of the
year, on the contrary, the southerly part of the belt has
reached southward, and the northerly half extends consid-
erably bej'ond the coast, parching the land and withering
vegetation. The tremendous updraught of warm air aids
materiall}' in giving strength to these winds. The " break-
ing" or change of the monsoon is usually attended b}- a
number of terrific storms.

Along the Gulf coast of the United States the deflected
Trade Winds of the summer season, noted on p. 219, that
flow up the Mississippi Yalley are replaced by Prevailing
Westerlies that are turned doivn the valley. These winds
may be regarded as monsoons, but they are neither so
regular nor so strong as the Indian monsoons.

Day and Night Breezes.— The difference between the
temperature of day and night is sufficiently great to result
in strongly marked local winds. Thus, along the coasts,
especially Avarm regions, the updraught of the land causes
a stiff on-shore wind during the day, while at night the
air over the land, being more quickly chilled, flows down
the slopes toward the sea.^ Thus there results a sort of
daily monsoon, or day and night local wind. Coast fisher-
men frequently take advantage of such winds; they go



THE ATMOSPHERE AXD ITS PROPEKTIES 223

out in the morning with an off-shore, and return at night
with an on-shore breeze.

Similarly, in mountainous countries the air upon the
higher slopes is commonly heated and cooled more rapidly
than in the valleys. As a result there is often a strong
wind blowing up the valley by day, and flowing dowmnard
at night. Mountain valleij winds of this character are very
common in almost every rugged countr}'. Which is the
better indication of the general direction of the wind — that
noted at the ground, or the movement of the clouds ?

Local and Variable Winds. — There are many winds
occurring at irregular intervals that are confined each to a
particular localit}'. In most instances these winds are
confined to desert regions and arid lands, or else they re-
sult from the proximity of the latter. Almost always they
are very " dry " winds.

Thus, the Northers of Texas and Mexico are cold winter
winds of several days' duration that blow from the high-
lands of the Plateau region. The Chinook and Santa Ana
winds of the western highlands of the United States are
descending, and therefore warm winds blowing from arid
regions upon fertile lands. In southern Europe they are
called Foehn winds.^*^ The Pamperos are similar winds
flowing from the cold slopes of the Andes over the arid
pampas of Argentina. The Punas of the Peruvian table-
lands are of the same nature.

In the vicinity of the African desert are the famous
Mistral and the Etesian winds, both blowing from the
snow-clad Alpine ranges toward the desert, while the
Sirocco, like the Chinook, is a hot wind that in summer
blows from the desert. The Harmattan is a warm winter
wind, blowing from the desert to the Guinea coast. Aside
from these there are several winds peculiar to desert r(v
gions. Chief among them is the Simoon, a fierce blast of



224 PHYSICAL GEOGEAPHY

hot air and rock waste, that neither man nor beast can face.
It is common both in the Okl World and the American
deserts. A mikler form of this wind along the lower Nile
valley is called the Khamsin. Classify these winds as
either hot blasts fro)ii the desert, or colds winds blowing
into it.

The most interesting of all desert winds, however,
are the sand whirls. These occur in the morning when
the air is still — never when wind is blowing. Under a hot
sun the air next the earth becomes considerably heated,
having a high temperature. Above the ground the air is
cooler at the rate of one degree F. for every three hundred
feet. Thus there is formed the very unstable condition
of a layer of heavy, cold air on a surface stratum that is
much lighter. Such a condition cannot last long, and
sooner or later some slight disturbance or other starts a
slender column of air upward.

Immediately the stratum of cold air begins to settle,
and, as it descends, it forces the warm air upward through
the self-made passage. The ascending column begins to
whirl, and soon its motion is rapid enough to carry with it
a cloud of dust and fine rock waste.

As a rule these whirls begin when the sun is two or
three hours high, and continue nntil the wind begins to
blow. The latter, by mixing the warm air with the cold,
prevents their formation until a calm again begins. Oc-
casionally such whirls develop into very vigorous " sand
spouts."

Physiographic Effects of Winds.— As an agent in
wearing away the surface of the land, the wind acts in dif-
ferent ways. It may alter the chemical composition of the
rock with which it comes in contact. It may carry minute
particles that cut away softer material. It may transport
material from one place to another. The chemical action



THE ATMOSPHERE AND ITS PROPERTIES 225



of air is due mainly to the water and carbon dioxide wbich
it contains. It is manifested in the gradual crumbling of
many rocks, when the latter are exposed to the air. The
rocks most aifected are certain iron ores and granite rocks.
Dry air may affect rocks by chemically withdrawing the
moisture they contain ; moist air niay affect other kinds by
chemically imparting water to them. In either case the
rock sooner or later crumbles.

The impact of minute particles carried by the wind is
especially noticeable in the w^estern highlands. In regions




i^fcS«eE



SAND-BEATHN ROCKS



where sand winds are prevalent the surfaces of the hardest
rocks are worn, channelled, and polished from this cause.
Many of the " needles " or rock spires of this region have
been scul])tured into fantastic forms l)y (volian or wiud-
l)l(j\vn rock waste."

The transporting power of the wind is confined chietly
to sea-shores and regions un]u-otected by vegetation.
AVhy? The wave-formed islands nnd barrier bea(;hes of






l>nYSICAL GEOGRAPHY



the Atlautic and Gulf coast have foimdations of sea sedi-
ments, but the above-water part consists of wind-blown
material. The sand-dunes of sea and lake shores are ex-
cellent illustrations, and in regions swept by monsoons the
dunes travel seaward during one season and landward the
other. A w'ave of sand about a mile long and seventy feet
high at one time inundated a part of Cape Henlopen.^^

Between the silt brought down by the Colorado River,
and the fierce winds of that region, the Gulf of California




DUNES OF SAND

has been cut in twain, and most of the severed portion
filled with rock waste to a height now considerably above
sea-level ; indeed, all through this region dunes are con-
stantly forming, shifting, and re-forming. In western
Nebraska, where the rainfall is not sufficient to grow pro-
tective vegetation, dunes are common.

Very notable examples of the transporting power of
wind occur in China. In the basin of the Hoang River
there are seolian deposits covering many thousand square
miles to a depth of several hundred feet. These deposits,



THE ATMOSPHERE AND ITS PROPERTIES 227

called loess — from a German word meaning " loose " — are
thought to come from the desert region to the westward.
In many places the rivers have cut their channels through
the loess, and the latter not only colors the water of the
river, but imparts a yellow tint to the sea into which it
flows.

^olian deposits have filled most of the valleys of the
Basin Region of the western highlands. The ranges stand
out in bold relief from an ocean of level rock waste. Many
of the valleys of the Rocky Mountains have been filled and
levelled in the same manner.



QUESTIONS AND EXERCISES.— Devise or describe an experi-
ment to show that air has weight ; show that it is elastic ; show
that heating a volume of air causes it to expand ; using a bicycle
pump, show that compressing air warms it.

What is the prevailing direction of the wind in the locality in which
you live? Consult the records of the nearest weather station and com-
pare- the number of days of westerly winds with the number in which
the wind is from other directions.

The tropical calm belts are regions of descending air-currents ; is the
air apt to be chilled or warmed by this movement ?

Read Stedman's poem, " The Simoon," and compare it with the
description in any standard cyclopedia.

Why are northerly winds apt to be cold ?

Explain the manner in which street whirlwinds are formed

Note any instance of the physiographic effects of winds in the lo-
cality with which you are best acquainted ; prepare a description of it.

In what way do the general winds affect the temperature of the
earth ?

Note any examples in which winds accomplish work that has an
economic value



COLLATERAL READING AND REFERENCE.

U. S. Coast Survey.— Atlantic Coast Pilot Cliart, for March
and September, or February and August — any year.
Le Conte.— Elements of Geology, pp. 1 -H.



238 PHYSICAL GEOGRAPHY



NOTES

' At a heipfht of fifteen thousand feet the air is so rare that
hreathin;^ is labored and the pulsations of the heart are very
rapid. Climbing becomes difficult and any form of exertion is
very wearying. Water boils at about 85° (185° F.), a temperature
so low that it is difficult to cook vegetables by boiling.

" Approximately the pressure is one-lialf a pound for every
inch in the iieight of the column of mercury. At the level of the
sea, the lieight of the barometer varies usually between 39 and
30.4 inches.

' It is well to bear in mind that the common expression, "hot
air rises, because it is lighter," is not strictly correct. The hot
air does not rise ; it is pushed upwards and floated ontlie surface
of the heavier air.

*The height to which the updraught rises before it turns
toward the pole is not known, except in two or three instances.
On the Island of Hawaii, the Trade Winds reach an altitude
of about twelve thousand feet. Above this elevation the winds
have almost an opposite direction ; they are the winds that, a
few degrees farther north, descend to become the Prevailing
Westerlies.

'' The Prevailing Westerlies are also called 7'eturn-trades, anti-
trades, and counter-trades. The name here used is now com-
monly emftloyed in meteorology.

" Many years ago, when most of the foreign carriage was
eflfected by sailing vessels, there was a brisk trade in horses
between the ports of the New England States and the West
Indies, nearly all the horses used in the latter country being
obtained from New England. Frequently the vessels were be-
calmed in this belt, and it became necessary to throw overboard
half the number of horses, in order to save the remaining
animals.

' The name is derived from a Malay word, meaning "season."

* On account of its inland position. Central Asia is marked by
great extremes of temperature. During summer its vast deserts
are almost like a furnace, and the updraught of heated air is so
enormous that it causes atmospheric disturbances two thousand
miles away. In winter the dry air is chilled many degrees below



THE ATMOSPHERE AND ITS PROPERTIES 239

that of the warm sea-air, and, being correspondingly heavier,
tlows outward toward the ocean. No other body of land pos-
sesses the qualities requisite to produce monsoons that compare
with those of Asia.

" A similar movement of air is noticeable in many large caves
— especially those that have openings at different levels. In the
daytime air in the cave is usually colder than that outside,
while at night it is warmer. As a result, at night there is a
strong in-draught of colder air at the lower entrance, and an up-
draught at the higher opening. In the daytime these move-
ments are reversed.

'" These three names are applied to winds that have certain
principles in common. Warm, moist air is pushed up the side of
a mountain-range ; being cooled either by its own expansion, or
by contact with the colder mountain top, its moisture is con-
densed ; the air then descending on the other (or possibly the
same) side warms very rapidly by its own compression. The
effect is very marked ; snow disappears very rapidly — hence the
popular name "snow-eaters." The descending air is not only
warm, but it is so dry that in summer it withers vegetation.
The Chinook wind gets its name from a locality in Oregon,
whence it seemed to come, but the name is now applied to warm
Avinds that flow from the Rocky Mountains out on the plains to
the East. Following a blizzard, it quickly melts the snow that
covers the scanty feed of the cattle herds. The Santa Ana is a
hot w'ind common in .southern California and Mexico.

" Some years ago the author left an octagonal steel drill in an
upright position exposed to the full sweep of a desert wind. Six
months afterward its angles had been almost obliterated by the
impact of rock waste. The telegraph poles in these regions are
frequently cut in two by the wind-blown ruck waste.

"■ It is likely that a fire, which in 1828 burned off the vegeta-
tion protecting the ridge, was responsible for starting this dune on
its travels. In 1H45, (ieiicral .Toscph K. .Johnston, then a govern-
ment engineer, noticed that north winds were very actively at
work in picking up sand from the seaward face of the dune and
carrying it over the crest to the landward side. Little by little
the wave of sand overwhelmed a strii) of pint; barrens and filled
a salt marsh })oyond. Then it advanced upon a heavy growth of
timber and, in time, covered all but the tallest trees, killing them



330 niYSICAL GEOGRAPHY

as eflFectually as though they had been swept by fire. As the
years passed by the wave steadily advanced, and tlie wind began
to uncover tlie buried surface in tlie rear. First the strip of
pine barrens re-appeared, and tlien tlie salt marsh was cleaned
out and promptly reclaimed by the tide. Even the pine barrens
began to show signs of life and a growth of young trees sprang
up. Within tlie past lew years the advancing sand has begun
to uncover the forest, and a border of dead trees now flanks the
rear slope. Near the eastern end of the dune is Cape Henlopen
light-house. A straggling ridge of the wave entered the yard,
covered up the oil-house and the garden, and then took posses-
sion of the keeper's cottage. The Government acknowledged
its inability to cope with the dune by erecting a new cottage on
the other side of the tower.



CHAPTEK XIII

THE MOISTURE OF THE ATMOSPHERE . SEASONAL
AND PERIODICAL DISTRIBUTION OF RAINFALL




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i r "''






1 4 - -au




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The vapor of water mingled with the atmosphere, iu a
way, may be considered a part of it ; ' but, if all the other
constituents were absent, the water vapor would exist as an
atmosphere in itself, and its movements would be the same

practically as those of the winds.
But while the proportion of oxygen
and nitrogen of the atmosphere do
not perceptibly varj^ that of Avater
vapor is subject to rapid changes.
The amount present depends on one
thing only — temperature. With a
high temperature there may be a
great deal of vapor mingled with the
air ; with a lo\v temperature there
can be but little.

Changes in humidity are usually

apparent to the sense of feeling, and

one readily learns the difference be-

S iiU^ tween moist and dry air.'^ In many

instances they may be forecast by

observing the clouds. If the latter

form rapidly, or if small patches of

cloud increase in size, the humidity

is increasing. On the contrary, if

the cloud area is becoming smaller, it is highly probable

that the humidity is decreasing.

231



Till- HYGROMETER.



233 PHYSICAL GEOGRAPHY

Tlie ainoimt of moisture is determined in various ways
Mostcommonly tlieA//(/vo//i<3^e/-, an instrument employed to
measure the amount of moisture, consists of two thermom-
eters, the bulb of one being covered Avith a single thickness
of wet cloth. If the air be dry, the water that saturates
the cloth evaporates rapidly and chills the bulb, so that
the reading of the thermometer is several degrees lower.
If the air is moist, on the other hand, very little evaporates,
and the difference in the reading of the thermometers is
very slight. From the difference in the readings of the two
thermometers the amount of moisture may be calculated.

Dew Point. — Table VII., Appendix, shows the amount
of water vapor there may be in the air at various tempera-
tures.- With the thermometer at 66° F., for instance,
there may be seven grains in each cubic foot of atmos-
phere. There might be less, but there can be no more ;
if more be added it would immediately condense — that is,
change to rain or snow. From this table find whether or
not there may be vapor in the air when the temperature is
below freezing-point of water. Compare the amount at
70° F. and 90° F. Learn the temperature at the time of
recitation, and find the amount of moisture there may be.
What is the general law shown in this table, so far as tem-
perature and the percentage of moisture are concerned ?

When all the vapor that can exist at a particular tem-
perature is present, the air is said to be saturated or at the
deiu-jwint.^ This condition is unusual, however, except
when rain is falling ; generally the amount present is con-
siderably less than that required for saturation. From the
amount present one may easily compute the relative hu-
midity ; thus, if half the quantity required to saturate the
air is present, the relative humidity is fifty per cent. What
is meant when the relative humidity is eighty per cent.?
If the amoimt is near the dew-point, the air is moist ; if



THE MOISTURE OF THE ATMOSPHERE 233

the relative humidity is low, it is dry. Air that is moist
at a given temperature may feel very dry at one that is
higher, even though no more moisture is present.

Latent Heat of Evaporation. — ^Yater is changed to
vapor by heat. When water boils it reaches the tem-
perature at which it begins to change rapidly to steam.
No matter how fierce the heat may be, the water (unless
it is confined under pressure) gets no hotter, and the steam


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