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Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

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waiian Islands are examples ; although in the torrid zone,
they are regions of perpetual spring, with no oxceasos of
temperature. The Leeward and Windward islands of the



290 PHYSICAL GEOGRAPHY

West luclian group are also examples. Though situated
oul}- a few degrees north of the equator their summer tem-
perature is less oppressive than that of New York City.

Prevailing Winds. — Winds are the chief medium for
the transmission both of moisture and warmth. Cold
winds from polar regions modify the excessive heat of low
latitudes, and tropical winds blowing into high latitudes
soften the rigors of polar climate. The mild temperature
of western Europe is due largely to southwesterly winds,
and the same is true of the equable climate of western
North America. Not only do the winds themselves trans-
fer a great amount of heat by convection, but the vapor of
water furnishes an enormous supply. For every pound of
water vaporized, enough heat is made latent to raise nearly
half a ton of water one degree (F.) in temperature. When
the vapor, mingled with the wind, is carried to higher lati-
tudes and there precij^itated, all this heat is again set free.
An inspection of the chart of winds (p. 221) readily gives
all the information necessary to determine roughly the
climate of a country. The regions invaded by sea winds
that have come from low latitudes are the regions of warm
and equable climate. Inland and polar regions are areas
of climatic extremes.

Changes in Climate. — As a rule, the climate of a
country is constant ; that is, it does not change materially
except after long intervals of time. The mean tempera-
ture of any given locality rarely varies more than a very
few degrees from one year to another, and the averages of
long periods show still less variation. Fluctuations in rain-
fall and cloudiness are considerably greater than those of
temperature. In regions of generous rainfall the precipi-
tation of very wet years may be nearly twice that of very
dry years, but in localities of deficient rainfall the differ-
ence may be greater.



CLIMATE AND ITS FACTORS 291

When time is reckoned by geological epochs, however,
it seems certain that great climatic changes have occurred
in every part of the earth, and that they have been of the
most radical character. The Glacial Epoch, already de-
scribed, is an example of a change in the climate that has
taken place in the North Temperate Zone. It is certain
that the rainfall of the Basin Region of the United States
is subject to periods of oscillation. The few scattered
sinks and salt lakes of the Great Basin itself are remnants
of two large lakes that existed there at no very remote
period, and these in turn are evidence of a much greater
rainfall than the region receives at the present time.

Definite knowledge of such changes, in the main, is cir-
cumstantial, and statistics regarding them are almost
wholly wanting. The cause or causes of such changes,
moreover, are unknown. A change in the inclination of
the earth's axis would be competent to account for changes
in temperature, and therefore in rainfall.^ Changes in in-
clination have certainly occurred, but their definite eftects
are not known. Changes in the level of a region are also
capable of producing variations in temperature, and it is
highly probable that elevation and depression have re-
sulted in many of the climatic changes of which there is
an unwritten record.^

Zones of Climate.— Zones or belts whose limits are
bounded by lines of equal average temperature are called
isothermal or climatic zones, and the lines bounding them
isothermal lines or isotherms. A comparison of the map of
the astronomical and the climatic zones shows that the
correspondence of the two is only general. Tiie former
are fixed and their boundary lines are parallels of latitude.
The latter change their i)Ositions with the apparent motion
of the sun, behaving in this respect like the zones of winds
and calms. In fact they are all governed by the sam.- 1m w




ISOTHERMS FOR JULY




ISOTHERMS FOR JANUARY



CLIMATE AND ITS FACTORS 293

and arise from the same cause— the iuelmation aud self-
paralleHsm of the earth's axis.

In the southern hemisphere the isotherms range ap-
proximately with the parallels. What may be inferred
from this concerning the uniformity of temperature with
respect to latitude ? In the northern hemisphere the iso-
therms are very irregular. In which direction do they
bend in crossing the great highlands of the earth ? Explain
the cause of this. In the North Atlantic warm ocean cur-
rents and their drifts cause a deviation of the isotherms ;
explain how and why.

By what isotherms is the climatic torrid zone limited
north and south ? ^ Compare the position in January and
July. In the spring and the fall its position corresponds
roughly Avith that of the astronomical zone. The hottest
areas are situated not on the equator, however, but north
of it. In the African desert, Arabia, and the arid lands of
the United States, the summer temperature is above 38°
(100° F.) and during unusual hot spells it sometimes
reaches 49° (120= F.).

The isothermal temperate zones are limited by the lines
of 21° (70° F.) and 0' (32° F.). The summer limit of the
northern zone extends high into the arctic regions. The
wdnter limit on land approximates the fortieth parallel, but
on the ocean it is much higlier. In the Pacific it reaches
to the sixtieth parallel ; in the Atlantic, owing to the drift
of the Oulf Stream it penetrates the polar latitudes.

Extremes of Climate. — The isotherm of highest
temperature that completely girdles the earth is theoreti-
cally the thermal equator. Its temperature is prol)ubly
between 27^ and 30^ (80'^ to 86° F.). There are several
isolated regions having a considerably higher temperature,
however. An extensive region in tlie Sahara has a mean
temperature of about 29° (85° F.), and in Hindustan and



294



PHYSICAL GEOGRAPHY



Africa there are others equally warm. In the American
continent an oval-shaped region extending southward from
the Gulf of California has about the same mean.

The regions of extreme cold are not in the viciuity of
the geographical pole, but considerably south of it. In the
American continent the area of extreme cold is near the
Arctic Archipelago. In Eurasia it is a little to the east-
ward of the Lena River. In both regions the mean tem-
perature is not higher than— 17° (0° F.). At Werchojansk/
Siberia, the temperature ranges from — 67° (—90° F.) to

32° (90° F.) a range
of one hundred and
eighty degrees — and
probably the great-
est on the earth.

Changes of Sea-
son. — Because the
earth's axis is in-
clined to the plane
of its orbit, and re-
mains parallel to it-
self while the earth
revolves around the
sun, it follow^s that the rays of the sun do not fall on a
given place always at the same angle.

From the accompanying diagrams find the time at
which the sun's rays are vertical at the tropic of Cancer.
What is the season at this time in the northern hemisphere ?
Are the sun's rays direct or slanting in the southern hemi-
sphere? What is the season there ? What are the seasons
when the sun's rays are vertical at the equator? On a
piece of thin paper trace, with a pencil, the isothermal hot
zone on the map for January, p. 292 ; cut it out along the
lines, and place it in its proper position {i.e., for January




POSITION OF HEAT-RAYS IN'JUNE



CLIMATE AND ITS FACTORS



295



on the map for July). The iDarts that overlap show the
region where summer is continuous all the j^ear. Compare
this result with the diagram on this page. What parts are
not covered bv the



SU N'S



-fi-ft-¥-S-



heat-belt? AYlien
the heat-belt is far
north what is the
season in the North-
ern Hemisphere ?
in the Soiithern ?
From the oscilla-
tion of the heat-
belt show how five
zones of tempera/
ture result.

The inclination
of the axis, together with its parallelism, as the earth re-
volves around the sun, bring the temperate zones, in turn,




POSITION OF HEAT-RAYS IN DECEMBER




ANNUAL MOVEMENT OF THE HEAT-BELT

to a position where the sun's rays are nearly vertical. It
is this movement that causes the shifting of the zones
of climate alternately north and south.



296 PHYSICAL GEOGRAPHY

The alternation of the four seasons is realized mainly in
the temperate zones. In the greater part of the Avestern
coast of North America the seasons are distinguished more
by the distribution of rain than by variations in tempera-
ture. Practically there are two seasons — a rainy and a
dry. Within the greater part of the torrid zone these are
also about the only distinctions of season. In the frigid
zones the distinctions of summer and winter are also those
of day and night, each of which is six months in duration.

Deserts. — There are many extensive areas that have
little or no rainfall. If the rainfall is so deficient that
irrigation is necessary to produce crops the region is said
to be arid ; if it is too dry for food crops, it is generally
considered a desert region. In many instances there is no
sharply drawn line between fertile and arid lands, or be-
tween arid lands and deserts. For instance, all that part
of the Mississippi basin east of the 97th meridian, or more
strictly, the 2,000-foot contour — produces an abundance of
food stuff. West of this contour, however, the climate be-
comes much drier, and beyond the 100th meridian or
2,500-foot contour, crops must depend mainly on irriga-
tion.

Farther west, turf grass is replaced by scanty bunch
grass, and beyond the crest of the eastern ranges of the
Kocky Mountains the character of the country in places
approaches that of a typical desert. In the northern part'
of the Basin Region, the cooler climate and the high
ridges wring a small amount of water from the clouds, but.
in the south almost all vegetation disappears and the region
is absolutely a desert. The same gradation is observed
in the great African desert. Both north and south of the
equatorial rain-belt, precipitation decreases little by little ;
fertile lands grade imperceptibly into arid belts, and the
latter into deserts.



CLIMATE AND ITS FACTORS 297

In the South American deserts the line, on the contrary,
is pretty sharply drawn, and the same is true of the North
American desert and the Sahara, if they are approached
from the western side. In each case a high moiintain-
range forms a barrier to the rain winds, sharply dividing
a fertile area from a desert.

Only a small part of the extensive desert areas is desti-
tute of vegetation, and in such parts the finely pulverized
rock waste, or "sand"' shifts hither and thither with the
winds. It is in such regions that the fierce simoon and sim-
ilar sand-storms prevail. The Colorado Desert, in south-
eastern Calif oi-nia, is an excellent example of the kind.

The climate of desert regions is marked b}' peculiarities
and extremes. The wands are hot sand-blasts and whirls ;
the scanty rains come usually in the form of cloud-biirsts ;
the temperature is one of frightful extremes ; the relative
humidit}" of the atmosphere rarely exceeds thirty percent,
of saturation. Notwithstanding all this, desert climate is
wonderfully healthful.

Any fertile spot in a desert is called an oasis, and the
latter is fertile because it is more or less abundantly sup-
plied with water. On account of the presence of water,
the oasis commonly 3'ields a goodly supply of food-stuffs.
Various causes contribute to the formation of oases. The
underlying strata may be im])ervi()us to water, thereby
preventing the latter from sinking deep into the soil,
or there may be a mountain-crest that is suffici<'iitly high
to condense and precipitate more or less moisture. The
water flowing down the slopes percolates through the fine
rock waste at the bottom, much of it being hcKl there in
suspension. The oases of the North American deserts are
of this character.

The distribution of deserts constitutes an inten^sting
study. There are practically two zones, situated mainly



298 PHYSICAL GEOGKAPHY

between the 20tli and SOLli parallels, north and south, that
contain nearly all the desert and arid lauds of the earth. In
Eurasia and Africa a belt of desert stretches from the west-
ern coast almost throusfh the continent.*^ In North America
this belt is nearly 1,000 miles east and west. The deserts
of the Southern Hemisphere are smaller in area only be-
cause of the smaller land area. In South America it lies
at the eastern base of the Andes ; in Africa, south of the
Kongo water-shed; in Australia, it extends almost across
the continent.

Various causes contribute to make arid and desert con-
ditions ; but in any case a desert is a desert, not because
of any natural sterility of the soil, but because of the lack
of moisture. ^ In some localities a high mountain -range
that faces the sea-winds condenses all the moisture thev con-
tain and the opposite slope with its outlying area is there-
fore a desert. Explain why the Peruvian desert of South
America is west of the Andes, and the desert of Argentina
lies to the east of these ranges. Why is the region east
of the Cascade and Sierra Nevada Ranges either arid or
desert? What effects have the Himalaya Mountains on
the rainfall of the region to the northward ?

In other instances the desert conditions arise from other
and more complex causes. Thus, between the 20th and
30th parallels there is a downward movement of atmos-
pheric currents ; explain why these may produce deficiency
or absence of rainfall (p. 288). In some localities the
winds blowing inland from the sea may enter localities
having a temperature higher than that of the winds them-
selves, and in such instances their moisture is not con-
densed. The Australian and African deserts result mainly
from one or the other, or both, of these causes. - They are
unfortunately situated with reference to latitude, and they
also are lacking in high mountain-ranges.



CLIMATE AND ITS FACTORS 299

QUESTIONS AND EXERCISES. -Referring to any good map, de-
termine the climate of South America from the following suggestions
giving a reason for each statement : What are the conditions of tem-
perature of the northern part ? How do those of the southern part
differ ? In which part is temperature the basis of the seasons ? In
which is rainfall? From which direction do the rains of the northern
part come ? of the southern part ? What is the effect of the Andes
Mountains on the distribution of the rainfall ? Give the location of
the desert and arid regions. Note the effects of altitude on the climate
of the highlands ; of the lowlands. What evidence does the map give
to show whether the rainfall of the Amazon basin is profuse or deficient ?
Explain why the basin of the Orinoco has two rainy and two dry
seasons.

Compare the Asian and American deserts as to origin. How do the
African deserts compare in this respect ?

Prepare a summary of the climatic conditions of the state or county
in which you live, noting especially any facts not ordinarily included
in the general outlines of the subject. From the United States Weather
Bureau obtain the following : highest temperature observed, lowest
temperature observed, mean for each month, mean annual rainfall,
mean for each month, number of rainy days for any year, general
direction of the winds, other relevant facts.



COLLATERAL READING AND REFERENCE.

Davis. — Elements of Meteorology.
Waldo. — Elementary Meteorology.
Greely. — American Weather.



NOTES

' To these may be added the effect of ocean currents. It is
sometimes stated that the warmth of western Euro|H> — the Brit-
ish Isles, especially — is due to the (rulf Stream, and that of the
western United States to the influence of the Japan (Jurrent.
So far as their temperature in general is concerned, such a state-
ment is untenable. Ocean currents juH'omi)lish very potent re-
sults, however, but not becau.se of their cfTects on climat*'.
Thus, the warm drift of the duU Stream is carried by the I'n -
vailing Westerlies into about every bay and cove of western



300 PHYSICAL GEOGRAPHY

Europe; and as a result the harbors — even as far north as
Hainmerfest — are free from ice all the year.

" A noticeable and highly important difference between a mari-
time and a continental climate, is the daily range of tempera-
ture. In a maritime climate this rarely exceeds twenty degrees
(F.), while in a few inland regions the fluctuations may be twice
as great.

^ That is, if the axis of the earth were to incline forty degrees,
tlien the polar and the tropical circles would have a correspond-
ing distance from the poles and from the equator, and the tem-
perate zones each would be ten degrees in width, instead of
forty-three. Or, if the longitude of perihelion were to change
materiallj% the winters of the northern hemisphere might be
longer by several days than the summers, thus causing the ice
and snow to collect faster than it would melt, thereby in time
causing far-reaching changes.

* The elevation of a region is thought to result in a lowering of
its mean temperature, and the depression of its surface, it is be-
lieved, has an opposite effect. The surface of New York and the
New England States was about 1,000 feet higher during the
Glacial Epoch than at present.

^ The mean annual temperature of a region reveals but little
concerning its actual conditions of temperature. These can
be studied only from monthly isotherms— that is, by comparing
the monthly range of temperature and climate. For this reason.
Instead of a chart of annual isotherms, it has been deemed wiser
to prepare two charts, one showing the isotherms for January,
the other for July.

" Werchojansk (or Verkoyansk) is four hundred miles north of
Yakutsk, Siberia. The two are in probably the coldest inhab-
ited region in the world. The highest temperature taken under
standard conditions — that is, shaded by a double roof with an
air-space between, and exposed at a distance from any radiating
surface — seems to have been recorded at Warglar, Algeria, where
the mercury marked 127° F. In the Colorado Desert an unoffi-
cial temperature of 136° has been noted. In this case, however,
it is doubtful if a properly exposed thermometer would have
registered so much by ten or fifteen degrees. No temperature
in this region recorded by the Weather Bureau has exceeded 123°,
though there are several localities, such as Salton Lake and



CLIMATE AND ITS FACTORS 301

Death Valley, where the temperature ranges higher than at any
of the Weather Bureau stations. The author has repeatedly
noted temperatures in the Colorado Desert varying from 130° to
145° registered by a thermometer exposed to the direct rays of the
sun. The experience of General Greely, U.S.A., Chief Signal
Officer, shows the range of human endurance. At Fort Conger,
Lady Franklin Bay, he and his party experienced no intolerable
discomforts with the temperature as low as — 6(j°, the same officer
served in Arizona where the shade temperature was 119° and that
of an unprotected thermometer 144°.

' The shifting soil of deserts is popularly regarded as sand. As
a matter of fact it consists of about every kind of rock waste
broken and pulverized by the impact it receives as it is blown
about by the wind. Doubtless it contains more or less quartz,
but in general, true quartz sand is rare. In the Colorado and
Mojave Desex'ts the detritus passing for sand is broken felspathic
rock ; in certain localities of the Arabian Desert it is a red. loamy
soil.

' ' ' The districts of the Sahara destitute of oases present a formid-
able aspect. The path which the feet of the camels have marked
out in the immense solitude points in a straight line toward the
spot which the caravan wishes to reach. Sometimes these faint
footmarks are covered with wind-blown rock waste, and the
travellers are obliged to consult the compass, the horizon, a dis-
tant sand-hill, a bush, a heap of camels' bones, or some other
indications which the practised eye of the Tuareg alone can un-
derstand as the means by which the road is recognized. Vege-
tation is rare, and the only plants to be seen are the scrub, con-
sisting mainly of thorny Mimosas ; in some sandy deserts there is
a complete aljsenee of all kinds of vegetation. The only animals
to be found are scorpions, lizards, vipers, and ants. During the
first few days of the journey a few indefatigable individuals of the
fly tribe accompany the caravan, but they are soon killed by the
heat; even the flea itself will not venture into these dreadful
regions. The intense radiation of the enormous white or red
surface of the desert dazzles the eyes; in tliis blinding light
every object appears to be clothed with a somi)re and preter-
natural tint. Occasionally tii(> tra\-eller, when sitting upon his
camel, is seized with a kind of brain fever, which cau.ses him to
see the most fantastical objects in Ills delirium. Even thotm



303 PHYSICAL GEOGRAPHY

who retain the entire possession of their faculties and clearness
of vision are beset by distant mirages ; palm-trees, groups of
tents, shady mountains and sparkling cascades seem to dance
before their eyes in misty vapor. When the wind blows hard,
the traveller's body is beaten by grains of sand which penetrate
even through his clothes and prick like needles. Stagnant pools
or wells, dug with great labor in some hollow or other, from the
sides of which oozes out a brackish moisture, point out each
day the end of the stage. But often this unwholesome swamp,
where they hoped to recruit their energies, is not to be found,
and the people of the caravan must content themselves with the
tainted water Avith which they filled thair flasks at the preceding
stage. It is said that in times of great need travellers have been
compelled to kill their dromedaries in order to quench their
thirst in the nauseous liquid contained in the stomachs of these
animals. " — Elisee Reclus.

° In popular literature the climate of deserts is supposed to
have baneful properties, and the expression ' ' poisonous emana-
tions ' ' has a prominent place in many newspaper accounts. As
a matter of fact, desert air is unsurpassed so far as salubrity is
concerned. It is so free from the germs that produce or hasten dis-
ease, that meat will not putrefy and food will not ferment. Sep-
ticaemia, or "blood-poisoning," rarely if ever follows accidental
wounds or surgical operations, and tuberculosis originating in
such localities is unknown.



CHAPTEK XYII
THE DISPERSAL OF LIFE

There are two lessons iu uatiu-e tliat probably every
human being of mature years has learned, namely — that
the earth is full of organisms endowed with that mys-
terious force called life, and that the life-forms are
grouped in kinds or species. Moreover, while the indi-
viduals of a species closely resemble one another, those
of different species are commonly very unlike.

Almost every living body or organism passes through
several stages or conditions.' It first appears in the form
of a germ enclosed in an envelope culled an egg, or per-
haps, a seed. Under the action of heat, or moisture, or
both heat and moisture, the egg or seed passes through
various stages of development in which it gradually ap-
proaches its mature form — the condition that immediately
precedes death. In general, the egg develops into a life-
form, known as an animal, the seed into a plant. Tlie egg
may contain both food and moisture as well within its en-
velope ; but the seed contains food only. The egg very
easily loses its vitality or life principle ; the seed nuiy re-
tain its vitality for months, or even years. The offsi)ring
of the egg almost always possesses the power of moving
from place to place in one or another of its forms of life ;
the offspring of the seed, on the contrary, cannot move ;
it spends its life in tlio spot in wliicli it d(^veh)i)(Ml into life.

The seed-form of the organism is remarkably adaj^tcd
f(jr transportation and dispersal. Commonly tlie seeds

303



304



PHYSICAL GEOGRAPHY



are strong enough to resist not a little mechanical force.
Those of some species will endure a temperature but little
lower than that of boiling water ; they will likewise endure
the severest cold, and almost ahvays they are enclosed in a
water-tight case. Tlie egg, on the other hand, will not
eudure extremes of temperature, nor will it survive the
slightest injury. As a rule, both seeds and eggs float on


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Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 20 of 25)