90 PHYSICAL GEOGRAPHY.

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has a small capacity for heat, and also because
the heat passes through but a comparatively thin
layer. Therefore,a comparatively short exposure
of land to heat produces a high temperature, and
a comparatively short exposure to cooling, a low
temperature. Water heats or cools slowly, ab-
sorbing or emitting large quantities of heat. This
is because water has a great capacity for heat.
The heat penetrates a comparatively deep layer,
and then, too, as soon as slightly heated, the warm
water is replaced by cooler water. Therefore, the
water can be exposed to either long heating or
long cooling without growing very hot or very
cold. Hence, the land is subject to great and
sudden changes of temperature; the water, to
small and gradual changes.

Places situated near the sea have, therefore, a
more equable, uniform climate than those in the
same latitude in the interior of the continent.
The former are said to have an oceanic climate ;
the latter, a continental climate.

In the polar regions, a preponderance of moder-
ately elevated land areas causes a colder climate than
an equal arew of water, because land loses heat
more rapidly than water.

In the tropics, a preponderance of land areas
causes a warmer climate than an equal area of water,
because land gains heat more rapidly than water.

(2.) The Distribution of the Relief Forms a
the Land Masses.

(1.) Elevation.—The temperature of the atmo-
sphere rapidly decreases with the elevation. The
decrease is about 3° Fahr. for every 1000 feet.

The increased cold is caused as follows:

(1.) Since the air receives so much of its heat indirectly
from the earth’s surface, the farther we go upward from
the surface, the colder it grows.

(2.) In the upper regions of the atmosphere the de-
creased density and humidity of the air prevent it from ab-
sorbing either the direct rays of the sun, or those reflected
or radiated from the earth. The effect of elevation is so
powerful that on the sides of high tropical mountains the

same changes occur in the vegetation that are observed in
passing from the equator to the poles.

(2.) Direction of the Slopes—That slope of
an elevation on which the sun’s rays fall in a di-
rection the more nearly at right angles to its sur-
face will be the warmest. ~

In the northern hemisphere the southern slope of a hill
is warmer in winter than the northern slope, because the
rays fall more nearly at right angles to its surface,

(3.) Position of the Mountain-Ranges,— A
mountain-range will make the country near it
warmer if the wind from which it shields it is

 

cold; it will one it colder if such wind is
warm.

The position of the mountain-ranges of a country also
greatly affects the distribution of its rainfall. Thus, the
tropical Andes are well watered and fertile on their east-
ern slopes, but dry and barren on their western. The pre-
vailing moist trade winds, forced to ascend the slopes,
deposit all their moisture on them in abundant showers,
and are dry and vaporless when they reach the other side.

(4.) Nature of the Surface—The temperature
of a tract of land is greatly affected by the nature
of its surface. If covered with abundant vege-
tation, like a forest, or if wet and marshy, its sur-
face heats and cools slowly, and has a compara-
tively uniform temperature; but if destitute of
vegetation, and dry, sandy, or rocky, it both
heats and cools rapidly, and is sup ject to great
extremes of temperature.

(8.) Distribution of Winds and Moisture —The
principal action of the winds, and their accom-
panying moisture, is to moderate the extremes of
temperature by the constant interchange between
the heat of the equatorial and the cold of the
polar regions. Both wind and vapor absorb and
render latent large quantities of heat in the equa-
torial regions, and give it out, in higher latitudes,
on cooling. In cold countries the climate is ren-
dered considerably warmer by the immense quan-
tity of heat thus emitted by the condensed vapor.

(4.) Ocean Currents.—Since the warm waters
move to the polar regions, and the cold waters to
the equatorial regions, the general effect of ocean
currents on climate is to reduce the extremes of
temperature.

The combined effects of the action of the winds,
moisture, and ocean currents are seen in the north-
ern continents, whose western shores, under the in-
fluence of the prevailing south-westerly winds,
copious rains, and tropical currents, are consider-
ably warmer than the eastern shores in the same
latitude.

The coasts of Great Britain are warm and fertile, while
Labrador, in the same latitude, is cold and sterile. The
island of Sitka, in the Pacific, is warmer than Kamtchatka
from similar causes,

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CHAPTER III.
The Winds.

241. Origin of Winds—Winds are masses of
air in motion. They resemble currents in the
ocean, and result from the same causes—differ-