Man's physical universe

xanabras

UNIT III

SECTION 3

THE PRINCIPAL AGENCY IN THE GRADATION OF THE

EARTH IS ITS ATMOSPHERE

Introduction.

The earth's atmosphere, when acted upon by the sun, produces the

changes which the earth's surface is constantly undergoing. The sun

not only produces changes in the temperature of the atmosphere that

cause winds and rains, but it also heats water and thus causes it to

evaporate into the atmosphere more rapidly. Water is the greatest

Fig 35. A rock worn by windparticles.

(Courtesy of the

borne

U. S. Geol. Survey.)

agent of erosion.

In the mountains,

in the plains, or on the seashore, one

can see water at work. Sometimes

it works gradually, and sometimes

it comes with the overwhelming

force of floods. Running water removes

soil from some places and

deposits it at other places, sometimes

where it is not wanted. Rain

water erodes the surface of the earth

as it runs ofi to the sea, and the

snow and ice form vast storage

reservoirs to maintain this run-off

during periods of little precipitation.

About one third of the rainfall

evaporates, another third runs off to

the ocean, and the last third soaks

into the ground, where it

The Atmosphere Acts on the Earth in Many Ways.

begins its

relentless work as ground water.

The unequal heating of the earth's surface causes movements in the

atmosphere. A continuous flow of water from the mountains to the

The winds

sea is assured by the water vapor that these winds carry.

are likewise responsible for the constant pounding of the waves on the

shores of lakes and oceans.

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