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UNIT 3

Weather Causes (2)

SECTION 5. CIRCULATION

1. Basic Circulation

The atmosphere tends to maintain an equal pressure over the entire earth just as the ocean
tends to maintain a constant level. Whenever the equilibrium is disturbed, air begins to flow
from areas of higher pressure to areas of lower pressure.

2. Causes of Atmospheric Circulation

a. The factor that disturbs the normal equilibrium is the uneven At the equator,heating
of the earth. the earth receives more heat than in areas to the north and south.This heat is
transferred to the Thus The atmosphere, warming the air and causing it to expand and rise.
expansion caused by the warming lowers the density of the air. an area of low pressure is
produced at the equator. The air at the poles is cold. Cooling causes air to contract cooler,
(become denser) and sink to the surface. This heavier, denser air from the north and south
moves along the earth's surface toward the equator to equalize the pressure. This air also
becomes warm and rises, thereby establishing a constant circulation. On a non-rotating earth
this circulation would consist of two circular paths, one in each hemisphere, followed by the
air rising at the equator, traveling aloft toward the poles, and returning along the earth's
surface to the equator.

Figure 1
Circulation

as it would be

on a non-rotating
earth. Intense

heating at the

equator lowers the
density. Denser air

at the poles flows

toward the equator
forcing the less

dense air upward

where it flows aloft
toward the poles.