The sun’s heat at the tropics not only warms air, it also evaporates ocean water into water
vapor that disperses into the air. The rising moisture-laden warm air rapidly expands and
cools as its pressure reduces (since atmospheric pressure is highest at the earth’s surface
and decrcases with height), and its moisture condenses back into water, falling as rain
that is largely confined to a belt about 10° north and 5 to 10° south of the equator. This is
why much of this belt, the central tropics, is extremely moist and covered with dense,
lush vegetation.
The now almost moistureless air continucs to gain height and flow northeast or
southeast, pushed by more hot, moist air rising at the central tropics. Gradually it moves
beyond about 20 to 25° north and south of the equator, into the subtropics, and becomes
cool enough to descend. As it does so, its pressure rises, rehcating the air, just as the air
squeezed in a bicycle pump becomes hot. This is the warm, dry, high-pressure air found
about 25 to 30° north and south of the equator. It is also the air that helps create most of
the world’s deserts.