Equatorial Rain Belt

The Earth is most strongly heated near the equator.

Surface air is heated and expands as the temperature increases, thereby decreasing the density of the air.

This low density air rises over the equator, and is replaced by surface air streaming from north and south of the equator.

The air pressure decreases as the hot equatorial air rises through the atmosphere because there is less and less weight of air overhead.

The rising air expands due to the decreasing pressure.

The air cools as it expands.

Condensation of water vapor into cloud droplets and precipitation occurs because cool air can hold less moisutre than warm air.

The equatorial air, after dropping off its excess moisture in the form of rain, moves away from the equator to the north and south at high altitude.

Desert Belts

The air sinks back to the surface at around 20 to 30 degrees latitude (both north and south of the equator) to balance the air rising at the equator.

The air is compressed as it sinks because of the increasing weight of air overhead.

The air temperature increases as the molecules are pressed more tightly together.

The air becomes very dry (relative humidity decreases) as it warms because warm air can hold more moisture than cool air.