Because more solar energy strikes the equator, the air over the equator is warmer than elsewhere on the planet. Warm air has low density, so it rises and forms a low pressure zone. At the top of the troposphere, half of the warm air moves toward the north pole and half toward the south pole along the top of the troposphere. At about 30 degrees north latitude, the air from the equator meets air flowing toward the equator from higher latitudes and descends to the ground, creating a high pressure zone. Once on the ground, the air returns to the equator. These air movements form a convection... Show more

Because more solar energy strikes the equator, the air over the equator is warmer than elsewhere on the planet. Warm air has low density, so it rises and forms a low pressure zone. At the top of the troposphere, half of the warm air moves toward the north pole and half toward the south pole along the top of the troposphere. At about 30 degrees north latitude, the air from the equator meets air flowing toward the equator from higher latitudes and descends to the ground, creating a high pressure zone. Once on the ground, the air returns to the equator. These air movements form a convection cell, called a Hadley cell, which is found between 0 and 30 degrees north latitude. A similar    
Hadley cell is also found between 0 and 30 degrees south latitude, except the air flows in the opposite directions.    
In addition to Hadley cells, there are two other major convection cells in each hemisphere. A Ferrell cell is located between 30 and about 60 degrees north or south latitude. In the Northern Hemisphere, air in this cell moves from north to south; it moves in the opposite direction in the Southern Hemisphere. A polar cell is located between about and 90 degrees north or south latitude. In the Northern Hemisphere, the air in this cell moves from south to north and descends at the north pole. It moves in the opposite direction in the Southern Hemisphere and descends at the south pole.    
 

Global circulation cells cause global wind belts. Global wind belts are enormous. They occur because of the flowing air at the bottom of the major circulation cells. In both hemispheres, the global wind belts are the trade winds, westerlies, and polar easterlies.

In the Northern Hemisphere, the trade winds flow from northeast to southwest at the bottom of the Hadley cell; the westerlies blow from southwest to northeast at the bottom of the Ferrell cell; and the polar easterlies blow from northeast to southwest at the bottom of the polar cell. The winds blow in the opposite directions in the Southern Hemisphere. The winds do not blow due north or south because of the Coriolis effect. It deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. 

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