Air above hot areas expands and rises. Air from cooler areas then flows in to replace the heated air. This process is called circulation. The circulation over the entire earth is the general circulation. Winds that occur only in one place are called local winds.
General circulation produces average winds that occur over large sections of the earth's surface.
These winds, called prevailing winds, vary with differences in latitude. Surface air moving in to replace the rising air produces two belts of prevailing winds. These belts lie between the equator and about 30 north and south latitude. The winds there are called trade winds because sailors once relied on them in sailing trading ships. The trade winds do not blow straight toward the equator.