The sun’s energy is concentrated in equatorial regions. This warms the air in these regions, causing it to rise. This creates a zone of low pressure in areas at or near the equator. In the subtropics, there are areas of high pressure located about 30 degrees north and 30 degrees south. Winds blow from areas of higher pressure to areas of lower pressure. Therefore, winds blow from subtropical areas of high pressure toward the area of low pressure near the equator. These winds are known as the trade winds.
The trade winds do not blow directly toward the equator. Because the Earth is rotating, they are deflected. In the northern hemisphere, they blow from the northeast and are called the northeast trade winds. In the southern hemisphere, they blow from the southeast and are called the southeast trade winds. The trade winds got their name because during the days of sailing ships, they were very important for world trade.
Near the equator atmospheric pressure is low and air rises. High level winds called the antitrades blow from equatorial regions toward the subtropical areas of high pressure. In these areas of high pressure, air from high up in the atmosphere sinks toward the ground. The trade winds blow from subtropical areas of high pressure toward the equator. Thus, there is a continuous circulation of air between areas near the equator and subtropical areas of high pressure (see diagram below). This circulation is known as the Hadley Cell and it helps to regulate the temperature of the Earth.
Diagram showing the movement of the trade winds and the circulation of the Hadley Cell.
Related Pages
Inter-Tropical Convergence Zone
References
http://www.infoplease.com/encyclopedia/weather/trade-winds.html
http://geography.about.com/od/physicalgeography/a/tradewinds.htm