The resulting band of cloudy and rainy weather near the equator is called the doldrums. Westerlies blow from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere. Westerlies steer storms from west to east across middle latitudes. Westerlies and trade winds blow away from the 30 degrees latitude belt. Over broad regions centered at 30 degrees latitude, surface winds are light or calm.
Air slowly descends to replace the air that blows away.
Descending air warms and is dry. The tropical deserts, such as the Sahara of Africa and the Sonoran of Mexico, occur under these regions of descending air. Polar winds blow from the northeast in the Arctic and from the southeast in the Antarctic. In the Northern Hemisphere, the boundary between the cold polar easterly winds and the mild westerly winds is known as the polar front. A front is a narrow zone of transition, usually between a mass of cold air and a mass of warm air.