Trade Winds, winds that blow westward and toward the earth’s equator in both the northern and southern hemispheres. The trade winds occur between approximately latitude 30° north and the equator and latitude 30° south and the equator. The name trade winds comes from an old sailing term that meant the winds could be counted on to blow steadily from the same direction at a constant speed. The trade winds are less regular over land areas than they are over the oceans.

The trade winds are a major part of the general circulation pattern of the earth’s atmosphere. They blow from a high-pressure belt near 30° latitude in each hemisphere, called a subtropical high, toward a low-pressure belt near the equator, called the equatorial low. In the equatorial low the air rises and travels aloft to the subtropical highs, where it then sinks. Because of the apparent deflecting force of the earth’s rotation, known as the Coriolis force, the trade winds do not blow due south or due north toward the equator. Instead, they blow from the northeast in the northern hemisphere and from the southeast in the southern hemisphere. The entire trade wind belt system moves about 5° of latitude northward during summer (July) and southward during winter (December). See also Meteorology: Energy Flow and Global Circulation; Wind: The Prevailing Winds.

Belts of calm air surround the trade wind belts. The belt of calm air in the equatorial low was called the doldrums by early sailors for the monotony of the weather. The belt of calm air in the subtropical highs often becalmed sailing ships. As food supplies dwindled, horses were either thrown overboard or eaten. Consequently, these regions were called the horse latitudes.

When trade winds move over warm tropical waters, they pick up moisture and bring heavy rainfall to the windward-facing slopes of mountainous areas. Parts of the Hawaiian Islands, for example, receive more than 11,680 mm (about 460 in) of rainfall a year.