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The trade winds are a persistent band of easterly winds that blow toward the equator in both hemispheres, covering most of the earth between 25 degrees N and 25 degrees S latitude. These winds originate on the equatorial sides of subtropical high-pressure systems that exist over the tropical and subtropical oceans and represent a major component of the general circulation of the atmosphere. The high-pressure areas force air to move toward a belt of low-pressure near the equator called the doldrums. The air converging at the doldrums rises high over the earth, recirculates toward the poles, and sinks back toward the earth’s surface to about 30 degrees latitude, thus completing a cycle. The surface air that flows from the subtropical highs toward the equator is deflected toward the west in both hemispheres because of the earth’s westto-east rotation. This results in the northeast trade winds in the Northern Hemisphere and southeast trade winds in the Southern Hemisphere.
The most reliable winds on earth are unquestionably the trade winds. They are extremely consistent in both direction and speed throughout the year, averaging about 11 to 13 miles per hour (18 to 21 kilometers per hour). These steady winds are called trade winds due to their ability to quickly propel trading ships across the ocean. The trade winds were named by the crews of sailing ships that depended on these winds during ocean navigation. The name trade winds derives from the Old English “trade,” meaning “path” or “track,” because of the regular course of the winds. These winds helped carry Christopher Columbus on his voyage to the New World in 1492. Mariners of the 16th century recognized early that the quickest and most reliable route for their sailing vessels from Europe to America lay in the belt of the northeasterly winds in the tropical North Atlantic Ocean.
The trade winds are best developed on the eastern and equatorial sides of the subtropical high-pressure systems, especially across the Atlantic Ocean. The trade winds are stronger and more consistent over the oceans than over land due to increased friction on the continental surfaces. When the trade winds reach the western edge of an ocean basin, they turn toward the poles and then loop back east to become part of the prevailing westerlies. The trade winds are primarily a surface wind and move north and south about 5 degrees with the seasons.
The trade winds originate as warm, dry winds capable of holding a tremendous amount of moisture. As they blow across the tropical oceans, they evaporate huge quantities of moisture. The trade winds are overlain by warmer and drier air, creating a temperature inversion in which temperatures increase with height. The temperature inversion often limits the vertical development of clouds, producing clear skies that make trade wind islands a popular tourist attraction. As the trade winds blow against mountain ranges, they are forced to rise and cool. This allows the moisture to condense and fall as rain. These conditions create large differences in rainfall due to topographic variations. Low-lying islands usually experience desert-like conditions, while the windward slopes of some islands are among the wettest places in the world.
- Edward Aguado and James Burt, Understanding Weather and Climate (Prentice Hall 2004);
- Frederick Lutgens and Edward J. Tarbuck, The Atmosphere (Prentice Hall, 2004);
- Tom L. McKnight and Darrel Hess, Physical Geography: A Landscape Appreciation (Prentice Hall, 2005);
- Brian J. Skinner and Stephen C. Porter, The Blue Planet: An Introduction to Earth System Science (John Wiley and Sons, 1994).