An increase of temperature with height is known as a temperature inversion. This warm, dry air is known as a superior air mass and normally resides above a maritime tropical (warm and moist) air mass. The subsident (sinking) air is relatively dry because as it descends, the temperature increases, but the moisture content remains constant, which lowers the relative humidity of the air mass. Around 30° in both hemispheres, air begins to descend toward the surface in subtropical high-pressure belts known as subtropical ridges. When located within a monsoon region, this zone of low pressure and wind convergence is also known as the monsoon trough. A low-pressure area of calm, light variable winds near the equator is known as the doldrums, near-equatorial trough, intertropical front, or the Intertropical Convergence Zone. See also: Air mass, Hadley cell, Humidity, Intertropical Convergence Zone, Monsoon, Monsoon trough, Near-equatorial trough, and Prevailing windsĪs part of the Hadley cell, surface air flows toward the equator while the flow aloft is towards the poles. Cause 3D map showing Hadley cells in relationship to trade winds on the surface Between 18, Matthew Fontaine Maury collected enough information to create wind and current charts for the world's oceans. Edmond Halley's map of the trade winds, 1686īy the 18th century, the importance of the trade winds to England's merchant fleet for crossing the Atlantic Ocean had led both the general public and etymologists to identify the name with a later meaning of "trade": "(foreign) commerce". For example, Manila galleons could not sail into the wind at all. During the Age of Sail, the pattern of prevailing winds made various points of the globe easy or difficult to access, and therefore had a direct effect on European empire-building and thus on modern political geography. The captain of a sailing ship seeks a course along which the winds can be expected to blow in the direction of travel. (This is because following the African coast southbound means sailing upwind in the Southern hemisphere.) In the Pacific Ocean, the full wind circulation, which included both the trade wind easterlies and higher-latitude westerlies, was unknown to Europeans until Andres de Urdaneta's voyage in 1565. They also learned that to reach South Africa, they needed to go far out in the ocean, head for Brazil, and around 30°S go east again. They could then turn northeast, to the area around the Azores islands, and finally east to mainland Europe. From West Africa, the Portuguese had to sail away from continental Africa, that is, to west and northwest. The Portuguese recognized the importance of the trade winds (then the volta do mar, meaning in Portuguese "turn of the sea" but also "return from the sea") in navigation in both the north and south Atlantic Ocean as early as the 15th century. The term originally derives from the early fourteenth century sense of trade (in late Middle English) still often meaning "path" or "track". See also: Age of Discovery, Volta do Mar, and Age of sail Its presence negatively impacts air quality by adding to the count of airborne particulates. When dust from the Sahara travels over land, rainfall is suppressed and the sky changes from a blue to a white appearance which leads to an increase in red sunsets. Sahara dust is on occasion present in sunsets across Florida. The trade winds also transport nitrate- and phosphate-rich Saharan dust to all Latin America, the Caribbean Sea, and to parts of southeastern and southwestern North America. The weaker the trade winds become, the more rainfall can be expected in the neighboring landmasses. Shallow cumulus clouds are seen within trade wind regimes and are capped from becoming taller by a trade wind inversion, which is caused by descending air aloft from within the subtropical ridge. In meteorology, they act as the steering flow for tropical storms that form over the Atlantic, Pacific, and southern Indian oceans and make landfall in North America, Southeast Asia, and Madagascar and East Africa. They enabled colonial expansion into the Americas, and trade routes to become established across the Atlantic Ocean and the Pacific Ocean. Trade winds have been used by captains of sailing ships to cross the world's oceans for centuries. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, strengthening during the winter and when the Arctic oscillation is in its warm phase. The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The westerlies (blue arrows) and trade winds (yellow and brown arrows) For other uses, see Tradewind (disambiguation).
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