Definitions

wind

wind

[n. wind, Literary wahynd; v. wind]
wind, flow of air relative to the earth's surface. A wind is named according to the point of the compass from which it blows, e.g., a wind blowing from the north is a north wind.

Wind Direction and Velocity

The direction of wind is usually indicated by a thin strip of wood, metal, or plastic (often in the shape of an arrow or a rooster) called a weather vane or weathercock (but more appropriately called a wind vane) that is free to rotate in a horizontal plane. When mounted on an elevated shaft or spire, the vane rotates under the influence of the wind such that its center of pressure rotates to leeward and the vane points into the wind.

Wind velocity is measured by means of an anemometer or radar. The oldest of these is the cup anemometer, an instrument with three or four small hollow metal hemispheres set so that they catch the wind and revolve about a vertical rod; an electrical device records the revolutions of the cups and thus the wind velocity. The pressure tube anemometer, used primarily in Commonwealth nations, is conceptually a Pitot tube mounted on a wind vane. As the wind blows across the tube, a pressure differential is created that can be mathematically related to wind speed. Doppler radar can be used to measure wind speed by shooting pulses of microwaves that are reflected off rain, dust, and other particles in the air, much like the radar guns used by the police to determine the speed of an automobile. Although the U.S. National Weather Service has estimated that tornado winds have reached a velocity of 500 mph (800 kph), the highest wind speeds ever documented, 318 mph (516 kph), were measured using Doppler radar during a tornado in Oklahoma in 1999.

The first successful attempt to standardize the nomenclature of winds of different velocities was the Beaufort scale, devised (c.1805) by Admiral Sir Francis Beaufort of the British navy. An adaptation of Beaufort's scale is used by the U.S. National Weather Service; it employs a scale ranging from 0 for calm to 12 for hurricane, each velocity range being identified by its effects on such things as trees, signs, and houses. Winds may also be classified according to their origin and movement, such as heliotropic winds, which include land and sea breezes, and cyclonic winds, which blow counterclockwise in low-pressure regions of the Northern Hemisphere and clockwise in the Southern Hemisphere.

Prevailing Winds and General Circulation Patterns

Over some zones around the earth, winds blow predominantly in one direction throughout the year and are usually associated with the rotation of the earth; over other areas, the prevailing direction changes with the seasons; winds over most areas also are variable from day to day so that no prevailing direction is evident, such as, for example, the day-to-day changes in local winds associated with storms or clearing skies. Around the equator there is a belt of relatively low pressure known as the doldrums, where the heated air is expanding and rising; at about lat. 30°N and S there are belts of high pressure known as the horse latitudes, regions of descending air; farther poleward, near lat. 60°N and S, are belts of low pressure, where the polar front is located and cyclonic activity is at a maximum; finally there are the polar caps of high pressure.

The prevailing wind systems of the earth blow from the several belts of high pressure toward adjacent low-pressure belts. Because of the earth's rotation (see Coriolis effect), the winds do not blow directly northward or southward to the area of lower pressure, but are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The wind systems comprise the trade winds; the prevailing westerlies, moving outward from the poleward sides of the horse-latitude belts toward the 60° latitude belts of low pressure (from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere); and the polar easterlies, blowing outward from the polar caps of high pressure and toward the 60° latitude belts of low pressure.

This zonal pattern of winds is displaced northward and southward seasonally because of the inclination of the earth on its axis and the consequent migration of the belts of temperature and pressure. In addition, the pattern is considerably modified by the distribution of land and water, especially in the temperate regions, where temperature differences between land and water are greatest. In winter, areas of high pressure tend to build up over cold continental land masses, while low-pressure development takes place over the adjacent, relatively warm oceans. Exactly the opposite conditions occur during summer, although to a lesser degree. These contrasting pressures over land and water areas are the cause of monsoon winds.

Superimposed upon the general circulation of winds are many lesser disturbances, such as the extratropical cyclone (the common storm of the temperate latitudes), the tropical cyclone, or hurricane, and the tornado; each of these storms moves generally along a path that follows the direction of the prevailing winds but within itself maintains a circulatory wind pattern.

See also chinook; climate; roaring forties; sandstorm; sirocco; weather.

Localized Influences on Wind Patterns

The diurnal, or daily, heating and cooling of land near a lake or ocean of fairly constant temperature causes air to blow toward the relatively warmer land during the day (sea breeze) and toward the relatively warmer water at night (land breeze). These breezes are shallow and seldom penetrate far inland or attain high velocity. Similar diurnal changes occur on mountain slopes, the air in the valley becoming heated and expanding so that it moves up the slope in the daytime, the cold air settling into the valley at night. Friction with the earth's surface, eddies caused by surface irregularities, and inequalities of heating with consequent convection currents tend to reduce wind velocity near the earth's surface and cause winds to blow in gusts.

Bibliography

See A. Watts, Instant Wind Forecasting (1988); P. Gipe, Wind Energy Comes of Age (1995); J. DeBlieu, Wind: How the Flow of Air Has Shaped Life, Myth, and the Land (1999).

Device for producing a controlled stream of air to study the effects on objects such as aircraft moving through air or the effects of moving air on models of stationary objects such as buildings. Applications of wind-tunnel research range from testing of airframes (the structures of aircraft and spacecraft) to research on the boundary layer, turbulence, drag, and lift. Measurements of air pressure and other characteristics at many points on the model yield information about how the total wind load is distributed. In addition to testing the effects of wind on aircraft and spacecraft, studies in wind tunnels have been used to solve design problems in automobiles, boats, trains, bridges, and buildings. Seealso aerodynamics.

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Sport of riding a sailboard, a modified surfboard with a movable mast. Steered from a standing position, sailboards are capable of moderately high speeds and are usually used on lakes, or close to shore—sometimes within the surf zone—on the ocean. The sport originated in the U.S. in the late 1960s and quickly grew in popularity. It was introduced at the Olympic Games in 1984.

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Rate of change of wind velocity with distance perpendicular to the wind direction. A very narrow zone of abrupt velocity change is known as a shear line. Wind shear is observed near the ground and in jet streams, where it may be associated with clear-air turbulence. Vertical wind shear is closely associated with the vertical flux of momentum, heat, and water vapour.

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Matching a specific air temperature (columns) with a wind speed (rows) will show the wind chill elipsisEncyclopædia Britannica, Inc.Matching a specific air temperature (columns) with a wind speed (rows) will show the wind chill elipsis

Matching a specific air temperature (columns) with a wind speed (rows) will show the wind chill elipsisEncyclopædia Britannica, Inc.Matching a specific air temperature (columns) with a wind speed (rows) will show the wind chill elipsis

Still-air temperature that would have the same cooling effect on exposed skin as a given combination of temperature and wind speed. As the wind speed increases, the wind chill equivalent temperature decreases; e.g., an air temperature of 30 °F (–1.1 °C) with a wind speed of 20 mph (32.2 kph) produces a wind chill of 17 °F (–8 °C). Wind chill is often included in weather reports to describe how cold it feels.

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Movement of air relative to the surface of the Earth. Wind is an important factor in determining and controlling climate and weather. It is also the generating force of most ocean and freshwater waves. Wind occurs because of horizontal and vertical differences in atmospheric pressure. The general pattern of winds over the Earth is known as the general circulation, and specific winds are named for the direction from which they originate (e.g., a wind blowing from west to east is a westerly). Wind speeds are often classified according to the Beaufort scale.

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Flux of particles, chiefly protons, electrons, and helium nuclei, accelerated by the hot solar corona's high temperatures to speeds high enough to allow them to escape the Sun. Solar flares increase its intensity. The solar wind deflects planets' magnetospheres and the ion tails of comets away from the Sun. The uninterrupted portion of the solar wind continues to travel to a distance of about 110–170 astronomical units, where it cools and eventually diffuses into interstellar space. Seealso heliopause.

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Mountain range, central Rocky Mountains, west-central Wyoming, U.S. The range extends for 100 mi (160 km) northwest-southeast to the Sweetwater River and is part of the Continental Divide. It contains many peaks above 12,000 ft (3,658 m); the highest is Gannett Peak at 13,804 ft (4,207 m). The Oregon Trail ran through the historic South Pass (7,743 ft, or 2,360 m). Parts of Bridger and Shoshone national forests and Wind River Indian Reservation are in the range. The Wind River flows from the eastern side into the Bighorn River; the Green River rises on its western slopes.

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National park, southwestern South Dakota, U.S. Established in 1903 to preserve limestone caverns and unspoiled prairie grassland in the Black Hills, it covers an area of 28,292 ac (11,449 ha). Its caves contain 83 mi (134 km) of explored passages and have beautiful rock formations called boxwork, formed by calcite deposition in unique patterns. The park is also a wildlife refuge.

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Wind, Sand and Stars (French title:Terre des hommes (Land of Men)) is a memoir by Antoine de Saint Exupéry published in 1939. The pilot and philosopher recounts several episodes from his years flying treacherous mail routes across the Sahara and the Andes. The central incident details the 1935 plane crash he survived in the Libyan Sahara Desert, between Benghazi and Cairo. Saint Exupéry, and his navigator André Prévot, are left almost without water and food, as chances of finding an oasis or help from the air gradually decrease. The book illustrates the author's view of the world and his opinions of what makes life worth living.

The charity Terre des hommes was named after this book.

Expo 67, the 1967 world's fair held in Montreal, used "Terre des hommes / Man and His World" as its theme, based on the philosophies expressed in Saint Exupéry's book.

Awards

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