wind vane: see weather vane.

wind tunnel, apparatus for studying the interaction between a solid body and an airstream. A wind tunnel simulates the conditions of an aircraft in flight by causing a high-speed stream of air to flow past a model of the aircraft (or part of an aircraft) being tested. The model is mounted on wires so that lift and drag forces on it can be measured by measuring the tensions in the wire. The paths of the airstream around the model can also be studied by attaching tufts of wool (which align themselves with the wind direction) to various parts of the model, by injecting thin streams of smoke into the tunnel to render the airflow visible, or by using certain optical devices. Pressures on the model surface are measured through small flush openings in its surface. Forces exerted on the model may be determined from measurement of the airflow upstream and downstream of the model. In wind tunnels operating well below the speed of sound, the airstream is created by large motor-driven vanes. At velocities near or above the speed of sound, the airstream is created either by releasing highly compressed air from a tank at the upwind end of the tunnel or by allowing air to rush through the tunnel into a previously evacuated vacuum tank at its downwind end. Sometimes these methods are combined, especially for the production of hypersonic velocities, i.e., velocities at least five times as great as the speed of sound. The effect of wind on other vehicles, e.g., automobiles, and on stationary objects such as buildings and bridges may also be studied in wind tunnels. In many instances, wind tunnels have been rendered obsolete by computer modeling.

wind shear, a sudden, drastic change in wind direction or speed over a comparatively short distance. Most winds travel horizontally, as does most wind shear, but under certain conditions, including thunderstorms and strong frontal systems, wind shear will travel in a vertical direction. Microburst wind shear is an extremely violent downward blast of air that hits the earth and radiates outward. With its sharp shifts in wind direction and relative wind speed, it can cause an aircraft to lose lift and crash, especially during takeoff or landing, when the slower speeds and closeness to the ground make altitude correction more difficult. Since 1996 all U.S. airliners have been required to be equipped with instruments that provide the pilot with advance warning of wind shear. See also weather and wind.

wind instrument, in music, any instrument whose tone is produced by a vibrating column of air. In the pipe organ the column of air is set into vibration by mechanical means. Other wind instruments are blown by the player and are divided into two groups, the woodwinds and the brass winds, or brasses. The woodwinds include the flute family, played without a reed, the clarinet family, having single-reed mouthpieces, and the oboe family, having double-reed mouthpieces (see reed instrument). The brass winds include the bugle, cornet, ophicleide, trombone, trumpet, and tuba, all having cup-shaped mouthpieces, and the French horn, having a funnel-shaped mouthpiece. In the brasses the lips of the player perform the function of reeds. The wind passage of a wind instrument is called the bore and may be conical or cylindrical; its flared edge is called the bell. Woodwind and brass instruments are now best distinguished according to their mouthpieces, since metal flutes and saxophones remain woodwinds regardless of the material used to make them.

wind chill, the cooling effect of wind and temperature combined, expressed in terms of the effect produced by a lower, windless temperature, also called wind chill factor, wind chill temperature, wind chill equivalent temperature, wind chill index, wind chill equivalent index, and wind chill temperature index. Wind chill is based on the rate of heat loss from exposed skin. Under windless conditions air provides an invisible blanket around the skin. As wind speed increases, this layer of heated air is carried away from the body at an accelerated rate, forcing the body either to work harder to generate more heat or cool down. If the actual air temperature is -5°F;(-21°C;) with a 20 mph (32 km/hr) wind, the wind chill temperature is -29°F;(-34°C;). Because wind chill is based the removal of heat from the human body, it does not reflect the increased rate of heat loss for inanimate objects such as automobile radiators under the same conditions but they also experience a faster heat loss with increasing winds.

The term wind chill was coined by the American geographer Paul A. Siple in his dissertation, Adaptation of the Explorer to the Climate of Antarctica, (1939). Subsequently, on the third Byrd Antarctic expedition, Siple and American geologist Charles Passel determined how quickly extreme conditions could produce frostbite on exposed skin. By 1945, Siple and Passel had published a set of numbers expressing heat loss as a function of temperature and wind speed.

A wind chill advisory is issued when the forecast projects a wind velocity of at least 10 mph (16 km/hr) producing a wind chill temperature of -15°F; or lower for 3 hours or more. At these values wind chill is more of a nuisance than it is life threatening. A wind chill warning is issued when the forecasted wind chill temperature is -25°F; or lower, which can be life threatening if the individual is not suitably dressed. Persons who go outside under such conditions may experience frostbite and other cold-related symptoms in a matter of minutes, even if properly clothed for normal winter conditions, and longer exposures may prove fatal.

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.

solar wind, stream of ionized hydrogen—protons and electrons—with an 8% component of helium ions and trace amounts of heavier ions that radiates outward from the sun at high speeds. The continuous expansion of the solar corona into the surrounding vacuum of space carries away from the sun about 1 million tons of gas per sec; this blows out like a wind through the solar system. During the days of quiet sunspot activity the wind at the sun has an approximate density of 1 billion atoms per cc and a temperature of about 1 million degrees Fahrenheit. During relatively quiet periods, the wind moves outward from the sun at velocities of 220 to 440 mi (350 to 700 km) per sec (averaging about 1 million mph/1.6 million kph). Near the earth it has a density ranging from 3 to 6 atoms per cc, a velocity of 450 mi (700 km) per sec, and a temperature of about 1,300°F; (700°C;); during periods of greater sunspot activity it shows corresponding increases in density, temperature, and velocity—reaching speeds of 2 million mph (3.2 million kph). The increased velocity is attributed to acceleration caused by magnetic waves spiraling from the sun. The wind is believed to extend out to between 100 and 200 AU (1 AU is the mean distance between the earth and the sun), far beyond Pluto (at 39 AU), where it is dispersed in the interstellar gases.

Many effects result from the solar wind. The characteristic that a comet tail always points away from the sun is explained by the pressure of the wind pushing it out. The intensity of the cosmic rays in the inner part of the solar system is reduced by the magnetic fields carried on the wind, which tend to deflect the rays, thus providing a shield against that radiation. The interaction of the wind with the earth's magnetic field is responsible in part for such phenomena as auroras and geomagnetic storms.

brass wind instrument: see wind instrument.

Wind River Range, part of the Rocky Mts., W Wyo., running southeastward c.120 mi (190 km) and constituting part of the Continental Divide. Gannett Peak (13,804 ft/4,207 m) is the highest point in Wyoming. A number of historic passes cross these mountains. South Pass (alt. c.7,550 ft/2,301 m), at the southern end of the range, was the most important pass on the Oregon Trail through the Rocky Mts.

Wind Cave National Park, 28,295 acres (11,459 hectares), in the Black Hills, SW S.Dak.; est. 1903. Wind Cave, discovered in 1881, was named for the strong air currents that blow alternately in and out of it depending on whether the atmospheric pressure is higher or lower than the air pressure inside the cave. There are 10.5 mi (16.9 km) of explored passageways. The cave's temperature remains constant at 47°F; (8°C;). Unusual calcite-crystal and boxwork formations make Wind Cave unique. The park's surface is characterized by rolling grasslands where bison, elk, deer, and pronghorn antelope can be found; several prairie dog towns are in the park. See National Parks and Monuments (table).

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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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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