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 radar, system or technique for detecting the position, movement, and nature of a remote object by means of radio waves reflected from its surface. Although most radar units use microwave frequencies, the principle of radar is not confined to any particular frequency
..... Click the link for more information. 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 tornado, dark, funnel-shaped cloud containing violently rotating air that develops below a heavy cumulonimbus cloud mass and extends toward the earth. The funnel twists about, rises and falls, and where it reaches the earth causes great destruction.
..... Click the link for more information. 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 Beaufort scale, a scale of wind velocity devised (c.1805) by Admiral Sir Francis Beaufort of the British navy. An adaptation of Beaufort's scale is used by the U.S.
..... Click the link for more information. , 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 doldrums (dŏl`drəmz) or equatorial belt of calms,
..... Click the link for more information. , 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 horse latitudes, two belts of latitude where winds are light and the weather is hot and dry. They are located mostly over the oceans, at about 30° lat. in each hemisphere, and have a north-south range of about 5° as they follow the seasonal migration of the
..... Click the link for more information. , regions of descending air; farther poleward, near lat. 60°N and S, are belts of low pressure, where the polar front polar front, zone of transition between polar and tropical air masses . Its average position during the winter is at about 30° lat. and during the summer at about 60° lat.
..... Click the link for more information. 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 Coriolis effect (kôr'ē-ō`lĭs) [for G.-G.
..... Click the link for more information. ), 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 trade winds, movement of air toward the equator, from the NE in the Northern Hemisphere and from the SE in the Southern Hemisphere. The trade winds originate on the equatorial sides of the horse latitudes , which are two belts of high air pressure, one lying between
..... Click the link for more information. ; 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 monsoon (mŏns
n) [Arab.
..... Click the link for more information. winds.
Superimposed upon the general circulation of winds are many lesser disturbances, such as the extratropical cyclone cyclone, atmospheric pressure distribution in which there is a low central pressure relative to the surrounding pressure. The resulting pressure gradient, combined with the Coriolis effect , causes air to circulate about the core of lowest pressure in a
..... Click the link for more information. (the common storm of the temperate latitudes), the tropical cyclone, or hurricane hurricane, tropical cyclone in which winds attain speeds greater than 74 mi (119 km) per hr. Wind speeds reach over 190 mi (289 km) per hr in some hurricanes.
..... Click the link for more information. , 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 chinook, warm, dry air mass that descends the eastern slopes of the U.S. and Canadian Rocky Mts. after having lost moisture by condensation over the western slopes. Chinooks occur mainly in winter.
..... Click the link for more information. ; climate climate, average condition of the atmosphere near the earth's surface over a long period of time, taking into account temperature, precipitation (see rain ), humidity , wind , barometric pressure, and other phenomena.
..... Click the link for more information. ; roaring forties roaring forties, name applied, especially by sailors, to the latitudes between 40°S and 50°S, where the prevailing westerly winds are strong and steady. Unlike the winds in the Northern Hemisphere, those in the roaring forties are not impeded by large land areas.
..... Click the link for more information. ; sandstorm sandstorm, strong dry wind blowing over the desert that raises and carries along clouds of sand or dust often so dense as to obscure the sun and reduce visibility almost to zero; also known as a duststorm.
..... Click the link for more information. ; sirocco sirocco (sərŏk`ō) [Ital., from Arab.
..... Click the link for more information. ; weather weather, state of the atmosphere at a given time and place with regard to temperature, air pressure (see barometer ), wind, humidity, cloudiness, and precipitation.
..... Click the link for more information. .
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).
wind
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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Wind
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