Introduction to Wind

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Wind, the movement of air over the earth's surface. The upward movement of air is known as an updraft; movement downward, as a downdraft. Wind is an important element of weather.

A wind is named for the direction from which it blows. Thus a wind blowing from southeast to northwest is a southeast wind. The direction from which wind blows is called windward. The direction toward which it blows is leeward. An object is upwind from a person when it is between the person and the source of the wind. The person is downwind from the object. Wind direction is important to weather observers, sailors, aviators, artillerymen, and hunters. It is shown by a weather vane.

Wind speed can vary greatly. Near the earth's surface, it is measured by an instrument called an anemometer. In the upper atmosphere, wind speed is determined by tracking a balloon or a balloon-borne object, usually by radar. Wind speed is measured in knots (nautical miles per hour), statute (land) miles per hour, or in meters or feet per second.

The Beaufort scale, based on a system devised by Admiral Sir Francis Beaufort in 1806, assigns numbers from 0 to 12 to wind speed—the higher the number the greater the speed. The Beaufort scale was widely used until 1949, when what is now the World Meteorological Organization adopted the knot as the international unit for measuring wind speed. The knot was adopted for use on all official weather charts in the United States in 1955.

What Causes Winds

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Wind is caused by differences in atmospheric pressure created, in large part, by the unequal heating of the earth's surface by the sun. Air moves from a region of higher pressure to one of lower pressure and this movement is wind. Any difference in pressure will cause wind, but the greater the difference the stronger the wind.

The direction that wind takes is influenced by the rotation of the earth. On a nonrotating earth wind would move in a straight path from a high- to a low-pressure area. It is deflected from this path—to the right in the Northern Hemisphere and to the left in the Southern—by the turning of the earth on its axis.

Local winds—those that affect a comparatively small area—are often caused by heat transferred by convection. Direct radiation from the sun does little to heat the air. It is warmed chiefly by heat radiated from the earth. Intense local heating of the land causes air directly above to become greatly heated and to expand. As a result, some of the air aloft flows away, lowering the pressure over the heated area and increasing the pressure around it. The cooler, heavier air near the earth then flows to the heated area.

In mountainous areas, winds tend to blow uphill during the day because the mountainside is heated more than the valley below it. At night, when the mountainside cools, the wind blows downhill. In summer, breezes tend to blow from oceans or large lakes to the warmer land during the day. They blow from the land at night, when the land cools.

The Primary Wind Belts

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The air moves not only as local winds, but also in a general circulatory system over the entire earth. This planetary circulation operates at two levels—at the earth's surface and in the upper portion of the troposphere, the layer of the atmosphere nearest to the earth.

Planetary circulation, like local wind, is caused by pressure differences. Convectional circulation is, in part, a cause of these differences, but there are other causes as well. Among these are the effects of one part of the circulatory system on the other parts.

The circulatory system can be divided into five general zones, called primary wind belts. The zones are not well defined, but are broken and constantly shift and migrate with the sun.

The Doldrums lie approximately between 5 north and south of the equator, the area receiving the largest amount of heat from the sun. Like other wind belts, the doldrums shift with the sun, reaching as far as 17 or 18 north latitude in July and far into the Southern Hemisphere in January. Air pressure is generally relatively low, and the air moves chiefly upward from the warm earth. At a high elevation, the air stops rising and moves north and south toward the polar areas. Weather in the doldrums is usually hot, humid, and rainy. Surface winds are light and variable.

The Trade Winds extend from the doldrums to as much as 30 north and south. These winds are created by surface air moving from high-pressure areas in the subtropics to replace air rising in the doldrums. Turned westward by the rotation of the earth, the trades blow with great steadiness from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. They are steadiest over oceans. (Trade, in this case, has the old meaning of "steady track or course.")

The Horse Latitudes, at about 30 north and south latitudes, form a belt between the trade winds and the prevailing westerlies. Here part of the high-altitude air flowing poleward accumulates, descends to earth, and becomes warmer because of compression. The barometer shows the air pressure to be high. Some of the world's largest deserts are located here.

Horse-latitude winds are weak and shifting. Calms are common. The weather is usually clear and bright. Part of the descending air feeds the trade winds; part flows to the prevailing westerlies.

The Prevailing Westerlies extend from the horse latitudes to about 60 north and south. Although the wind is highly variable in direction because of traveling high-pressure areas (anticyclones) and low-pressure areas (cyclones), the wind comes from the west more often than from other directions. Air flowing toward the poles from the horse latitudes is here deflected eastward to become a west wind. In the middle latitudes of the Southern Hemisphere, where there are few land areas to interfere with the westerlies, wind speeds are greatest. The region from 40 to 50 south is notorious among seamen as "the roaring forties."

The Polar Easterlies extend from about 60 north and south latitude to the poles. Near the poles the shrinkage of air due to cooling favors the inflow of air aloft from warmer latitudes. This causes a polar high-pressure area from which winds move toward the equator to the subpolar low-pressure areas, located at about 60 north and south. These winds are deflected by the earth's rotation to become east winds, or the polar easterlies.

The Jet Streams

Jet streams are areas of high-speed winds that occur at about 25,000 to 50,000 feet (7,600 to 15,200 m) above the earth. Several distinct jet streams are known, and all seem to be associated with weather and climate at the earth's surface. The course, speed, and altitude of each stream varies from day to day. Jet streams are normally 100 miles (160 km) or more wide and sometimes extend completely around the earth. Wind speed is highest at the core of each stream, where it may reach 150 to 300 miles per hour (240 to 480 kmh).

Commercial and military airplanes sometimes use jet streams as powerful tailwinds in order to achieve higher than normal airspeeds or to conserve fuel.

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