Wind

I

Introduction

Wind, air in motion. The term is usually applied to the natural horizontal motion of the atmosphere; motion in a vertical, or nearly vertical, direction is called a current. Winds are produced by differences in atmospheric pressure, which are primarily attributable to differences in temperature. Variations in the distribution of pressure and temperature are caused largely by unequal distribution of heat from the sun, together with differences in the thermal properties of land and ocean surfaces (see Meteorology). When the temperatures of adjacent regions become unequal, the warmer air tends to rise and flow over the colder, heavier air. Winds initiated in this way are usually greatly modified by the earth's rotation.

Winds may be classified into four major types: the prevailing winds, the seasonal winds, the local winds, and the cyclonic and anticyclonic winds (see Cyclone; Hurricane; Tornado).

II

The Prevailing Winds

Near the equator is a low-pressure belt, known as the doldrums, that lies roughly between latitudes 10° South and 10° North. Within this belt, sometimes called the equatorial belt of calms, the air is hot and sultry. At about 30° from the equator in both hemispheres are the horse latitudes, which are high-pressure belts of calms, or light variable winds. Surface air, moving from the horse latitudes toward the low-pressure equatorial belt, constitutes the trade winds, which are the prevailing winds of the lower latitudes. In the northern hemisphere, the northerly wind blowing toward the equator is deflected by the rotation of the earth to become northeasterly and is known as the northeast trade wind. In the southern hemisphere, the southerly wind, which is similarly deflected, becomes southeasterly and is known as the southeast trade wind.

On the polar side of the horse latitudes in either hemisphere, the atmospheric pressure diminishes toward low-pressure centers in middle and high latitudes. The winds set in motion poleward by these pressure systems are deflected toward the east by the earth's rotation. Because winds are known by the direction from which they blow, the winds in middle latitudes are known as the prevailing westerlies. These westerlies are greatly affected by traveling cyclonic and anticyclonic disturbances that cause their actual direction to change greatly from day to day.

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The colder regions at the poles tend to be high-pressure centers, particularly in the southern hemisphere, and winds spreading out from these areas are deflected to become the polar easterlies.

The strongest wind ever reliably measured on the surface of the earth was 362 km/h (225 mph), recorded on Mount Washington, New Hampshire, on April 12, 1934. Considerably stronger winds, however, occur near the centers of tornadoes.

With increasing elevation above the surface of the earth, the prevailing westerlies increase in speed, and their range of latitudes expands toward the equator and the pole. The trade winds and polar easterlies are thus relatively shallow, and are generally replaced by westerlies above a few thousand meters. The strongest westerlies occur at heights of about 10 to 20 km (about 6 to 12 mi), and tend to be concentrated in a rather narrow belt called the jet stream, where wind speeds as high as 550 km/h (about 340 mph) have been measured.

III

The Seasonal Winds

The air over the land is warmer in summer and colder in winter than the air over the adjacent ocean during the same seasons. During the summer, the continents thus become seats of low pressure, with winds blowing in from the colder oceans. In the winter, the continents are seats of high pressure, with winds directed toward the warmer oceans. These seasonal winds are typified by the monsoons of the China Sea and the Indian Ocean. See Monsoon.

IV

The Local Winds

Corresponding with the seasonal variations in temperature and pressure over land and water, diurnal changes occur, which exercise a similar but more local effect. Especially in the summer, the land is warmer than the sea by day and colder than the sea by night. The variations of pressure thus induced evoke a system of breezes directed landward during the daytime and seaward at night. These land and sea breezes penetrate to a distance of about 50 km (about 30 mi) on and off the shore.

Similar daily changes in temperature occur over irregular terrain and cause mountain and valley breezes. Other winds induced by local phenomena include whirlwinds and winds associated with thunderstorms.