Wind is air moving across the earth’s surface. Wind may blow so slowly and gently that it can hardly be felt. Or it may blow so fast and hard that it smashes buildings and pushes over large trees. Strong winds can whip up great ocean waves that damage ships and flood land. Wind can blow away soil from farmland so crops cannot grow. Sharp grains of dust carried by wind wear away rock and change the features of land.
Wind is also a part of weather. A hot, moist day may suddenly turn cool if a wind blows from a cool area to the hot area. Clouds with rain and lightning may form where the cool air meets the hot, moist air. Later, another wind may blow the clouds away and allow the sun to warm the land again. Wind can carry a storm great distances.
Winds are named according to the direction from which they blow. For example, an east wind blows from east to west. A north wind blows from north to south.
Causes of wind
Wind is caused by the uneven heating of the atmosphere (the air around the earth) by energy from the sun. The sun heats the surface of the earth unevenly. Air above hot areas expands and rises. Air from cooler areas then flows in to replace the heated air. This process is called circulation. The circulation over the entire earth is the general circulation. The smaller-scale circulations that cause day-to-day wind changes are known as synoptic-scale circulations. Winds that occur only in one place are called local winds.
General circulation
produces average winds that occur over large sections of the earth’s surface. These winds, called prevailing winds, vary with differences in latitude. Near the equator, heated air rises to about 60,000 feet (18,000 meters). Surface air moving in to replace the rising air produces two belts of prevailing winds. These belts lie between the equator and about 30° north and south latitude. The winds there are called trade winds because sailors once relied on them in sailing trading ships.
The trade winds do not blow straight toward the equator. Instead, they blow somewhat from east to west. The westward part of their motion is caused by the spinning of the earth. The earth and the air around it rotate eastward together. Each point on the earth’s surface travels around a complete circle in 24 hours. Points near the equator travel around larger circles than points near 30° north or south latitude, because the earth is larger at the equator. So, the points that are near the equator travel faster.
As air moves toward the equator, it reaches faster-moving points on the earth’s surface. Because these surface points are moving eastward faster than the air, a person standing on the earth feels a wind blowing westward.
There are no prevailing winds near the equator and up to about 700 miles (1,100 kilometers) on either side of it, because the air rises there instead of moving across the earth. This calm belt is called the doldrums. Often the trade winds converge (come together) in a narrow zone which is called the intertropical convergence zone (ITCZ).
Some of the air that rises at the equator returns to the earth’s surface at about 30° north and south latitude. Air moving downward there produces no wind. These areas are called the horse latitudes, possibly because many horses died on sailing ships that were stalled by the lack of wind there.
Two other kinds of prevailing winds result from the general circulation in the atmosphere. The prevailing westerlies blow somewhat from west to east in two belts between latitudes of about 30° and 60° north and south of the equator. These winds result from surface air moving away from the equator and reaching slower-moving points nearer the poles. Prevailing westerlies carry weather eastward across the northern United States and southern Canada. The polar easterlies blow somewhat from east to west in two belts between the poles and about 60° north and south latitude. Surface air moving away from the poles moves westward across faster-moving points nearer the equator.
Synoptic-scale circulations
are air motions around relatively small regions of high and low pressure in the atmosphere. These regions form within the larger general circulation. Air flows toward low-pressure regions called lows or cyclones. Air flows away from high-pressure regions called highs or anticyclones. Viewed from above, the wind moves clockwise around a high and counterclockwise around a low in the Northern Hemisphere. These directions are reversed in the Southern Hemisphere.
Highs and lows generally move with the prevailing winds. As they pass a given spot on the earth, the wind direction changes. For example, a low moving eastward across Chicago produces winds that shift from southeast to northwest.
Local winds
arise only in specific areas on the earth. Local winds that result from the heating of land during summer and the cooling of land during winter are called monsoons. They blow from the ocean during summer and toward the ocean during winter. Monsoons control the climate in Asia, producing wet summers and dry winters. A warm, dry, local wind that blows down the side of a mountain is called a chinook in the western United States and a foehn in Europe. These three local winds, as well as the harmattan and the sirocco, are discussed in articles listed in the Related articles at the end of this article.
Measuring wind
Two features of wind, its speed and its direction, are used in describing and forecasting weather.
Wind speed
is measured with an instrument called an anemometer. Several kinds of anemometers are used today. The most common kind has three or four cups attached to spokes on a rotating shaft. The spokes turn the shaft as the wind blows. The wind speed is indicated by the speed of the spinning shaft.
In the United States, wind speeds are stated in miles per hour or in knots (nautical miles per hour). In many other countries, they are stated in kilometers per hour.
Wind direction
is measured with an instrument called a weather vane. A weather vane has a broad, flat blade attached to a spoke pivoted at one end. Wind blowing on the blade turns the spoke so that the blade lines up in the direction of the wind. The wind direction may be indicated by an arrow fastened to the spoke, or by an electric meter remotely controlled by the vane.
Wind directions are often indicated by using the 360 degrees of a circle. On this circle, north is indicated by 0°. An east wind blows from 90°, a south wind blows from 180°, and a west wind blows from 270°. Winds at various altitudes often differ in speed and direction. For example, smoke from a chimney may be blown northward while, at the same time, clouds higher in the sky are blown eastward.
Winds high above the earth’s surface are measured by sending up helium-filled balloons. A balloon moves with the same speed and in the same direction as the wind. The balloon’s motion is measured by sight or by radar. The balloon’s altitude is sometimes determined by noting the atmospheric pressure, as measured by a barometer attached to the balloon. Cloud motions determined from satellites are also used to estimate winds, especially over the ocean, where few balloons are launched.
The Beaufort wind scale
is a series of numbers, ranging from 0 to 17, that are used to indicate wind speeds. The Beaufort wind scale was devised in 1805 by British Rear Admiral Sir Francis Beaufort. Beaufort defined the numbers in terms of the effect of various winds on sailing vessels. Today, the Beaufort scale is defined in terms of wind speeds measured 10 meters (about 33 feet) above the ground. The scale is sometimes used to estimate wind speeds, but the Beaufort numbers are little used in the United States.
