Causes of wind. Measuring wind

Wind is air moving across 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 a part of weather. A hot, humid 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 ea5fwind 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 Earth) by energy from the sun. The sun heats the surface of 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 planet 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 of air around Earth. Prevailing winds result from the general circulation of air around Earth. In this drawing, the circulation has been greatly simplified. At the equator, air is heated by the sun and rises, as shown by the blue arrows. In the upper atmosphere, this air flows away from the equator. When the air returns to Earth's surface, it flows across the surface, as shown by the black arrows.

This moving surface air produces the six belts of prevailing winds around Earth. The turning of Earth causes the winds to blow toward the east in belts where the air moves away from the equator. In belts where the air moves toward the equator, the prevailing winds blow toward the west

General circulation produces average winds that occur over large sections of 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 Earth. Earth and the air around it rotate eastward together. Each point on 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 Earth is larger at the equator. So, the points near the equator travel faster.

As air moves toward the equator, it reaches faster-moving points on Earth’s surface. Since these surface points are moving eastward faster than the air, a person standing on 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 Earth. This calm belt is called the doldrums. Often the trade winds converge(come together) in a narrow zone called the intertropical convergence zone (ITCZ).

Some of the air that rises at the equator returns to 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. Airflows 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 Earth, the wind direction changes. For example, a low moving eastward across Chicago produces winds that shift from south-east to northwest.

Local winds arise only in specific areas on 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 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 to 17, that are used to indicate wind speeds. The Beaufort wind scale was devised in 1805 by British Rear Admiral Sir Francis Beaufort. He 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.