Frank Warren, age 8, of Bellingham, Wash., for his question:
WHAT CAUSES WIND?
Wind is air that moves across the earth's surface. Wind is caused by the uneven heating of the atmosphere, or the air around the earth, by energy that comes from the sun.
The sun heats the surface of the earth unevenly. Then air above the hot areas expands and rises. At the same time, air from cooler areas nearby flows in to take the place of the heated air. This process is called circulation.
Two kinds of circulation can produce wind: general circulation extending around the earth and smaller secondary circulation. Winds that occur only in one place are called local winds.
General circulation happens over large sections of the earth's surface. It produces what we call prevailing winds. Near the equator, heated air rises to about 60,000 feet. Surface air moving in to replace the rising air produces two belts of prevailing winds.
The two belts lie between the equator at about 30 degrees north and south latitute. The winds there are called trade winds because sailors once relied on them in sailing trading ships.
Some of the air that rises at the equator returns to the earth's surface at about 30 degrees 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. Also, the name may have come from a Spanish sailing term that called the winds there unpredictable, supposedly like a female horse.
Secondary circulation is the motion of air around relatively small regions of high and low pressure in the atmosphere. Air flows toward low pressure regions called lows. Air flows away from high pressure regions called highs.
Local winds arise only in specific areas on the earth. Winds from the heating of land during summer and cooling of land during winter are called monsoons.
Monsoons blow from the ocean during summer and toward the ocean during winter. These winds 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.
Wind speed is measured with an instrument called an anemometer. Several kinds 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.
Wind speeds are stated in miles per hour or in knots in the United States. Knots is a nautical miles per hour scale. In other countries, however, wind speeds are stated in kilometers per hour.
Wind direction is measured with an instrument called a weather vane.