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.