The speed of wind is usually measured using an anemometer. An anemometer is a device that is used to measure the speed of wind. It works by rotating in response to the wind, and the speed of rotation is proportional to the speed of the wind. The most common type of anemometer used today is the cup anemometer, which has three or four cups mounted on horizontal arms that spin when exposed to the wind. The number of rotations per minute is measured, and this is used to calculate the wind speed. Other types of anemometers include the vane anemometer, which uses a propeller and wind vane to measure both wind speed and direction, and the hot-wire anemometer, which uses a heated wire to measure the rate of cooling caused by the wind. However, the most commonly used anemometer is the cup anemometer. Therefore, to measure the speed of wind, an anemometer is used.