An anemometer measures ________ speed.

Correct answer: Wind.

An anemometer is a meteorological instrument used to measure wind speed. (Wind direction is measured separately by a wind vane, also called a weather vane.) The most common type, the cup anemometer, has hemispherical cups mounted on horizontal arms that rotate when the wind blows; the rate of rotation is proportional to the wind speed. It is widely used in weather stations, airports, and wind-energy installations.

Why the other options are wrong: the speed of a flowing water current is measured with a current meter (an ammeter only measures the amount of electric current, in amperes, not a speed); the intensity of an earthquake is recorded by a seismograph (Richter scale); and a cyclone is a large-scale weather system whose track and intensity are monitored using satellites and radar, not a single quantity read off an anemometer.