Anemometer, an instrument for measuring wind speed. Anemometers are used not only in weather recording and forecasting, but also in mines, tunnels, and ventilation systems; in aircraft testing and other experimental work; and in aerial navigation.

The cup anemometer is the most widely used type. It consists of several metal cups (usually three) attached to the ends of horizontal arms mounted on a vertical shaft. Wind catching in the cups causes them to revolve. This action turns the shaft, which is connected to a device that gives the wind speed in miles per hour, kilometers per hour, or knots. In one common type of cup anemometer, the shaft is connected to an electrical generator. The amount of current produced by the generator varies with the speed of the wind.

In the propeller anemometer, the wind turns a propeller instead of cups. Small propeller anemometers are used to check the flow of air in ventilating shafts. A propeller anemometer that is used where the direction of the wind changes, as it does out-of-doors, must be connected to a weather vane. The propeller blades indicate the correct wind velocity only when they face the flow of the wind. The vane, which also shows the direction of the wind, keeps the propeller properly aligned.

The hot-wire anemometer consists of an electrically heated wire exposed to the wind. The speed of the wind affects the rate at which the wire loses heat. Typically, the wind speed is determined by measuring the electrical current necessary to maintain the wire at a constant temperature. The hot-wire anemometer is used mainly in experimental work.