An anemometer is a meteorological device that is used to measure wind velocity. Anemometers come in a variety of forms, but the simplest type is the cup anemometer. The cup anemometer consists of a vertical pole with four horizontal arms attached to the top. Cups are attached to the ends of the four arms, and the wind causes the cups to rotate the arms around the central pole. The cup anemometer is prone to friction, which makes it less accurate than the more advanced versions.
Cup anemometers, however, are sufficient for personal use. A person using one can calculate the wind speed by multiplying the cups' revolutions per minute by the circumference of the circle the cups create. The product will be a close estimate of the wind's speed per minute.
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The windmill anemometer is similar to its namesake, the windmill, in that it must be parallel to the direction of the wind in order to work properly. A wind vane, attached to the tail of the anemometer, is pushed around until the propeller is facing into the wind. The wind then turns the propeller, which rotates a mechanism that calculates the wind speed.
A thermoelectric anemometer uses a very thin wire which is heated to a temperature that is higher than the temperature of the surrounding air. The wind cools the wire, and electronics within the body of the instrument calculate the wind speed based on the electrical resistance of the wire. Anemometers of this type are rather delicate instruments, but are also quite accurate at determining wind speed.
Laser Doppler and ultrasonic anemometers are high-tech and very accurate. Laser Doppler anemometers measure wind speed by detecting how much of the light from a laser beam has been reflected off of the moving air particles. Ultrasonic anemometers determine wind speed by sending sound waves between a pair of transducers and determining how their speed is affected. Because ultrasonic anemometers are capable of measuring both wind speed and wind direction, they are popular for use on ships, airplanes, meteorologists' weather stations, and scientific wind turbines.