An anemometer is an instrument which measures wind. They come in two main types, those which measure wind speed and those which measure wind pressure. Many anemometers also show wind direction. Wind speed and direction are part of the data collected by weather stations to predict the weather. This information is also needed by airports and aircraft pilots to safely take off and land. For this reason, there is a wind sock and anemometer beside all helicopter landing pads.
WIND PRESSURE ANEMOMETERS
Plate anemometers consist of a suspended plate which can be deflected by the wind. They are the oldest form of anemometer. Later versions place a spring against the plate to measure the wind force more precisely. They must be faced against the wind to obtain accurate results.
Tube anemometers consist of a U-shaped glass tube, partly filled with liquid, with one open horizontal end which faces into the wind and the other end facing up. The stronger the wind pressure, the further the liquid will rise in the vertical arm of the U. This is very similar to the design of a barometer, and the wind pressure is measured in the same way. These anemometers have no moving parts, and are very low maintenance.
WIND SPEED ANEMOMETERS
Cup anemometers are the simplest kind of anemometer, but they are also the least precise. They consist of a horizontal cross mounted at the center on a vertical shaft, with a hollow hemispherical cup at the end of each arm. A more accurate design uses three differently-sized arms instead. This design is used with a windsock to indicate wind direction.
Propeller anemometers measure wind speed by measuring the speed of a propeller driven by the wind. They must always be faced parallel to the wind to obtain accurate results. For this reason, a wind vane is included as part of the design. In enclosed areas where the wind always comes from the same direction, they are very accurate.
Thermal anemometers derive wind speed indirectly from wind chill. Wind passes over a pipe containing a heated wire made of tungsten or platinum. Its electrical resistance at a fixed temperature is known. Wind speed is derived by measuring the amount of energy needed to maintain that temperature. Because they are very precise, they are also used to measure other types of flow rates, such as in natural gas pipelines.
Laser Doppler anemometers split a laser beam in two, sending one beam outside the anemometer to be scattered and reflected back by air particles. The greater the wind speed, the greater the scatter. Wind speed is derived by comparing the Doppler shift of the reflected beam with that of the inside beam. This results in very accurate measurement of speed, but not direction.
Ultrasonic anemometers use sound waves to measure the path length between transducers. Unlike many designs, they can measure wind speed vectors in 3D. They have no moving parts and are very precise, and are used in automated weather stations.