Wind Anemometers – How to Measure Wind Speed Accurately

For a science that is constantly in the lives of everyday folk, wind speed measurement certainly manages to keep out of the public eye. The measuring of wind speed happens to be an important part of a number of everyday technologies. Of course there is meteorology, the measuring of weather phenomena, that wholly depends on the gauging of wind speed; but a surprising number of other everyday specialties depend on wind speed measurements too, chief among them being aviation and marine and navigation, stability management in skyscrapers, environmental sciences and disaster management. Wind measurement is done with a device known as a wind anemometer; though it might be argued that that is a redundancy since anemometer comes from the Greek Anemos = wind.

Any device that measures wind speed is bound to sense the pressure of it too. For this reason, many anemometer designs are successful when used as pressure meters too in addition. A version of anemometer is known to have existed since around 1450. The modern wind anemometer though, has been around for more than a century and a half now; the first successful design was one that used a structure with four arms fanned out, each one with a cup attached that caught the wind and spun the structure. The inventor, Dr. John Robinson, held the impression when he made his invention that any cup anemometer would share the characteristic that it would spin at a third of the speed of the wind blowing past it, no matter what size it was built to be. Researchers took his word at its face for quite a while before it was discovered that the size of design used always affected the results. Researchers who had used the inventor’s figures for their calculations for years had to start over from scratch.

Cup anemometers, these simple devices, are remarkably accurate machines today nevertheless; the best examples can approach a 99% accuracy level, and still be no more expensive than about $1000. But the cup anemometer is still a mechanical technology that is prone to maintenance lubrication issues, friction, mechanical damage and ice formation. There are competing technologies that attempt to eliminate the problems seen in the mechanical design. One of the most popular wind anemometer technologies in use today is the ultrasonic kind. The principle of the ultrasonic design is this: the speed of sound depends on the speed and the direction of the air that it passes through. A headwind slows sound down, and a tail wind speeds it up. An ultrasonic wind anemometer fires high-frequency sound pulses back and forth between two receivers. If the pulse takes more time travelling in one direction than the other, that is a sign that the slower trip had a headwind working against it. The time differential helps calculate the wind speed. You’ll find these in use on tall buildings, on weather buoys and at weather stations.

Another wind anemometer design that is particularly ingenious is the constant-temperature anemometer. A thin wire held between two electrodes is heated up electrically to hold a constant temperature. A sensor measures the amount of current needed to hold the temperature at ambient temperature levels. Any loss of temperature that is faster than would be explained by the ambient temperature levels would have to come from wind speed. This is a particularly accurate method of measurement of wind turbulence. However, like the laser measurement method below, this can be a quite inexpensive device to buy and maintain.

Ultrasonic and constant temperature anemometers may be accurate enough for most purposes; but laser Doppler anemometers offer extremely tight accuracy. A laser anemometer uses two laser beams; one that travels through a sealed and clean pathway, and one that travels through exposed air. The beam that travels through the exposed air encounters dust particles that are borne along at the speed of the wind at the point. The laser bounces off those dust particles, and measures by Doppler shift the speed at which the particle has been traveling. The Doppler shift is compared to what is measured for the beam traveling through the sealed tube and a relative measurement is made.

It would appear from these descriptions that anemometers always need to be large and permanent installations; as it happens though, small and inexpensive handheld versions with digital displays exist for use by field researchers and trainer pilots. The most striking feature of these is the way they recognizably use nothing other than the same mechanics and structures of the professional devices, only miniaturized for handheld use.