Anemometer

An anemometer (from the Greek word "anemos" meaning wind), is a device for measuring either the velocity or the pressure of the wind, and is one of the instruments found in a weather station.

Notes on wind measurements

The connection between the velocity and the pressure of the wind is one that is not yet known with absolute certainty. Many text-books on engineering give the relation P = 0.005 v^2 when P is the pressure in pounds force per square foot and v the velocity in miles per hour (equivalent to P = 0.048 v^2 for P in pascals and v in m/s). The history of this untrue relation is curious. It was given about the end of the 18th century as based on some experiments, but with a footnote stating that little reliance could be placed on it. The statement without the qualifying note was copied from book to book, and at last received general acceptance. There is no doubt that under average conditions of atmospheric density, the 0.005 should be replaced by 0.003, for many independent authorities using different methods have found values very close to this last figure. It is probable that the wind pressure is not strictly proportional to the extent of the surface exposed. Pressure plates are generally of moderate size, from a half or quarter of a square foot up to two or three square feet (3 m² to 32 m²), are round or square, and for these sizes, and shapes, and of course for a flat surface, the relation P = 0.003 v^2 (or P = 0.029 v^2 for pascals and m/s) is fairly correct.

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In the tube anemometer also it is really the pressure that is measured, although the scale is usually graduated as a velocity scale. In cases where the density of the air is not of average value, as on a high mountain, or with an exceptionally low barometer for example, an allowance must be made. Approximately 1½% should be added to the velocity recorded by a tube anemometer for each 1000 ft (5% for each kilometer) that it stands above sea-level.

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Anemometers, such as the one shown below, at Deconism Gallery, are often used in conjunction with windmills, so that the wind speed and power generated by the turbine (windmill) can be logged together in a data logger.

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Other types of anemometers include:

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• hot wire or hot plate sensors, which measure the cooling of a heated element immersed in the wind
• ultrasonic sensors, which measure the Doppler shift of sound waves travelling across the moving air.