The tools used to measure up wind are recognized as anemometers and can record wind speed, direction and also the strength of gusts.

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The regular unit that wind rate is the knot (nautical mile per hour = 0.51 m sec-1 = 1.15 mph). Wind direction is measured loved one to true north (not magnetic north) and also is reported from wherein the wind is blowing. One easterly wind blows from the eastern or 90 degrees, a southerly from the south or 180 degrees and a westerly from the west or 270 degrees.

Wind speed generally increases with height over the earth's surface and also is much affected by such factors as the roughness of the ground and the visibility of buildings, trees and also other obstacles in the vicinity.

The optimal exposure for the measurement of wind is over level ground of uniform roughness through no large obstacles within 300 m of the tower. In practice couple of sites in the observing network accomplish this requirement specifically for all event wind directions, but most are sensibly representative that an open up site.

Cup anemometer

Wind rate is typically measured through a cup anemometer consist of of 3 or 4 cups, conical or hemispherical in shape, an installed symmetrically around a vertical spindle. The wind blowing into the cups causes the spindle come rotate. In standard tools the design of the cups is such that the rate of rotation is proportional to the speed of the wind come a saturated close approximation.

At intervals the no much longer than five years, anemometers are calibrated in a wind tunnel come identify any kind of departures in the relationship in between spindle rotation and also wind rate specified by the manufacturer. Calibration correction are used to the measure wind speed.

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Measuring wind direction

Wind direction is measure by a vane consist of of a slim horizontal arm transferring a vertical flat plate at one finish with the edge come the wind and also at the other end a balance weight which also serves as a pointer. The eight is carried on a vertical spindle mounted on bearings which permit it to turn easily in the wind. The anemometer and also wind vane are each attached to a horizontal sustaining arm in ~ the height of a 10 m mast (see image above).

Sonic anemometer

Where wind dimensions are make in too much weather conditions, such together on the optimal of mountains, a heated sonic anemometer is used (see over image) having no moving parts. The instrument steps the speed of acoustic signal transmitted between two transducers located at the finish of slim arms. Dimensions from 2 pairs of transducers deserve to be an unified to productivity an calculation of wind speed and direction.

The distortion that the air flow by the structure supporting the transducers is a problem which have the right to be minimized by applying corrections based on calibrations in a wind tunnel.

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Measuring gusts and wind intensity

Because wind is an element that varies swiftly over very short periods of time it is sampled at high frequency (every 0.25 sec) to capture the soot of gusts, or short-lived peaks in speed, i beg your pardon inflict greatest damages in storms. The gust speed and direction are characterized by the best three 2nd average wind speed developing in any kind of period.

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A much better measure that the overall wind intensity is defined by the average speed and also direction end the ten minute duration leading approximately the report time. Mean wind over other averaging durations may likewise be calculated. A gale is defined as a surface ar wind of mean speed of 34-40 knots, average over a period of ten minutes. Terms such as 'severe gale', 'storm', etc are also used to define winds of 41 knots or greater.