The instruments used to measure wind are well-known as anemometers and can record wind speed, direction and the strength of gusts.

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The common unit of wind rate is the node (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 is reported from wherein the wind is blowing. One easterly wind blows indigenous the east or 90 degrees, a southerly native the southern or 180 degrees and a westerly native the west or 270 degrees.

Wind speed normally increases v height above the earth's surface and is much impacted by such factors as the roughness that the ground and also the presence of buildings, trees and also other obstacles in the vicinity.

The optimal exposure because that the measure up of wind is end level ground of uniform roughness through no big obstacles in ~ 300 m of the tower. In practice few sites in the observing network meet this requirement precisely for all event wind directions, but most are reasonably representative that an open up site.

## Cup anemometer

Wind speed is generally measured by a cup anemometer consists of three or 4 cups, conical or hemispherical in shape, mounted symmetrically about a vertical spindle. The wind blowing right into the cups reasons the spindle come rotate. In standard tools the architecture of the cups is such the the rate of rotation is proportional to the speed of the wind to a saturated close approximation.

At intervals that no longer than 5 years, anemometers space calibrated in a wind tunnel come identify any type of departures in the relationship between spindle rotation and also wind speed specified by the manufacturer. Calibration corrections are applied to the measure up wind speed.

### Measuring wind direction

Wind direction is measured by a vane consisting of a slim horizontal arm transporting a vertical level plate at one end with that edge come the wind and at the other finish a balance weight which also serves together a pointer. The arm is lugged on a vertical spindle placed on bearings which allow it come turn easily in the wind. The anemometer and also wind vane space each attached to a horizontal supporting arm in ~ the peak of a 10 m mast (see image above).

## Sonic anemometer

Where wind dimensions are make in too much weather conditions, such as on the height of mountains, a cook sonic anemometer is supplied (see above image) having no relocating parts. The instrument measures the speed of acoustic signal transmitted in between two transducers situated at the end of slim arms. Dimensions from 2 pairs the transducers deserve to be linked to productivity an calculation of wind speed and direction.

The distortion that the air circulation by the structure supporting the transducers is a problem which can be decreased by applying corrections based upon calibrations in a wind tunnel.

## Measuring gusts and wind intensity

Because wind is an element that varies promptly over really short durations of time it is sampled in ~ high frequency (every 0.25 sec) to record the intensity of gusts, or short-lived peaks in speed, which inflict greatest damage in storms. The gust speed and direction are defined by the preferably three 2nd average wind speed emerging in any period.

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A far better measure that the in its entirety wind soot is defined by the average speed and also direction over the ten minute period leading up to the report time. Mean wind over other averaging periods may likewise be calculated. A gale is defined as a surface wind of typical speed that 34-40 knots, averaged over a period of ten minutes. State such as 'severe gale', 'storm', etc are also used to define winds of 41 knots or greater.