Measurement of Surface Wind
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Definition of wind velocity
Wind velocity is a three-dimensional vector quantity with small-scale random fluctuations in space and time superimposed upon a larger-scale organized flow. In general, surface wind is considered mainly as a two-dimensional horizontal vector quantity specified by two numbers representing direction and speed. Surface wind is usually measured by a wind vane and a cup or propeller anemometer.
Meteorological requirements
Due to the effects of friction, wind speed increases considerably with height. For this reason, a standard height is defined for the exposure of anemometers above open terrain. The standard exposure of anemometers over level and open terrain is 10 m above the ground. Open terrain is defined as an area where the distance between the anemometer and any obstruction is at least 10 times the height of the obstruction.
Units of measurement
Wind speed: metres per second(m/s), kilometresper hour(km/h),or nautical miles per hour(knots)
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Conversion factors: | ||
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1 m/s = 3.6 km/h |
1 km/h = 0.278 m/s |
1 knot = 0.514 m/s |
Wind direction: Wind direction is defined as the direction from which the wind blows, and is measured clockwise from geographical north.
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Compass points |
N |
NE |
E |
SE |
S |
SW |
W |
NW |
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Angular bearing |
360o |
045o |
090o |
135o |
180o |
225o |
270o |
315o |
Cup Anemometer and Wind Vane
A cup generator and wind vane are used to measure the wind speed and wind direction respectively. The cup generator normally has three cups mounted symmetrically about a vertical axis so that the diametric plane of each cup is vertical. As the force on the concave side of the cup, due to the wind, is greater than on the convex side, the cup wheel rotates. The wind speed is determined by the rotation rate of the cups. Wind direction is indicated by means of a wind vane. It consists basically of an asymmetrically-shaped object mounted at its center of gravity about a vertical axis. The end which offers the greater resistance to the motion of air moves to the downwind position.