The arm is carried on a vertical spindle mounted on bearings which allow it to turn freely in the wind. Wind direction is measured by a vane consisting of a thin horizontal arm carrying a vertical flat plate at one end with its edge to the wind and at the other end a balance weight which also serves as a pointer. Calibration corrections are applied to the measured wind speed. In standard instruments the design of the cups is such that the rate of rotation is proportional to the speed of the wind to a sufficiently close approximation.Īt intervals of no longer than five years, anemometers are calibrated in a wind tunnel to identify any departures in the relationship between spindle rotation and wind speed specified by the manufacturer. The wind blowing into the cups causes the spindle to rotate. Wind speed is normally measured by a cup anemometer consisting of three or four cups, conical or hemispherical in shape, mounted symmetrically about a vertical spindle. In practice few sites in the observing network meet this requirement exactly for all incident wind directions, but most are reasonably representative of an open site. The optimal exposure for the measurement of wind is over level ground of uniform roughness with no large obstacles within 300 m of the tower. Wind speed normally increases with height above the earth's surface and is much affected by such factors as the roughness of the ground and the presence of buildings, trees and other obstacles in the vicinity. An easterly wind blows from the east or 90 degrees, a southerly from the south or 180 degrees and a westerly from the west or 270 degrees.

Wind direction is measured relative to true north ( n o t magnetic north) and is reported from where the wind is blowing. The normal unit of wind speed is the knot ( nautica l mile per hour = 0.51 m sec-1 = 1.15 mph).

Anemometer units