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3.2.1.4. Measurement of Wind

Wind is usually measured by a cup anemometer which rotates about a vertical axis perpendicular to the direction of the wind. The exposure of wind instruments is important (Johnson & Linacre, 1978); any obstruction close by will affect measurements. Wind direction is also measured by means of a vane, accurately balanced about a truly vertical axis, so that it does not settle in any particular direction during calm conditions.