Wind

Wind speed is measured with an anemometer, which is made up of three hemispherical cups on a vertical shaft ideally set 10 metres above the ground (see Figures 1.7 and 2.1). The wind puts a greater pressure on the inside of the concave surface than on the convex one so that the shaft is spun round; the rotation is displayed on a dial usually calibrated in knots (nautical miles per hour) or metres per second. An older and still much used visual method is the Beaufort Scale; originally based on observations made at sea, it is used to indicate the wind forces at sea or on land (see Table 2.7).

Table 2.7 The Beaufort Scale

Wind direction is indicated with a wind vane, which is often combined with an anemometer. Decorative wind vanes are a familiar sight, but the standard meteorological design comprises a pointer with a streamlined vertical plate on one end mounted so that it can rotate freely. The arrow shape points into the wind and the movements over a minute or so are averaged. The direction the wind is coming from is recorded as the number of degrees read clockwise from true north, i.e. a westerly wind is given as 270, south-easterly as 135 and a northerly one as 360 (000 is used for recording no wind).