A windsock, or wind cone, is a conical textile tube that resembles a giant sock. Windsocks can be used as a basic guide to wind direction and speed.
At many airports, windsocks are lit at night, either by floodlights on top surrounding it or with one mounted on the pole shining inside it.
Wind direction is the opposite of the direction in which the windsock is pointing, so a windsock pointing due north indicates a southerly wind. Wind speed is indicated by the windsock’s angle relative to the mounting pole; in low winds, the windsock droops; in high winds it flies horizontally.
Alternating stripes of high visibility orange and white were initially used to help to estimate the speed of wind. Each stripe adds up 3 knots to the estimated wind speed. However, some circle frames mountings cause windsocks to be held open at one end, indicating a velocity of 3 knots, even though anemometers would show no wind speed. A fully extended windsock suggests a wind speed of 15 knots (28 km/h; 17 mph) or greater.
ICAO (Annex 14)/UK CAA
Size: 3.60 m (12 ft) in length and 0.9 m (36 inches) throat diameter at large end.
Height: At a 6.0 m (20 ft) mast height, the taper of the fabric windsock from the throat to the trailing end must be designed to cause the windsock to fully extend when exposed to a wind of 15 knots (28 km/hr or 17 mph.)
“Wind direction indicators, including supplemental indicators, must be lighted if the airport is open for air carrier operations at night.”
Windsocks must rotate freely around a vertical shaft, must indicate true wind direction +/- 5 degrees, and indicate 15 knots of wind when fully extended. Wind socks must be white, yellow or orange to contrast with surroundings.
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