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.
Windsock at Freiburg airport, Germany. Source: wikicommons 2012
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.
- Size 1: 2.5 m (8 ft) in length and 0.45 m (18 inches) throat diameter at large end.
- Size 2: 3.60 m (12 ft) in length and 0.9 m (36 inches) throat diameter at large end.
- FAA type L-806 support - those mounted on low mass supporting structures - maximum of 3.0 m (10 ft) mast height.
- FAA type L-807 support - those mounted on rigid supporting structures - Maximum of 4.8 m (16 ft) height to windsock.
AC 150/5345-27E states that a properly-functioning windsock will orient itself to a breeze of at least 3 knots (5.6 km/h; 3.5 mph) and will be fully extended by a wind of 15 knots (28 km/h; 17 mph). In addition, wind cone assemblies must be designed to operate in wind speeds of up to 75 knots (140 km/hr or 86 mph) and ambient temperatures between -67° F (-55° C) and 131° F (+55° C.)
TC AIM-AGA-Aerodromes-5.9 "Wind Direction Indicators", requires that a 15-knot (28 km/h; 17 mph) wind will fully extend the windsock, a 10-knot (19 km/h; 12 mph) wind will cause the windsock to be 5° below the horizontal, a 6-knot (11 km/h; 6.9 mph) wind will cause the windsock to be 30° below the horizontal.