Windsock

At many airports, windsocks are externally or internally lit at night. Wind direction is opposite the direction in which the windsock is pointing. Wind speed is indicated by the windsock's angle relative to the mounting pole‍— in low winds it droops; in high winds, it flies horizontally.

Design

Alternating stripes of high-visibility orange and white were initially used to help estimate wind speed, with each stripe extended adding 3 knots (5.6 km/h; 3.5 mph) to the estimated speed. Some circular frame mountings cause windsocks to be held open at one end and the first stripe extended, indicating a velocity of 3 knots even when stripes are not present. A fully extended windsock suggests a wind speed of 15 knots (28 km/h; 17 mph) or greater.

Standards

Per FAA standards, a properly functioning windsock orients itself to a breeze of at least 3 knots (5.6 km/h; 3.5 mph) and fully extends in a wind of 15 knots (28 km/h; 17 mph).

Per Transport Canada standards, a 15-knot wind fully extends the windsock; a 10-knot (19 km/h; 12 mph) wind raises it to 5° below the horizontal; and a 6-knot (11 km/h; 6.9 mph) wind raises it to 30° below the horizontal.

ICAO standards specify a truncated cone-shaped windsock at least 3.6 metres (12 ft) long and 0.9 metres (3 ft) in diameter at the large end. It should be readable from an altitude of 300 metres (980 ft) and ideally be of a single colour. If it is necessary to use two colours, they should ideally be orange and white, arranged in five alternating bands, with the first and last darker in tone. In wind speeds of 3 knots (5.6 km/h; 3.5 mph) or more, they must indicate wind direction to within ±5°.