A windsock is a conical textile tube which resembles a giant sock. Windsocks can be used as a basic guide to wind direction and speed, or as decoration.
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Uses
Windsocks are used to tell wind speed and the direction of the wind speed itself. Windsocks typically are used at airports to indicate the direction and strength of the wind to pilots and at chemical plants where there is risk of gaseous leakage. They are sometimes located alongside highways at windy locations.
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.
Airport Wind Sock Video
Function
Wind direction is the opposite of the direction in which the windsock is pointing (note that wind directions are conventionally specified as being the compass point from which the wind originates; 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.
Interchangeable 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.
Standards
Per FAA standards, 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).
Per Transport Canada standards: a 15-knot (28 km/h; 17 mph) wind will fully extend the wind sock, a 10-knot (19 km/h; 12 mph) wind will cause the wind sock to be 5° below the horizontal, a 6-knot (11 km/h; 6.9 mph) wind will cause the wind sock to be 30° below the horizontal.
Source of the article : Wikipedia
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