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Why are windsocks striped?
If you’ve paid attention at an airport, heliport, or by the side of a road/building that sometimes experiences strong winds you’ve probably seen a windsock.

Image: Matt Brown
What is a windsock? A windsock is a truncated textile conical tube (frustrum), vaguely resembling a sock with no toes (hence the name). It is used as basic guide to determine wind strength and direction (and also sometimes used just for decoration!)
The sock, having a smaller diameter at its tail than the opening for the mouth, experiences drag when exposed to the wind, and naturally orientates itself to the wind direction. When the wind blows, the sock partially inflates and aligns with the wind.
The direction the sock points is the orientation in which the wind is blowing. This means that a sock that is pointing North is experiencing a wind from the South (This is opposite to the conventional way that wind is described; that from where the wind originates).
The stronger the wind, the more drag, and the more horizontal the sock becomes. Thus, a simple windsock gives a rough approximation of the vector of the wind (direction and speed). It does this in an intuitive and visual way that is easy to interpret. Being a good analog device, it also gives indication about rate of change (if the wind is changing direction rapidly, or if it is gusting), or wind shear.


So why the stripes?
Windsocks are typically manufactured with banded stripes of alternating high visibility orange and white fabric. These stripes certainly help provide strong contrast to make them easy to see, but there is more to the design than just prominence.
Windsocks are calibrated so that, as a first approximation, each ring represents about 3 knots of windspeed. A windsock experiencing no wind will droop down. As the speed increases, the drag increases, and more of the sock becomes horizontal. By counting the number of rings of the sock that are turgid, a rough estimate of the speed of the wind can be determined.
Each horizontal band is approximately 3 kts of wind.
When all the rings of the sock are inflated, the wind speed is a minimum of 15 knots.
Of course if the sock is entirely missing, it’s either been stolen, or you’re not in Kansas anymore Dorothy! — FAA requirements are that a windsock must be safe up to at least 75 knots (140 km/h, 85mph), so if it’s missing, it’s probably very breezy outside!