We never like starting the answer to a question like "How do you use a Windsock to measure windspeed" with "It Depends!". However it really does.
Windsocks provide an excellent indication of wind direction and a broad indication of wind speed. There is no universally standardised Windsock that precisely indicates wind speed however the following provides guidance based on our experience:
CASA (Civil Aviation Safety Authority) Regulations
Under CASA (AC 91-02) a Windsock:
- at a 45° angle to the horizon indicates a windspeed of ~ 15 knots (~ 28 km/hr)
- that is horizontal indicates a windspeed of ~ 25-30 knots (46 - 56 km/hr)
It should be noted that the CASA Aviation Grade Windsocks we manufacture are very large (at 3.65m in length), contain a significant amount of material (almost 6.5 m2) and we also use heavier material (almost 300gr/m2), triple stitching and a reflective strip on the tail.
From our experience:
Based on the above, and our experience:
- even the slightest wind will move a Windsock (assuming it is properly mounted) and therefore indicate the wind direction
- smaller Windsocks (with less material weight) will be more responsive to windspeed and achieve their 45° angle and be horizontal at lower windspeeds that those shown above;
- a Windsock that is horizontal (eg, in the CASA diagram above) shows you that the windspeed is at least 30 knots, but doesnt show you how much above 30 knots the true windspeed might be;
- Windsocks that are worn or shredded do not provide accurate windspeed or even wind direction indications;
- trees of other obstructions, even if, say, 30m away, severely impact the performance of Windsocks. We have done many experiments where the prevailing wind is 20 knots and the obstructed Windsock is hardly moving;
- weather, humidity, moisture, and how dry the Windsock is will also have a large bearing on the Windsock performance on any given day.
Calibrate the Windsock:
If determining windspeed is important to you then you will need to calibrate the Windsock using an Anemometer, Bureau of Meteorology data or other wind readings (from an airport for instance). It is possible to produce a more accurate Windsock response to varying windspeeds in a trial and error approach varying:
- the length of a Windsock;
- the material from which the Windsock is manufactured;
- applying extra material weight into the tail of the Windsock.
Windsock traffic light system in place - red section fully elongated indicates a "stop work" condition.
The International Experience:
You can find on line references to striped "Airport Windsocks" that are calibrated and will respond and extend in 3-5 knot increments to increasing windspeed.
In our view, it doesn't work this way because there is so much variation in length, material and local conditions!
Interestingly, CASA, the FAA, and ICAO do not mandate the use of striped Windsocks but prioritise a Windsocks visibility and typically requiring solid colours: white, yellow, orange.
Windsocks Australia does however manufacture Dual SS Windsock Frames with striped Windsocks for particular customer requirements. Again, they key is calibrating the performance of the Windsock.
Finally
For more accurate wind speed measurements, it's recommended to use an anemometer to calibrate the Windsock in its specific location.
Remember that Windsock readings should be considered as rough estimates rather than precise measurements, especially for critical operations like aviation.
Good luck out there!