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I know that airport windsocks are calibrated to reach full erection at a particular wind velocity. At half that speed, the first half of the windsock should be erect and the second half should droop sorrowfully. At one third the speed it will be one third, and so on.

Trouble is that not all windsocks are calibrated the same way. How do I find out what wind velocity will bring my airport windsock to a fully erect state?

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    $\begingroup$ It means a smokin' hot wind just blew by. $\endgroup$
    – Jason C
    Commented Aug 8, 2014 at 1:58
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    $\begingroup$ . . . It means your windsock is very happy. The local pilots maybe not so much. $\endgroup$
    – voretaq7
    Commented Aug 8, 2014 at 2:36
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    $\begingroup$ Now I understand the origin of the phrase "A stiff breeze". $\endgroup$
    – Digital Trauma
    Commented Aug 9, 2014 at 20:24
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    $\begingroup$ If it lasts longer than 4 hours, consult a meteorologist. $\endgroup$
    – corsiKa
    Commented Aug 10, 2014 at 3:16

4 Answers 4

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Windsocks come in various sizes and speeds, but an FAA Standard Wind Sock aligns with the wind at 3 knots, and is fully extended at 15 knots like CGCampbell pointed out.

A typical wind sock (at least around here) looks something like this:
Wind Sock

The stripes are actually useful:

  • The first stripe indicates a 3-knot breeze
    (The wind sock has turned and aligned with the wind - usually you should look for an anemometer near the wind sock to determine if there's any appreciable wind because often the first segment is held open by the frame.)
  • The second stripe is ~ 6 knots
  • The third stripe is ~ 9 knots
  • The fourth stripe is ~ 12 knots
  • The last stripe is 15 knots or higher
  • If the windsock is missing you probably shouldn't be at the airport.
  • If the pole is missing you're probably on your way to Oz.

(You can actually get a far more precise estimation if you're willing to do some trigonometry, but this works well enough for most purposes.)

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    $\begingroup$ Hey, I'm willing to do some trigonometry. What kind of calculation are we talking about here? $\endgroup$
    – user3132
    Commented Aug 8, 2014 at 20:09
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    $\begingroup$ @AlexanderGruber Nothing hard - this works better with a "floppy" wind sock that doesn't have its throat held open (or something like a wind streamer / flag), but basically sin(windsock_angle) * full_extension_speed should give you the current wind speed (the angle is from the pole, so hanging limp is 0 degrees, and sticking straight out is 90 degrees). See also "Wyoming Windsock" :-) $\endgroup$
    – voretaq7
    Commented Aug 8, 2014 at 20:26
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    $\begingroup$ @voretaq7 "First officer, bring us in close, and fetch me my protractor!" $\endgroup$
    – Bassinator
    Commented Aug 9, 2014 at 13:07
  • $\begingroup$ @voretaq7: Just a heads-up, but the "Wyoming Windsock" link is broken. $\endgroup$
    – Vikki
    Commented May 8, 2019 at 3:23
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According to the FAA, Advisory Circular 150/5345-27E, dtd 26 Sep 2013, Paragraph 3.2.2,

3.2.2 Dimensions

The taper or 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).

The paragraph, 4.2.6 Windsock Extension gives the variance of wind speed allowed which should still cause a full extension of the wind sock, but that paragraph appears to need editing to put in the starting values for mph and km/hr, 17 and 28 respectively, which are indicated above.

4.2.6 Windsock Extension.

Test the windsock to assure that it extends fully when subjected to a wind of 15 (+2,-1) knots (+3.7,-1.8) km/hr or (+2.3,-1.2 mph) ).

So, for instance, the sock should be fully extended from 15.8 mph through 19.3 mph.

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  • $\begingroup$ I also read a comment in a blog that indicates that the EASA was coming out with (new) regulations about windsocks, but am having difficulty finding anything at the current time. $\endgroup$
    – CGCampbell
    Commented Aug 8, 2014 at 2:40
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    $\begingroup$ Yeah I wasn't able to find anything about an EASA Standard Windsock. Canadian windsocks are the same 15-knot standard as the FAA uses though. (That site also gives the alternate, and maybe easier, explanation of "5ks for every orange stripe sticking out", but I always like the 3kts-per-segment one I gave in my answer because it's a little more precise) $\endgroup$
    – voretaq7
    Commented Aug 8, 2014 at 5:12
  • $\begingroup$ I'd feel a lot more confident about that FAA Circular you cite if it didn't seem to be missing the actual threshold values in km/h and mph. Presumably, it should say something like "15 (+2,-1) knots (27.8 (+3.7,-1.8) km/hr or 17.3 (+2.3,-1.2) mph)." $\endgroup$
    – Ilmari Karonen
    Commented Aug 8, 2014 at 19:05
  • $\begingroup$ @IlmariKaronen I edited my answer to fill in the blanks you noticed. Thanks for that. $\endgroup$
    – CGCampbell
    Commented Aug 8, 2014 at 20:09
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The main function of the windsock is to show from which direction the wind blows. The faster the wind blows, the straighter and more horizontally the wind extends.

As per FAA standards a 15-knot (28 km/h; 17 mph) wind will fully extend the properly functioning windsock. A 3-knot (5.6 km/h; 3.5 mph) breeze will cause the properly functioning windsock to orient itself according to the wind. If the windsock has blown away, the wind is greater than the 75 knot design limit.

http://www.faa.gov/documentLibrary/media/Advisory_Circular/150_5345_27e.pdf

Windsocks are accurate only within a small radius. In those airports where there is only one windsock and it's alongside the midpoint of the runway, the only thing of which you can be certain , is that, the wind at either end of the runway will NOT necessarily match. This explains why many airports have windsocks at both ends of the runway. The sock in the middle tells traffic which runway to use, but the windsock at the end of the runway tells the pilot what to expect on approach.

Note that the correspondence between wind speed and degree of extension and straightening will varies from one model to another. You can calibrate the windsock with the help of a anemometer.

After installing the windsock on the ground , use the anemometer to calibrate the shape and orientation of the windsock to specific wind speeds. The anemometer (wind meter) should be held at the same height as the windsock’s larger opening . Point it into the wind and record the wind speed reading.

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Myth alert! There is a somewhat widespread belief in the most accepted answer here. It is not true that there is a 3kt relationship per segment of the windsock. Those who claim so provide diagrams of this but never photographs. Also the FAA does not specify this in their specification for windsocks. Windsocks are designed to fly straight, it is the angle of the dangle which indicates windspeed. The first segment (the one nearest the mast) is often rigid so as to keep the rest of the windsock from entangling itself around the mast. But other than that first rigid section you will not see a bent windsock. FAA spec: http://www.faa.gov/documentLibrary/media/advisory_circular/150-5345-27D/150_5345_27d.pdf

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    $\begingroup$ Welcome to Aviation.SE! You have to pardon my partial skepticism, but where in the document you link is there indicated that the "3 kt rule" is a myth? it is neither refuted nor mentioned, and it could have been behind the design of the sock size (length, diameter, fabric selection, etc.). Moreover, the first segment being rigidly connected is already addressed in the accepted answer $\endgroup$
    – Federico
    Commented Jul 26, 2016 at 12:03
  • $\begingroup$ in addition, the document you link has been superseded by the newer edition linked in D_S' answer $\endgroup$
    – Federico
    Commented Jul 26, 2016 at 12:05
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    $\begingroup$ Actually there is more than one patent for windsocks (actually, there are several). One design operates as you indicate; the whole thing beyond the stiffener section droops at an angle corresponding to wind speed. Another patent is designed to operate as Voretaq mentions, with wind speed indicated by which section is drooping. $\endgroup$
    – CGCampbell
    Commented Jul 26, 2016 at 15:02

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