Btw: did you know and is this true?

[HansR]

240129 Windsnelheid.jpeg

[freddie]

Yes it is, but it does depend on the wind sock. The ones used on UK airfields (maybe just military) indicate 20 knots when extended.

[HansR]

OK. I never knew. These socks are everywhere yet most people don't know. A well kept secret in that sense.
And that the British ones deviate ... could it be otherwise

[freddie]

it's not so much that they're British - you can get wind socks calibrated to all sorts of speeds, the 20 knot ones are just the ones I'm familiar with.