I have come across this image on Twitter on how to measure wind speed with a windsocket. I noticed that the rings look equally long in the photograph (and others) and measure a speed increase of 3 knots per ring. I found it surprising that this linear relationship exists here, but was not able to find any good explanation/computation or could come up with one myself.
Is there a straightforward way to show this relationship? I realize this is a very open-ended question, but I could not really make progress on it.
