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Lets do some physics!
For large Reynolds number (https://en.wikipedia.org/wiki/Reynolds_number) the
drag coefficient for a sphere is about Cd=0.5. The typical mass and diameter of
a ping-pong ball is about 2.7g and 40mm, respectively, and thus with a
density of 0.081g/cm^3
The drag force is
Fd = 0.5 rhoair C_d A v^2
and the weight is
Fg=4 pi/3 R^3 g rhoball
For a given angle of deflection alpha one gets Fd=Fg tan(alpha) and thus
v=8.5524 sqrt(tan(alpha))
So by measuring the angle of deflection of the ball we can measure the wind
velocity.