How to make, read, and calibrate a ping-pong ball anemometer?
The drag coefficient varies with Reynolds number and hence with wind speed. Most fluid-dynamics textbooks have a graph of the drag coefficient of a sphere versus Reynolds number. Hopefully that could help. (A sphere such as a ping-pong ball is the best shape since its shape is the same in all orientations --- only a sphere has complete omnidirectional symmetry.) Because wind is rarely steady but usually includes gusts and lulls, it may be necessary to account for overshooting. A sudden gust can cause the ping-pong ball to temporarily overshoot to a higher angle than in a steady wind at the same speed as the gust.