How to make, read, and calibrate a ping-pong ball anemometer? I'd like to make a ping-pong ball anemometer to make estimates or approximate measurements of wind and surface air currents at low to moderate speeds. I don't own an anemometer now, but I'd estimate the speeds of interest to be between zero and a maximum of order of 10 or 15 mph (feels similar to riding a (non-racing) bicycle down hill on an otherwise windless day).

If I understand correctly, a ping-pong ball anemometer probably refers to the use of a ping-pong ball or other tell-tale normally used to indicate wind direction to also try to use it to estimate the speed of the wind.

If this were a problem on a test, I'd set up the equations for force and balance the vertical and horizontal forces separately, assuming the wind is horizontal. For the horizontal force, I'd use a simple equation for drag force as a function of velocity from Wikipedia:

$$F_D(v)= \frac{1}{2} \rho v^2 C_D A$$

where the density of air at low elevations and moderate temperature is roughly 1.2 kg/m^3, $A$ is the cross-sectional area of the ping-pong ball $\pi R_{p-pb}^2$ and $C_D$ would be the drag coefficient, which is where all of the reality and the complications of aerodynamics are actually hidden in practice. $C_D$ is not really a constant, and is often plotted against velocity and other parameters. However, for some range of Reynolds numbers, a perfect sphere would have a roughly constant $C_D$ of about 0.47.

But a 2.7 gram, 40mm diameter ping-pong ball anemometer, or similar device is a real-world device. Maybe I should use a heavier ball, but I think the smoothness of the ball is critical if I want to use a theoretical $C_D$ and estimate speeds using equations rather than doing a real calibration against a known anemometer or riding in/on a vehicle at a known speed. 

Are there any instructions for how to build, use, and calibrate or interpret results for this kind of simple anemometer? Ideally some dimensions and a suggested scale would be great! Or if this has been actually measured for a pingpong ball or another object, that would also be a great answer as well.





If the concept is good enough for a Mars lander weather station, then it's good enough for me!



above: GIF from images transmitted from Mars. From the The Telltale project page on the Mars Simulation Laboratory. Also archived here.



above: Instrument as delivered to NASA. From the The Telltale project page on the Mars Simulation Laboratory. Also archived here. That's a 45 degree mirror below the telltale providing a simultaneous view from below.
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.