Anemometer is blowing on the back of the cup on the opposite end of the cross. Since a hollow hemisphere has a drag coefficient of .38 on the spherical side and 1.42 on the hollow side, more force is generated on the cup that is presenting its hollow side to the wind. Because of this asymmetrical force, torque is generated on the axis of the anemometer, causing it to spin. Theoretically, the speed of rotation of the anemometer should be proportional to the wind speed, because the force produced on an object is proportional to the speed of the fluid flowing
Anemometer cups and arms, and may have a value between two and a little over three. Every previous experiment involving an anemometer had to be repeated after the error was discovered. The three-cup anemometer developed by the Canadian John Patterson in 1926 and subsequent cup improvements by Brevoort & Joiner of the United States in 1935 led to a cupwheel design with a nearly linear response and had an error of less than 3% up to . Patterson found that each cup produced maximum torque when it was at 45° to the wind flow. The three-cup anemometer also had a more