Editing Anemometer (section)

====Four cup====
With a four-cup anemometer, the wind always has the hollow of one cup presented to it, and is blowing on the back of the opposing cup. Since a hollow hemisphere has a [[drag coefficient]] of .38 on the spherical side and 1.42 on the hollow side,<ref>{{Citation|url=https://archive.org/details/FluidDynamicDragHoerner1965 |title=Sighard Hoerner's Fluid Dynamic Drag|pages=3–17, Figure 32|date=1965}} (pg 60 of 455)</ref> more force is generated on the cup that presenting its hollow side to the wind. Because of this asymmetrical force, [[torque]] is generated on the anemometer's axis, causing it to spin.

Theoretically, the anemometer's speed of rotation should be proportional to the wind speed because the force produced on an object is proportional to the speed of the gas or fluid flowing past it. However, in practice, other factors influence the rotational speed, including turbulence produced by the apparatus, increasing drag in opposition to the torque produced by the cups and support arms, and friction on the mount point. When Robinson first designed his anemometer, he asserted that the cups moved one-third of the speed of the wind, unaffected by cup size or arm length. This was apparently confirmed by some early independent experiments, but it was incorrect. Instead, the ratio of the speed of the wind and that of the cups, the ''anemometer factor'', depends on the dimensions of the cups and arms, and can have a value between two and a little over three. Once the error was discovered, all previous experiments involving anemometers had to be repeated.