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
Complete measure Naïvely, we would take the "σ"-algebra on R to be "B" ⊗ "B", the smallest "σ"-algebra containing all measurable "rectangles" "A" × "A" for "A" ∈ "B". While this approach does define a measure space, it has a flaw. Since every singleton set has one-dimensional Lebesgue measure zero, for "any" subset "A" of R. However, suppose that "A" is a non-measurable subset of the real line, such as the Vitali set. Then the "λ"-measure of {0} × "A" is not defined, but and this larger set does have "λ"-measure zero. So, this "two-dimensional Lebesgue measure" as just defined is not