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
Gibbs measure property. Measures with this property are sometimes called Markov random fields. More strongly, the converse is also true: "any" positive probability distribution (nonzero density everywhere) having the Markov property can be represented as a Gibbs measure for an appropriate energy function. This is the Hammersley–Clifford theorem. What follows is a formal definition for the special case of a random field on a lattice. The idea of a Gibbs measure is, however, much more general than this. The definition of a Gibbs random field on a lattice requires some terminology: We interpret as the contribution to the total energy (the Hamiltonian)