The use of winds in the earth’s atmosphere to drive machinery, especially to drive an electrical generator. Practical land-based wind generators (aerogenerators) are probably capable of providing some 10²⁰ J (10¹⁴ kW h) of energy per year throughout the world and interest in this form of renewable energy is increasing. The power, P, available to drive a wind generator is given by P=kd²v³, where k is the air density, d is the diameter of the blades, and v is the average wind speed. Wind farms now exist in many parts of the world; California, for example, has the capacity to produce over 1200 MW from wind energy.