The power available in wind, measured in watts, passing through a cylinder of a given cross-sectional area is proportional to the cube of the wind speed v. The kinetic energy,
Ec = (1/2) mv2 ,
accounts for the exponent 2 (where m is the mass of air contained in the cylinder), while the volume of air per second,
Vair = Area (cross section) × v,
accounts for the exponent 1. In other words, if the wind strength is halved, the power is divided by eight.