Most of the prospective sites in these classes are concentrated in the Great Plains states. In contrast, class 3 and 4 areas are distributed much more widely around the country and, according to the study could supply from 1.7 to 6 times the current U.S. electricity demand. Other constraints may reduce these figures considerably. Two factors not considered in the study were possible conflicts between wind power plants and bird habitats and migration routes and constraints on the transmission capacity needed to carry wind power to population centers. The intermittent nature of wind energy also may impose limitations on its use, although probably not until it begins to generate more than 10 percent of a utility's power needs.
Wind is caused by uneven heating of the earth's surface by the sun.
The heat absorbed by the ground or water is transferred to the air, where it causes differences in air temperature, density and pressure. These differences, in turn, create forces that push the air around. On a global scale, the temperature difference between the tropics and the poles drives the trade winds, which act as a giant heat exchanger to keep the equator from becoming even hotter and the poles from becoming even colder. On a much smaller scale, temperature differences between land and sea and between mountains and valleys often create strong breezes. Wind direction and speed are affected by other factors, as well, such as the earth's rotation, local topographical features and the roughness of terrain.