Other things being equal, a site with steady winds may yield more energy than another location with the same average wind speed but more variable winds. Likewise, higher average wind speeds and air densities usually yield more energy than lower ones. Because air density decreases with altitude, somewhat higher average wind speeds are required at high altitudes to yield the same energy as lower altitude sites with lower average wind speeds. On the other hand, trees, plants, buildings, and topographical irregularities tend to impede the flow of air near the ground and thus reduce wind speed. Consequently, wind power turbines are mounted on towers to raise them well above ground level. Wind resource maps usually identify areas by wind power class. In general, areas identified as class 4 and above are regarded as potentially economical for wind energy production with current technology. Nevertheless, some areas identified with class 3 wind resources are being developed in the United States.
Many regions of the country offer at least some usable wind resources. The Great Plains States have abundant wind resources, followed by other parts of the Midwest, the West, and the Northeast. Although there is some potential for wind energy development in the South, the wind resources there are not as significant as in the other regions of the United States.