The primary causes of horizontal atmospheric air motion, or wind, are the uneven heating of the earth and its atmosphere by solar radiation and the earth's rotation. Large diurnal or time-of-day variations in wind occur in both tropical and temperate latitudes. The wind energy potential (WEP) or potential gross annual wind energy production for a specific site and a specific wind turbine can be calculated with a wind speed distribution and the turbine power curve, properly adjusted for the local air density. The greatest environmental issues that the wind industry has had to face are the possible impacts of wind facilities on birds and bats and their habitats. Three things that must be present before a commercial wind energy generation plant can be successfully developed: a site with a good wind resource, access to a transmission line with the capacity to accept the plant output, and a buyer to purchase the energy generated.