Energy from moving air
Wind is caused by the uneven heating of the earth's surface by the sun. Because the earth's surface is made of different types of land and water, it absorbs the sun's heat at different rates. One example of this uneven heating can be found in the daily wind cycle.
The daily wind cycle
During the day, the air above the land heats up faster than the air over water. The warm air over the land expands and rises, and the heavier, cooler air rushes in to take its place, creating wind. At night, the winds are reversed because the air cools more rapidly over land than over water.
In the same way, the atmospheric winds that circle the earth are created because the land near the earth's equator is heated more by the sun than the land near the North Pole and the South Pole.
Wind energy for electricity generation
Today, wind energy is mainly used to generate electricity, although water pumping windmills were once used throughout the United States.