In the simplest sense, wind is air in motion produced by the uneven heating of the Earth's surface by the sun. Because our planet is made of various land and water formations, from the severe heights of K2 to the placid Chicago River, it absorbs the sun's radiation unevenly, creating differences in air pressure and the ensuing movement of air from high-pressure areas to lower ones. Wind is also one of our most valuable natural resources: As an alternative to fossil fuels, wind is plentiful, renewable, widely distributed, clean, produces no greenhouse gas emissions, and uses little land. While wind power may be more important today than ever before, it is by no means a new concept.