The ultimate source of most energy used by humans is the sun. Fossil fuels, for example, are substances in which solar energy has been converted and stored as plants or animals that have died and decayed. When energy experts look for alternative energy sources, they often search for new ways to use solar energy. Wind energy, although not technically classified as solar energy, is produced by the sun, and when converted accounts for approximately 1–2% of the total energy of the sun.

When sunlight strikes the earth, it heats objects such as land, water, and plants, but it heats them differentially. Dark-colored objects absorb more heat than light-colored ones and rough surfaces absorb more heat than smooth ones. As these materials absorb more or less heat, they also transmit that heat to the air above them. When air is warmed, it rises into the air until it...