What is the urban heat island?
Most cities are warmer than surrounding rural areas, especially on calm, clear nights. Some inner cites might be 10 to 20°F warmer than low-lying rural areas. The urban warmth results from waste heat from energy use, changes in the heat capacity of the surface (concrete holds more heat than does grass), smoke acting as an insulating blanket, and reduced cooling due to less water evaporation. One small contribution to possible global warming might be urban sprawl. In the United States, over 750,000 acres of land are urbanized each year.