Lawrence Berkeley National Laboratory

Modern urban areas have darker surfaces and less vegetation than their surroundings, affecting urban climate, energy use, and habitability. Dark roofs heat up more and thus raise the summertime cooling demands of buildings. Collec tively, dark surfaces and reduced vegetation warm the air over urban areas, creating urban "heat islands." On a clear summer afternoon, the air temperature in a typical city can be as much as 2.5 degrees higher than surrounding rural areas. Peak urban electric demand rises by 2-4% for each 1 degree C rise in daily max. temperature above 15-20 degrees C. Thus, the additional air-conditioning use caused by higher urban air temperature is responsibile for 5-10% of urban peak electric demand, costing U.S. ratepayers several billion dollars annually.