As modern urban areas have grown, there has been a corresponding growth in darker surfaces and a decline in vegetation, affecting urban climate, energy use, and habitability. Dark roofs heat up more and thus raise the summertime cooling demands of buildings, collectively with reduced vegetation, warming the air over urban areas and creating "heat islands." On a clear summer afternoon, the air temperature in a typical city can be as much as 2.5ºC (4.5ºF) higher than in surrounding rural areas. (The peak heat island effect occurs during cold winter evenings and is caused primarily by the rapid cooling of the rural areas - the thermal storage of pavements and dark-roofed buildings is much greater than greenery.) Peak urban electric demand rises by 2 to 4 percent for each 1ºC (1.8ºF) rise in daily maximum temperature above 15 to 20ºC (59 to 68ºF), so the additional airconditioning
use caused by higher urban air temperature is responsible for 5 to 10 percent of urban peak electric demand, costing U.S. ratepayers several billion dollars annually. Temperatures in cities are increasing. Figure I depicts summertime monthly maximum
and minimum temperatures between 1877 and 1997 in downtown Los Angeles, clearly indicating that these maximum temperatures are now about 2.5ºC (4.5ºF) higher than in 1920. Minimum temperatures are about 4ºC (7.2ºF) higher than in 1880. In California from 1965 to 1989, the average urban-rural temperature differences, measured at thirtyone pairs of urban and rural stations, have increased by about 1ºC (1.8ºF). In Washington, D.C., temperatures rose 2ºC (3.6ºF) between 1871 and 1987. This recent warming trend is typical of most U.S. metropolitan areas and exacerbates demand for energy. In Los Angeles, we estimate a heat-island induced increase in power consumption of 1 to 1.5 GW, costing ratepayers $100 million per year. Besides increasing systemwide cooling loads, summer heat islands increase smog production. Smog production is a highly temperature-sensitive process. At maximum daily temperatures below 22ºC (71.6ºF), maximum ozone concentration in Los Angeles is below the California standard of 90 parts per billion. At 35ºC (95ºF), practically all days in Los Angeles are smoggy (see Figure 2).