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 2.5°C (4.5°F) 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 1°C (1.8°F) rise in daily maximum temperature above 15 to 20°C (59 to 68°F), so the...