Urban heat islands are not inevitable, but the product of dark roofs, black pavement, and loss of vegetation. A “cool communities” approach would lower air-conditioning use and make the air healthier.

On a summer afternoon, central Los Angeles registers temperatures typically 5°F higher than the surrounding suburban and rural areas. Hot roofs and pavements, baked by the sun, warm the air blowing over them. The resulting urban “heat island” causes discomfort, hikes air-conditioning bills, and accelerates the formation of smog.

Heat islands are found in many large cities, including Chicago, Washington, and (as the Olympic athletes and fans can attest) Atlanta. The effect is particularly well recognized in cities that quote two airport temperatures on the weather report. Thus Chicago-Midway airport is typically a few degrees hotter than suburban O’Hare, and the same difference applies between Washington National airport and Dulles.

Contrary to popular opinion, heat islands do not arise mainly from heat leaking out of cars, buildings, and factories. In summertime, such anthropogenic heat gain accounts for a mere 1 percent of the heat island’s excess temperature. (The fraction rises in the winter to about 10 percent, when heat does leak out of buildings.) Rather, dark horizontal surfaces absorb most of the sunlight falling on them. Consequently, dark surfaces run hotter than light ones. The choice of dark colors has caused the problem; we propose that wiser choices can reverse it.