Hot spots in Washington show up as red areas in this satellite image. The presence of such heat islands increases energy use and raises smog levels. The largest red patch is at the site of a convention center. The coolest areas (green) are those covered by grass and trees. Credit: S. Stetson, Global Environmental Management

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.

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