delware environmental institute

NSF: Summertime — hot time in the city

It's the first day of summer, a hot time in the desert city of Phoenix. And in cities across the United States--and the Northern Hemisphere. Heat islands, as these urban hot spots are called, are metropolitan areas significantly warmer than surrounding rural areas. Why?

"Us," says sociologist Sharon Harlan of Arizona State University (ASU). "It's all due to the effects of humans. We've modified the surface of the land in ways that retain heat."

Urban heat islands are the result. Soil and grass have been replaced with materials such as asphalt and concrete that absorb heat during the day and re-radiate it at night, causing higher temperatures.