This is an excerpt from EERE Network News, a weekly electronic newsletter.

July 10, 2002

Study Confirms That Urban Heat Islands Cause Downwind Rain

Residents of flooded rural areas near San Antonio, Texas, have one more reason to complain about their city-dwelling neighbors: a new study shows that the heat generated by cities helps produce rainstorms downwind from cities. The study, led by the U.S. National Aeronautics and Space Administration's Goddard Space Flight Center, used a rain-measuring satellite to verify similar results obtained in previous ground-based studies. The satellite data demonstrated that major cities - including Nashville, Atlanta, Dallas, and, yes, San Antonio - increased downwind rainfall by about 28 percent within 18 to 36 miles from the cities. In some cities, the downwind area rainfall increased as much as 51 percent. On average, maximum rainfall rates in downwind regions exceeded the maximum values in upwind regions by up to 116 percent.

Cities basically act as giant solar collectors: during summer months, the dark roofs, concrete, asphalt, and other surfaces in the cities absorb a large amount of heat. The resulting "heat island" effect can boost urban temperatures by 5 to 9 degrees Fahrenheit. During the warmer months, the added heat creates wind circulations and rising air that can produce clouds or enhance existing ones. Under the right conditions, these clouds can evolve into rain-producers or storms. See the Goddard Flight Center press release.

Urban heat islands have long been of concern, since they drive up the use of air conditioners, increase ozone levels, and exacerbate health problems. In fact, the city of Toronto hosted the North American Urban Heat Island Summit in early May. The presentations from that summit are now posted on the City of Toronto Web site.

For more information about the urban heat island effect, see the Heat Island Group, part of DOE's Lawrence Berkeley National Laboratory.