Hot nights in the city. (Climate Change).

U.S. cities average 10 more hot summer nights--classified as above 70[degrees]F in the East, South, and Midwest, and above 80[degrees]F in the Southwest--than they did 40 years ago, climate researchers have found. While long-time city dwellers may have already suspected as much, the recent study surprised researchers by showing, nationwide, a 300% greater rate of warming in cities than in the rural countryside.

The study, by Arthur DeGaetano, an associate professor of Earth and atmospheric sciences at Cornell University, and Robert J. Allen, then a research support specialist at the university, appeared in the November 2002 issue of the Journal of Climate. The scientists analyzed historical data on daily high and low temperatures from 361 weather stations across the United States from 1910 to 1996, adjusting for omissions, differences in observation times, and other discontinuities (weather stations have been manned largely by volunteers since the station networks inception around 1900). The result is a first-of-its-kind analysis of day-to-day extremes--the hottest 10%, 5%, and 1% of all the daily high or low temperatures recorded by a station over its period of operation.

For the period 1960-1996, the pair found that 75% of stations showed an average increase in both hot summer days and hot summer nights. The rate of warming was greatest in the East and least in the central section, reflecting those regions' relative population growth and urbanization. The results also showed that cities are warming at more than triple the rate of rural locales.

What's to blame for the sharp rise in hot city nights--urbanization? global warming? natural climate variation? thermal pollution (cities generating and retaining more heat)? All contribute, DeGaetano says, "but how big each component is, I'm not sure."

Kenneth Kunkel, head of the Atmospheric Environment Section of the Illinois State Water Survey at the University of Illinois, says that the very hot nights may have been a major factor in the number of deaths during the 1995 Chicago heat wave. "There was no chance for people to get a break--the effect on the body accumulated over time," he says. That heat wave killed more than 700 people in just four days. Yet the average temperature that month was close to normal.

Extreme day-to-day temperatures also cause buckling railroads, failed crops, empty reservoirs, and power blackouts, DeGaetano says. Understanding extreme trends could help power companies plan for increased generation capacity and emergency management services for opening more shelters.

"Everybody looks at annual average temperatures for the globe, but it's much more relevant to look at extremes," says David Easterling, chief of scientific services at the National Oceanic and Atmospheric Administration's National Climatic Data Center. "You have to start looking at what is changing on shorter and shorter time scales, such as changes in daily temperature extremes, to get an idea of what factors are really influencing the climate."