Soaring temperatures and high humidity have killed more people in the last decade than other weather phenomenon, including destructive storms, according to National Weather Service data. One reason for the fatalities may be that prolonged periods of excessive heat have become more common. Analyzing the temperatures from 172 U.S. weather stations over roughly the last 50 years, from 1948-1999, the study found a threefold increase in the number of heat waves across the country. A heat wave is defined as a four-day period when average temperatures are above the 85th percentile for summer temperatures in the area. That means the threshold for a heat wave in Seattle would be lower than the temperature level of a Louisiana heat wave.
Scientists say urban areas stay hot in the summer because heat is trapped in concrete and metal.
The solid blue swatch of color flowing down the left side is the Mississippi River. The study was released by two nonprofit groups, Ozone Action and Physicians for Social Responsibility. They contend the steady rise in temperatures is a result of global warming caused by human pollution. Honolulu topped the list of cities with the most frequent heat waves. Fla. Tucson, Ariz., Atlanta and Midland, Texas.