The Nature and Impacts of the July 1999 Heat Wave in the Midwestern United States: Learning from the Lessons of 1995

Abstract

The July 1999 heat wave in the Midwest was an event of relatively long duration punctuated by extreme conditions during its last 2 days. The intensity of the heat wave on 29 and 30 July rivaled that of the 1995 heat wave that killed more than 1000 people in the central United States. In 1999, however, the death toll was about one?fourth of this amount in the same region. The 1999 heat wave 2?day maximum apparent temperature was slightly less than during the 1995 heat wave at most Midwestern first?order stations. In addition, the 2?day peak was preceded by several hot days that allowed some short?term acclimatization to occur prior to the intense final days. In Chicago, conditions during the peak of the 1999 heat wave were very similar to those during the 1995 heat wave peak, especially the extreme nocturnal conditions of temperatures and humidity. Therefore, it seems unlikely that the reduction in the heat wave death toll in Chicago from about 700 in 1995 to 114 in 1999 is due solely to meteorological differences between the two heat waves. In St. Louis, the 1999 heat wave was intense for a much longer duration than the 1995 heat wave, thus partially explaining the increase in heat?related deaths there from the 1995 event to the 1999 event. An examination of heat wave response efforts in both Chicago and St. Louis leads to the conclusion that both cities were quite effective at mitigating their respective heat wave mortality rates, which in the 1999 event were almost exactly the same in both metropolitan areas. This represents a great improvement for the city of Chicago compared to the 1995 heat wave. Suggestions are made for further improving municipal heat wave response efforts based on the 1999 experience.