Summer Heatwaves Intense summer heatwaves are the nation's and Chicago's leading cause of weather deaths. Extreme heat, when it occurs in northern sections of the U.S. that are unaccustomed to dealing with it, is especially threatening to the urban elderly. When accompanied by high humidity, the deadly nature of extreme heat is greatly magnified. A four-day siege of stifling heat and humidity in mid-July 1995 resulted in more than 700 deaths in Chicago and 160 in Milwaukee.