Relating changes in agricultural practices to increasing dew points in extreme Chicago heat waves

The temporal distribution of surface atmospheric water vapor levels during shortduration extreme heat events at Chicago was assessed. Heat events were characterized by at least 3 consecutive days with a minimum temperature greater than 24C (75F) and a maximum temperature greater than 35C (95F). Thirteen such heat events were identified during the 75 yr record at Chicago Midway Airport located inside the city. Average dew points in events fluctuated considerably over time, but generally increased. Event average hourly dew-point values before the mid 1980s were generally below 21C, rising to an average of 24C during the mid 1990s. The temporal changes in dew-point values were related to 2 factors: (1) the existence of average to aboveaverage regional precipitation anomalies occurring in May and during the 20 d period prior to the heat event, and (2) changes in agricultural practices that have led to enhanced evapotranspiration rates in the Midwest.