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

The temporal distribution of surface atmospheric water vapor levels during short-duration extreme heat events at Chicago was assessed. Heat events were characterized by at least 3 consecutive days with a minimum temperature greater than 24degreesC (75degreesF) and a maximum temperature greater than 35degreesC (95degreesF). Thirteen such heat events were identified during the 75 yr (1928-2002) 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 21degreesC, rising to an average of 24degreesC during the mid 1990s. The temporal changes in dew-point values were related to 2 factors: (1) the existence of average to above-average 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.