THE JULY 1995 HEAT-WAVE IN THE MIDWEST - A CLIMATIC PERSPECTIVE AND CRITICAL WEATHER FACTORS

A brief but intense heat wave developed in the central and eastern Uni ted States in mid-July 1995, causing hundreds of fatalities. The most notable feature of this event was the development of very high dewpoin t temperature (T-d) over the southern Great Lakes region and the Upper Mississippi River Basin. At many locations, hourly values of T-d set new records. The combination of high air and dewpoint temperatures res ulted in daily average apparent temperatures exceeding 36 degrees C ov er a large area on some days. A comparison with past heat waves shows that this was the most intense short-duration heat wave in at least th e last 48 years at some locations in the southern Great Lakes region a nd Upper Mississippi River Basin. An analysis of historical data for C hicago, where the majority of fatalities occurred, indicates the inten sity of this heat wave was exceeded only by a few periods in the 1910s and 1930s. Impacts in the Chicago urban center were exacerbated by an urban heat island that raised nocturnal temperatures by more than 2 d egrees C. An analysis of radiosonde data indicates that maximum daytim e boundary layer mixing depths were only a few hundred meters in the c ore region of the heat wave. Simulations using a single-column version of a three-dimensional mesoscale model strongly suggest that this con tributed to the very high values of T-d since soil moisture in the cen tral United States was near to above average and evapotransporation wa s likely high, causing a rapid moistening of the shallow boundary laye r.