THE 1995 CHICAGO HEAT-WAVE - HOW LIKELY IS A RECURRENCE

The deadly heat wave of July 1995 that affected much of the U.S. midwe st, most notably Chicago, Illinois, has been put into historical persp ective. The heat wave has been found to be remarkably unusual, but onl y partially because of the extreme high apparent temperatures (an inde x of the combined effect of temperature and humidity on humans), when the authors calculate a return period of the peak apparent temperature of less than or equal to 23 yr. Of greater significance were the very high temperatures that persisted day and night over an extended 48-h period. Analysis presented here indicates that for Chicago such an ext ended period of continuously high day and night apparent temperature i s unprecedented in modem times. The 2-day period where the minimum app arent temperature failed to go below 31.5 degrees C (89 degrees F) is calculated to be an extremely rare event (probability of occurrence < 0.1%) based on a 10 000-yr-long simulation of a four-parameter (temper atures related to the mean, the intraseasonal daily variance, the inte rannual variance, and the day-to-day persistence of temperature) proba bilistic model. Such unusual heat waves evoke questions related to the future course of the climate and whether this recent event was merely an extreme anomaly or part of an ongoing trend toward more extreme he at waves. A Monte Carlo analysis of trends (1948-95) for various quant iles of the hourly apparent temperatures during the most severe heat w aves each year from 26 midwestern stations reveals a modest, statistic ally insignificant increase of apparent temperatures for a wide range of quantiles without the inclusion of 1995 data. There is a statistica lly significant increase in apparent temperature with its inclusion, r eflected most strongly for upper quantiles or daytime temperatures. It is argued, however, that because of the impact of changes in instrume ntation at primary National Weather Service stations, the potential af fects of urbanization, and little trend of summer mean temperatures, i t is unlikely that the macroscale climate of heat waves in the Midwest or in Chicago is changing in any significant manner. Trends notwithst anding, the authors demonstrate the difficulty associated with project ing changes in the frequency and severity of similar types of events, even if the mean apparent temperature could be accurately predicted fo r the next century, for example, global warming projections. This is d emonstrated using Chicago temperatures. The authors show that accurate projections of the frequency, severity, and duration of heat waves in the Midwest require accurate projections not only of the mean, the in terannual variance, the intraseasonal variance, and day-to-day persist ence, but also the interrelationships among these quantities within di fferent synoptic-climatic regimes.