Why was the Chicago Heat Wave of 2012 so much less deadly than that of 1995?

Why was the Chicago Heat Wave of 2012 so much less deadly than that of 1995?

In July 1995 some 500-750 people died in Chicago during a torpid 5-day heat wave on July 12-16 that hit the city. This summer, similar temperatures occurred in Chicago on July 3-7 but there were relatively few heat-related fatalities associated with it. Why?



A steamy Chicago during the great July 1995 heat wave. An estimated 500-750 deaths have been attributed to the event. Photo by Gary Braasch, ‘Earth Under Fire’.

A Meteorological Comparison Between July 1995 and July 2012 in Chicago

Comparing the actual temperatures measured during each heat wave we can see that statistically the warmest five consecutive days were virtually the same. The table below shows the temperatures measured at Midway Airport. Midway is a much more relevant site than O’Hare Airport since it rests in South Chicago in the midst of, or close, to the very neighborhoods that most of the fatalities occurred in 1995.



A comparison of actual temperatures over the course of the five hottest days in July 1995 and July 2012 at Midway Airport on the south side of Chicago.

However, a key difference between the two heat waves was the humidity factor. The THI (Temperature-Humidity-Index) was considerably higher in 1995 and the periods of THI above key thresholds much longer. As NWS-Chicago put it in a recent statement on the subject:

“Meteorologically speaking, the most striking difference between the 2 heat waves was certainly the heat indices experienced during the afternoon hours. Generally speaking, heat indices between 95°F and 105°F can be tolerated with appropriate precautions, but when heat indices get over 105°F and particularly up to 115°F the ability of the body to keep its internal temperature regulated dramatically decreases”.

The charts below compare the average daytime heat indices between the 2 heat waves. This clearly shows that the 1995 heat wave was much more intense so far as THI was concerned during the daytime hours:







The maximum THI reached at any one time was 111° in 2012 and 119° in 1995 (the warmest THI on record for Chicago).



A comparison of THI indices over the course of the warmest four and a half days during the heat waves of 1995 and 2012. Graph from NWS-Chicago.

Non-Meteorological Factors Contributing to the Difference in Heat-related Fatalities

The American Journal of Public Health published an article in September 1997 (Vol. 87, No. 9, pp. 1515-1518) titled Mortality in Chicago Attributed to the July 1995 Heat Wave by Steven Whitman et al. One issue the authors put to rest was whether or not the criteria used by the Cook County medical examiners office to certify heat-related deaths were over estimated (Mayor Daley at the time claimed this was the case). Examination of the excess mortality data indicated that this was not the case. The article then focused on how weather played a roll in the excessive death rate. They reported that it was not just the temperature that contributed to the high mortality rates but rather a synoptic combination of temperature, cloud cover, wind speed, and, of course, humidity. But this was not the key problem.

Ultimately the report concluded that weather alone was insufficient to explain the excessive mortality rate (especially when compared to earlier extreme heat waves in Chicago). The real culprit was “a severe public health problem at the time of occurrence”, an oblique reference at the ineptitude of the response of the local government to the heat emergency.



A day-to-day comparison of the daily heat-related deaths to the actual temperature and heat THI index from July 10-20, 1995. Note how the death rate peaked following the actual peak of the highest temperatures. Death from heat does not happen suddenly but is rather a function of time (of consistent extreme heat). Graph from “Mortality in Chicago Attributed to the July 1995 Heat Wave’, American Journal of Public Health, Vol. 87, No. 9, p. 1516.

One thing that apparently has vastly improved in Chicago since 1995 is the local emergency response to extreme weather events (not just heat waves). There is a much improved heat watch/warning/advisory system in place now compared to 1995 thanks to the NWS and the city’s emergency response facilities. And finally of course, and of critical importance, many more inner city households have air-conditioning units at their disposal now than they did in 1995.

Conclusion

As we have seen in the past, Hurricane Katrina comes to mind, it is not necessarily just the weather that plays a roll in mass casualties during extreme weather events, but also non-meteorological factors as well.

Being prepared for ‘worst-case scenarios’, so far as the weather is concerned, seems to be a lesson that takes time to catch on. In the Chicago instance, at least, the lesson seems to have been learned.

REFERENCES: National Weather Service, Chicago ‘A Meteorological Comparison Between July 1995 and July 2012 Heat Wave’, July 23, 2012.

‘Mortality in Chicago Attributed to the July 1995 Heat Wave’, American Journal of Public Health, Vol. 87, No.9, September 1997 by Steven Whitman, PhD; Glen Good, MS; Edmund R. Donahue, MD; Nanette Benbow, MAS; Wenyan Shou, MS; and Shanxuan Mou, MS.

An excellent book about the Chicago 1995 heat wave is ‘Heat Wave: A Social Autopsy of Disaster in Chicago’ by Eric Klineberg.

KUDOS: Steve Gregory for bringing this to my attention.

Christopher C. Burt
Weather Historian