A Meteorological Comparison Between July 1995 and July 2012 Heat Waves

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While the heat wave earlier this July (3rd-7th )was certainly significant and one of the hottest 3 day periods in Chicago history, the death toll was dramatically less than that of the historic July 12-15, 1995 heat wave. Many non-meteorological factors likely contributed significantly to the lower death toll including: better awareness of heat related dangers, improved response of local government, many more households with access to air conditioning in 2012 than in 1995, and an improved heat watch/warning/advisory system.
 
While the actual air temperatures during the 2012 heat wave were in many instances higher than in 1995, there was substantial difference in dewpoint temperatures. Dewpoints are an absolute measure of how much moisture is in the air and are much more representative than relative humidity of how humid it feels. The higher dewpoints led to much higher heat indices in 1995, particularly during the afternoon hours. Peak heat indices in 1995 reached levels that put much larger percentages of people at risk of succumbing to heat illness, much more so than in the 2012 heat wave.
 
 

comparison of hourly heat indices

 
It’s clear from the chart that there were many more hours with higher heat indices in 1995 than in 2012, but breaking it down further into the number of hours with heat indices above 100°F and above 110°F really tells the story!
 
 
 
 
Number of Hours
 
1995
2012
Heat Index >100°F
42
25
Heat Index >110°F
16
1
 
 
The average temperatures and dewpoints over the 96 hour long periods of most intense heat both years show rather similar values overall, but the story is really in nearly 5°F difference in average dewpoint. These much higher dewpoints more than compensated for the approximately 2°F difference in actual air temperatures.
 
 
 
4 Day Averages
 
Temp
Dewpoint
Heat Index
1995
88.6°F
73.7°F
97.6°F
2012
90.5°F
69.1°F
95.3°F
 
 
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 chart below compares the average daytime heat indices between the 2 heat waves. This clearly shows that the 1995 heat wave was much more intense during the daytime hours.
 
 
Average Daytime (10AM-7PM) Heat Index
 
1995
2012
Day 1
98.8°F
96.6°F
Day 2
114.2°F
103.1°F
Day 3
112.1°F
99.6°F
Day 4
101.3°F
107.5°F
 
 
 
 
 


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