Heat Index Extremes for Illinois

Since heat and humidity is on everyone’s mind these days, I pulled out the record high heat index values that we calculated for the Illinois Climate Atlas from a few years ago.
For that analysis we looked at the few sites with long-term temperature and humidity records. Here is what we found for the highest heat index value at each site:

• Chicago’s record is a heat index of 118 degrees on July 13, 1995 (temperature 100°F, relative humidity 50%)
• Rockford’s record is a heat index of 119 degrees on July 13, 1995 (temperature 98°F, relative humidity 57%)
• Peoria’s record is a heat index of 121 degrees on July 13, 1995 (temperature 99°F, relative humidity 53%)
• Springfield’s record is a heat index of 118 on July 15, 1980 (temperature 98°F, relative humidity 56%)
• St. Louis’s record is a heat index of 119 on July 13, 1995 (temperature of 100°F, relative humidity 51%)

As you may have noticed, most of the cities set their record during the deadly July 1995 heat wave. I included St. Louis because it is just across the river.
I have seen unofficial heat index values even higher from locations at smaller airports.  Some of those sites have reported heat index values in the mid to upper 120s. However, we don’t normally use them for record keeping. For one thing the humidity sensor has a reputation of becoming unreliable at times. For another thing the archive of those observations extends back to 15 years or less at most sites.