We officially hit 101 degrees today here in Champaign-Urbana, Illinois, according to the long-term site at the Illinois State Water Survey. That’s the warmest we have been since August 18, 1988. However, it does not beat the record for this date, which was 104 degrees set in 1901.
100 Degree Mark in Illinois
Based on preliminary reports, Geneseo and Bentley Illinois reached 101°F yesterday. Another nine places reached 100°F. They include Illinois City, Moline, Mt. Carroll, Rockford, Prairie City, Normal, Rantoul, Streator, and Urbana. The Chicago Botanical Garden was close with 99°F.
Here is the list with the last time they saw 100°F:
- Geneseo: July 25, 2005
- Bentley: August 11, 2010
- Illinois City: July 24, 2005
- Moline: July 17, 2006
- Mt. Carroll: August 18, 1988
- Rockford: July 10, 1989
- Prairie City: July 26, 2005
- Normal: July 26, 2005
- Rantoul: July 22, 2002
- Streator: June 26, 2009
- Urbana: July 13,
20051995 (thanks Chris G.)
By the way, the last time Chicago at O’Hare reported 100°F was on July 24, 2005.
You notice all these sites were in central and northern Illinois instead of southern Illinois. Much of central and northern Illinois have been dry. As a result, more of the sun’s energy is devoted to warming up the surface and lower atmosphere and less for evaporation and transpiration in plants.
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.
Dangerous Heat and Humidity in Illinois
The National Weather Service (NWS) has issued a variety of heat warnings and advisories for Illinois this week. The combination of temperatures in the upper 90s and high levels of humidity mean a greater risk of heat-related illnesses and death.
One measure that combines both the effects of temperature and humidity is the heat index. Explanations of the heat index can be found on Wikipedia and NWS. Below is a chart showing the heat index for a given temperature and relative humidity. If you like to do your own calculations, here is the NWS heat index calculator.
While the heat index incorporates relative humidity to give a better idea of what the temperature feels like, there are some important underlying assumptions. It assumes a person who is 5′ 7″, 147 lbs, walking at 3 mph, wearing long pants and a short-sleeve shirt, in the shade with a light breeze. It is estimated that working out in the sun would increase the heat index by 15°F.
Here are some resources to consider for monitoring the heat and what to do during the current heat wave:
- National Weather Service: warnings and advisories
- Center for Disease Control: extreme heat guidelines
- Keep Cool Illinois: beat the heat in Illinois
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