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.
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:
Record of Days At or Above 80 Degrees for Chicagoland
THE PREVIOUS RECORD FOR CONSECUTIVE 80 DEGREE OR ABOVE DAYS WAS 42
DAYS IN A ROW...SET IN 1955. ON AUGUST 13 2010...THE CHICAGOLAND
AREA HIT 43 CONSECUTIVE DAYS...AND THE NUMBER CONTINUES TO
INCREASE. JULY 2 2010 WAS THE FIRST 80 DEGREE DAY OF THE CURRENT
RECORD. AS OF 251 PM CDT TODAY...WHEN THE TEMPERATURE AT OHARE
INTERNATIONAL AIRPORT OFFICIALLY REACHED 80 DEGREES DURING
AFTERNOON HEATING...TODAY IS THE 46 CONSECUTIVE DAY FOR THIS
THE TOP TEN RECORDS ARE AS FOLLOWS
1. 46 DAYS...WITH THE STREAK STILL CONTINUING ON AUGUST 16 2010
2. 42 DAYS...WITH THE STREAK ENDING ON AUGUST 7 1955
3. 34 DAYS...WITH THE STREAK ENDING ON SEPTEMBER 6 1995
4. 32 DAYS...WITH THE STREAK ENDING ON AUGUST 8 1983
5. 30 DAYS...WITH THE STREAK ENDING ON JULY 19 1921
6. 29 DAYS...WITH THE STREAK ENDING ON AUGUST 18 1988
7. 27 DAYS...WITH THE STREAK ENDING ON AUGUST 6 1999
8. 27 DAYS...WITH THE STREAK ENDING ON JULY 14 1966
9. 25 DAYS...WITH THE STREAK ENDING ON AUGUST 15 2007
10. 24 DAYS...WITH THE STREAK ENDING ON JULY 26 2005
The state-wide temperature in Illinois for the first half of August (August 1-15) was 79.5 degrees, 5.7 degrees above normal. That should come as no surprise to anyone who experience the hot, humid conditions in August.
The state-wide precipitation for the first half of August was 2.32 inches, just 0.41 inches above normal. The wettest conditions were in the northern third of Illinois with most locations reporting 2 to 5 inches of rain. Meanwhile parts of eastern and southeastern Illinois have been on the dry side, reporting less than 1.0 inches so far in August. The rest of the state was close to normal (around 2 inches).