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, 2005 1995 (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:

HEAT INDEX
RELATIVE HUMIDITY (%)
Temp. 40 45 50 55 60 65 70 75 80 85 90 95 100
110 136
108 130 137
106 124 130 137
104 119 124 131 137
102 114 119 124 130 137
100 109 114 118 124 129 136
98 105 109 113 117 123 128 134
96 101 104 108 112 116 121 126 132
94 97 100 103 106 110 114 119 124 129 135
92 94 96 99 101 105 108 112 116 121 126 131
90 91 93 95 97 100 103 106 109 113 117 122 127 132
88 88 89 91 93 95 98 100 103 106 110 113 117 121
86 85 87 88 89 91 93 95 97 100 102 105 108 112
84 83 84 85 86 88 89 90 92 94 96 98 100 103
82 81 82 83 84 84 85 86 88 89 90 91 93 95
80 80 80 81 81 82 82 83 84 84 85 86 86 87
Category Heat Index Possible heat disorders for people in high risk groups
Extreme Danger 130° or higher Heat stroke or sunstroke likely.
Danger 105 – 129° Sunstroke, muscle cramps, and/or heat exhaustion likely. Heatstroke possible with prolonged exposure and/or physical activity.
Extreme Caution 90 – 105° Sunstroke, muscle cramps, and/or heat exhaustion possible with prolonged exposure and/or physical activity.
Caution 80 – 90° Fatigue possible with prolonged exposure and/or physical activity.

Record Streak of Warm Days in Chicago

From the National Weather Service

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
RECORD.
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

First Half of August – Warm

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).

Illinois rainfall map
Rainfall map for Illinois for August 1-15, 2010

July 1936 – Hot in Chicago

While this summer has been hot and humid, it pales in comparison to the hottest month in Illinois history – July 1936. Here are the records from July of that year at Midway Airport in Chicago. The temperature hit the triple digit mark nine times that month.  That’s more times than for the entire 1954 to present-day period at Midway Airport. Also, eight of those nine days in July were consecutive.
It was also very dry with only 0.31 inches of precipitation that month. A “T” in the precipitation column means the a trace event – enough to wet the sidewalk but not enough to measure.
The second table has the reports from the University of Chicago site. Those numbers show that conditions closer to the lake were a little more tolerable that summer but still very warm. And this was all before air conditioning was in widespread use.

(111577) CHICAGO MIDWAY AP_IL
Date Precip
(in)
MaxT
(F)
MinT
(F)
1936-07-01 0.00 86 55
1936-07-02 0.00 79 63
1936-07-03 0.00 86 60
1936-07-04 T 91 64
1936-07-05 T 82 68
1936-07-06 0.00 92 68
1936-07-07 0.00 102 70
1936-07-08 0.00 106 73
1936-07-09 0.06 100 71
1936-07-10 0.00 106 77
1936-07-11 T 107 76
1936-07-12 0.00 100 76
1936-07-13 0.00 102 73
1936-07-14 0.00 104 69
1936-07-15 0.00 91 74
1936-07-16 0.00 91 72
1936-07-17 0.00 93 66
1936-07-18 0.14 86 70
1936-07-19 0.05 80 62
1936-07-20 0.00 80 57
1936-07-21 0.00 88 60
1936-07-22 0.00 95 59
1936-07-23 0.06 99 69
1936-07-24 T 78 66
1936-07-25 T 94 64
1936-07-26 T 102 71
1936-07-27 T 85 67
1936-07-28 0.00 98 60
1936-07-29 0.00 74 65
1936-07-30 0.00 78 51
1936-07-31 0.00 83 50
Sum: 0.31 2838 2046
Count: 31 31 31
Average: 0.01 91.5 66.0
M = Missing, T = trace
(111572) UNIVERSITY OF CHICAGO
Date Precip
(in)
MaxT
(F)
MinT
(F)
1936-07-01 0.00 82 58
1936-07-02 0.00 72 63
1936-07-03 T 81 61
1936-07-04 T 82 64
1936-07-05 0.01 74 66
1936-07-06 0.00 84 67
1936-07-07 0.00 95 72
1936-07-08 0.00 95 76
1936-07-09 0.03 95 74
1936-07-10 0.00 102 80
1936-07-11 0.00 97 80
1936-07-12 0.00 89 76
1936-07-13 0.00 91 76
1936-07-14 0.00 96 75
1936-07-15 0.00 86 74
1936-07-16 0.00 82 73
1936-07-17 0.00 84 70
1936-07-18 0.10 81 71
1936-07-19 T 75 70
1936-07-20 0.00 76 67
1936-07-21 0.00 79 66
1936-07-22 0.00 89 66
1936-07-23 0.05 95 70
1936-07-24 T 72 68
1936-07-25 0.00 83 66
1936-07-26 T 90 71
1936-07-27 0.03 86 68
1936-07-28 0.00 87 65
1936-07-29 0.00 73 65
1936-07-30 0.00 74 64
1936-07-31 0.00 78 58
Sum: 0.22 2625 2140
Count: 31 31 31
Average: 0.01 84.7 69.0
M = Missing, T = trace