Like most sites in Illinois, Chicago has experienced large temperature swings in October. The month started with below-normal temperatures on October 2-5. The departures were as large as 8 degrees below normal on October 3 and 4.
This cold period was followed by a period of much above normal temperatures on October 6-12. The departures were as large as 16 degrees above normal on October 9 and 10. In fact, the high of 86 degrees reported at O’Hare on October 9 tied the record set in 1962.
Slightly above-normal temperatures prevailed on October 13-18. The early cold period and later warm periods resulted in an average of 58.7 degrees for the first 18 days in Chicago, 3.9 degrees above normal.
September Climate Summary
Illinois experienced temperatures close to normal for September. The statewide average temperature was 66.8 degrees, just 0.6 degrees above normal.
During the month, many stations in southern Illinois had reported high temperatures in the mid to upper 90s. Fairfield and Grayville both reported the highest temperatures in the state with 99 degrees on September 21 and 22, respectively.
The statewide average precipitation was 4.0 inches, 0.8 inches above normal. However, the rainfall was unevenly distributed throughout the state. The largest rainfall totals for the month occurred across west-central Illinois. Quincy Lock and Dam 21 reported 8.10 inches, the most rainfall in the state. This was followed closely by Springfield with 7.94 inches.
Meanwhile, far southern Illinois continued to struggle with drought conditions. The U.S. Drought Monitor lists several counties in southeast Illinois in the category of moderate drought.
Historical Record High Temperatures
With the recent warm weather, have you wondered what the record high temperatures are for Chicago and elsewhere? In the last few years, a group composed of NWS, Regional Climate Centers, and State Climatologists stitched together the weather records for 270 major metropolitan areas. The results of this project can be found at http://threadex.rcc-acis.org/
The sites in Illinois include Chicago, Moline, Peoria, Rockford, and Springfield. Amongst other things, the program reports the daily record high and low temperature and precipitation.
Here I selected the record highs for Chicago and pulled out the results for September. The program gives you the top three candidates and their dates.
Threaded Climate Extremes for Chicago Area, IL Period of record: 1872 - 2009 Date Highest Maximum Temperatures (degrees F) Top Record 2nd Record 3rd Record 9/1 101 in 1953 96 in 1984 95 in 1960+ 9/2 101 in 1953 98 in 1922 97 in 1913 9/3 97 in 1953 95 in 1960 95 in 1947 9/4 95 in 1983 95 in 1960 95 in 1954 9/5 98 in 1899 97 in 1954 95 in 1983+ 9/6 97 in 1960 97 in 1954 96 in 1990+ 9/7 100 in 1960 100 in 1939 99 in 1985 9/8 96 in 1960 96 in 1959 96 in 1933+ 9/9 95 in 1983 95 in 1959 95 in 1955 9/10 95 in 1983 94 in 1964 94 in 1931+ 9/11 95 in 1952 92 in 1908 92 in 1895 9/12 96 in 1952 94 in 1939 93 in 1962 9/13 98 in 1939 95 in 1927 94 in 2005+ 9/14 99 in 1939 95 in 1927 95 in 1893 9/15 99 in 1939 94 in 1927 92 in 1955 9/16 92 in 1955 92 in 1931 89 in 1948+ 9/17 93 in 1955 90 in 1988 90 in 1891 9/18 94 in 1955 92 in 1953 90 in 1963+ 9/19 93 in 1955 92 in 1963 92 in 1948 9/20 91 in 1931 91 in 1895 90 in 1980+ 9/21 92 in 1970 90 in 1931 90 in 1924+ 9/22 92 in 1956 90 in 1986 90 in 1959+ 9/23 91 in 1937 88 in 1892 87 in 1945+ 9/24 91 in 1891 90 in 2007 90 in 1920 9/25 90 in 1933 89 in 1920 89 in 1900 9/26 90 in 1998 87 in 1973 86 in 1999+ 9/27 91 in 1971 89 in 1987 89 in 1954 9/28 92 in 1953 90 in 1952 89 in 1971 9/29 99 in 1953 87 in 1921 87 in 1898 9/30 92 in 1971 88 in 1943 87 in 1952 + indicates same value also occurred in a previous year.
Cooling Degree Days Up in Illinois This Year
As of August 16, cooling degree days (CDD) are well above normal this year in Illinois. The statewide average is 1051 CDDs, which is 268 CDDs above normal or 134 percent of normal. Last year the statewide average through this date was only 640 CDDs.
| City | Accumulated | Departure from Normal | Percent of Normal |
|---|---|---|---|
| Chicago | 961 | +284 | 145% |
| Rockford | 778 | +175 | 129% |
| Moline | 1028 | +278 | 137% |
| Peoria | 1044 | +280 | 137% |
| Quincy | 1123 | +294 | 135% |
| Springfield | 1214 | +338 | 139% |
| St Louis | 1583 | +442 | 139% |
Cooling degree days are calculated by subtracting the mean daily temperature by 65 degrees. Results above zero are accumulated over time. So if the mean temperature for the day was 75 degrees, then 75 – 65 = 10 cooling degree days. Accumulated over a season and compared to normal gives a relative idea of potential cooling costs. The higher the number, the higher the cooling costs.
Other factors that can influence cooling costs are humidity levels and solar radiation. Personally, I ran the A/C at home pretty much every day since mid-May. Last year I ran it for about two weeks the entire summer.