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.

Cooling Degree Day accumulations at selected cities.
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.