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.