| Month | High (°F) | Low (°F) |
|---|---|---|
| January | 34.7 | 18.6 |
| February | 39.7 | 22.4 |
| March | 51.3 | 31.5 |
| April | 63.9 | 41.3 |
| May | 74.1 | 52.4 |
| June | 82.9 | 61.5 |
| July | 85.8 | 65.1 |
| August | 84.4 | 62.8 |
| September | 78.5 | 55.0 |
| October | 66.1 | 43.5 |
| November | 51.3 | 33.0 |
| December | 39.4 | 23.7 |
| Entire Year | 62.8 | 42.7 |
| City | Average (°F) | Record (°F) | Yearly (inches) | |||
|---|---|---|---|---|---|---|
| High | Low | High | Low | Precipitation | Snow | |
| Aledo | 59.8 | 39.3 | 113 | -33 | 36.5 | 21.3 |
| Aurora | 60.0 | 40.2 | 111 | -26 | 39.1 | 31.2 |
| Belleville | 68.7 | 46.7 | 110 | -27 | 42.9 | 11.6 |
| Bloomington-Normal | 62.0 | 40.4 | 114 | -24 | 38.9 | 20.9 |
| Carbondale | 66.9 | 45.2 | 113 | -25 | 48.4 | 11.3 |
| Carlyle | 65.0 | 45.3 | 113 | -22 | 44.5 | n/a |
| Champaign | 62.3 | 42.6 | 109 | -25 | 40.7 | 20.7 |
| Charleston | 64.8 | 44.8 | 110 | -27 | 43.6 | 20.6 |
| Chicago | 60.2 | 44.0 | 106 | -25 | 40.9 | 39.4 |
| Danville | 63.1 | 43.1 | 112 | -26 | 42.4 | 13.0 |
| Decatur | 63.6 | 44.2 | 113 | -25 | 39.8 | 14.7 |
| Dixon | 59.1 | 39.6 | 110 | -32 | 40.2 | 34.1 |
| Du Quoin | 66.8 | 46.5 | 113 | -22 | 46.9 | 11.0 |
| Elgin | 58.9 | 39.2 | 103 | -27 | 39.3 | 31.7 |
| Elizabeth | 58.1 | 36.1 | 103 | -35 | 39.2 | 33.8 |
| Fairfield | 66.6 | 45.2 | 113 | -23 | 45.5 | 13.6 |
| Freeport | 58.0 | 38.0 | 112 | -31 | 38.3 | 32.8 |
| Galesburg | 60.2 | 41.6 | 112 | -28 | 38.8 | 23.6 |
| Geneseo | 60.6 | 41.7 | 107 | -25 | 38.9 | 27.9 |
| Hoopeston | 62.2 | 42.8 | 111 | -25 | 40.5 | 17.8 |
| Jacksonville | 64.0 | 41.3 | 114 | -28 | 39.0 | 18.4 |
| Joliet | 60.0 | 41.2 | 104 | -26 | 37.0 | 16.7 |
| Kankakee | 61.5 | 41.2 | 109 | -29 | 37.8 | 20.5 |
| Kewanee | 59.2 | 39.0 | 105 | -27 | 39.0 | 30.0 |
| Lincoln | 62.8 | 41.4 | 113 | -34 | 39.0 | 19.3 |
| Moline | 61.2 | 41.0 | 104 | -33 | 38.3 | 36.1 |
| Monmouth | 62.0 | 41.2 | 110 | -27 | 38.8 | 25.1 |
| Morris | 60.7 | 41.5 | 109 | -26 | 39.0 | 25.9 |
| Morrison | 60.3 | 38.2 | 112 | -35 | 39.1 | 32.2 |
| Mt. Vernon | 64.2 | 44.7 | 114 | -22 | 45.0 | 11.3 |
| Olney | 65.3 | 44.5 | 112 | -25 | 47.8 | 11.6 |
| Ottawa | 61.1 | 40.9 | 112 | -26 | 35.1 | 22.5 |
| Pana | 64.9 | 43.6 | 115 | -25 | 42.7 | 15.8 |
| Paris | 62.5 | 43.0 | 109 | -23 | 42.5 | 18.8 |
| Peoria | 61.9 | 42.9 | 105 | -25 | 37.5 | 26.0 |
| Quincy | 62.6 | 43.7 | 114 | -29 | 36.2 | n/a |
| Rantoul | 63.3 | 41.2 | 109 | -27 | 39.4 | 10.9 |
| Rockford | 59.0 | 39.5 | 112 | -31 | 37.2 | 38.0 |
| Springfield | 63.7 | 43.7 | 112 | -24 | 38.0 | 21.6 |
| Tuscola | 63.1 | 43.2 | 113 | -26 | 40.9 | 18.0 |