| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 33°F | 25°F | 2.9" |
| February | 17°F | 32°F | 24°F | 2.5" |
| March | 31°F | 52°F | 39°F | 4.5" |
| April | 39°F | 65°F | 50°F | 6.4" |
| May | 50°F | 76°F | 62°F | 3.5" |
| June | 61°F | 86°F | 73°F | 6.1" |
| July | 61°F | 86°F | 73°F | 7.8" |
| August | 64°F | 85°F | 73°F | 9.2" |
| September | 55°F | 82°F | 67°F | 3.9" |
| October | 45°F | 69°F | 56°F | 1.9" |
| November | 30°F | 52°F | 40°F | 4.2" |
| December | 17°F | 31°F | 23°F | 1.5" |
| Index | Milledgeville | Illinois | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 2,097,740 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 364 |
| Days with good air quality | 268 |
| Days with moderate air quality | 96 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.060% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.001% |
| Mercury | 0.300% |
| City | Population | Air Quality Index | National Avg. |
| Shannon, IL | 745 | 42 | 16.0% |
| Morrison, IL | 3,850 | 42 | 16.0% |
| Sterling, IL | 14,774 | 42 | 16.0% |
| Forreston, IL | 1,392 | 42 | 16.0% |
| Chadwick, IL | 529 | 42 | 16.0% |
| Milledgeville, IL | 987 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Shelby, AL | 987 | 53 | 6.0% |
| Alamosa East, CO | 987 | 16 | 68.0% |
| Arcadia, IA | 987 | 37 | 26.0% |
| De Soto, IA | 987 | 39 | 22.0% |
| Sharon Springs, KS | 987 | 15 | 70.0% |
| Milledgeville, IL | 987 | 42 | 16.0% |
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