| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 21°F | 39°F | 29°F | 3.2" |
| February | 22°F | 41°F | 31°F | 2.9" |
| March | 35°F | 56°F | 44°F | 3.7" |
| April | 44°F | 68°F | 55°F | 6.8" |
| May | 55°F | 76°F | 65°F | 5.2" |
| June | 64°F | 85°F | 74°F | 8.0" |
| July | 66°F | 87°F | 76°F | 5.2" |
| August | 64°F | 85°F | 74°F | 6.3" |
| September | 57°F | 80°F | 68°F | 6.6" |
| October | 46°F | 68°F | 56°F | 3.3" |
| November | 36°F | 55°F | 45°F | 5.0" |
| December | 26°F | 41°F | 33°F | 5.3" |
| Index | Chapin | Illinois | National |
| Air quality index | 40 | 41 | 42 |
| Pollution index | 479,423 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 360 |
| Days with good air quality | 287 |
| Days with moderate air quality | 73 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Lynnville, IL | 127 | 40 | 20.0% |
| Winchester, IL | 1,717 | 40 | 20.0% |
| Murrayville, IL | 582 | 40 | 20.0% |
| Virginia, IL | 1,382 | 40 | 20.0% |
| Exeter, IL | 72 | 40 | 20.0% |
| Chapin, IL | 536 | 40 | 20.0% |
| City | Population | Air Quality Index | National Avg. |
| Holly Grove, AR | 536 | 43 | 14.0% |
| Cumming, IA | 536 | 39 | 22.0% |
| Bismarck, IL | 536 | 42 | 16.0% |
| Fithian, IL | 536 | 42 | 16.0% |
| Collegeville, IN | 536 | 42 | 16.0% |
| Chapin, IL | 536 | 40 | 20.0% |
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