| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 27°F | 44°F | 35°F | 2.5" |
| February | 29°F | 46°F | 37°F | 1.6" |
| March | 40°F | 59°F | 49°F | 4.1" |
| April | 50°F | 70°F | 59°F | 10.7" |
| May | 58°F | 78°F | 68°F | 10.5" |
| June | 67°F | 87°F | 77°F | 13.3" |
| July | 70°F | 88°F | 79°F | 0.5" |
| August | 68°F | 88°F | 77°F | 0.4" |
| September | 61°F | 81°F | 70°F | 1.2" |
| October | 50°F | 70°F | 59°F | 0.8" |
| November | 39°F | 58°F | 48°F | 5.8" |
| December | 32°F | 47°F | 39°F | 1.2" |
| Index | Marion | Illinois | National |
| Air quality index | 43 | 41 | 42 |
| Pollution index | 125,359 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 344 |
| Days with good air quality | 269 |
| Days with moderate air quality | 74 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.003% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Carterville, IL | 5,861 | 43 | 14.0% |
| Pittsburg, IL | 842 | 43 | 14.0% |
| Colp, IL | 234 | 43 | 14.0% |
| Johnston City, IL | 3,414 | 43 | 14.0% |
| Thompsonville, IL | 527 | 43 | 14.0% |
| Marion, IL | 17,539 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Godfrey, IL | 17,540 | 54 | 8.0% |
| Lithia Springs, GA | 17,545 | 52 | 4.0% |
| Bostonia, CA | 17,532 | 64 | 28.0% |
| Wantagh, NY | 17,548 | 51 | 2.0% |
| Manassas Park, VA | 17,548 | 47 | 6.0% |
| Marion, IL | 17,539 | 43 | 14.0% |
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