| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 35°F | 26°F | 3.6" |
| February | 19°F | 38°F | 27°F | 3.0" |
| March | 33°F | 53°F | 42°F | 3.4" |
| April | 43°F | 67°F | 54°F | 5.4" |
| May | 53°F | 76°F | 64°F | 7.4" |
| June | 63°F | 84°F | 73°F | 10.8" |
| July | 65°F | 85°F | 75°F | 8.6" |
| August | 63°F | 84°F | 73°F | 8.8" |
| September | 55°F | 80°F | 66°F | 8.0" |
| October | 44°F | 68°F | 55°F | 8.3" |
| November | 33°F | 54°F | 42°F | 6.4" |
| December | 23°F | 38°F | 30°F | 5.1" |
| Index | Marietta | Illinois | National |
| Air quality index | 41 | 41 | 42 |
| Pollution index | 1,001,710 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 285 |
| Days with moderate air quality | 78 |
| Days w/ poor A.Q. for sensitive groups | 2 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.004% |
| Benzene | 0.040% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.010% |
| Mercury | 0.020% |
| City | Population | Air Quality Index | National Avg. |
| Industry, IL | 540 | 29 | 42.0% |
| Good Hope, IL | 405 | 29 | 42.0% |
| Adair, IL | 179 | 29 | 42.0% |
| Bardolph, IL | 277 | 29 | 42.0% |
| Bushnell, IL | 2,789 | 29 | 42.0% |
| Marietta, IL | 61 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Klukwan, AK | 61 | 20 | 60.0% |
| Jennette, AR | 61 | 45 | 10.0% |
| Yucca, AZ | 61 | 13 | 74.0% |
| Mount Hebron, CA | 61 | 29 | 42.0% |
| Rodriguez Camp, CA | 61 | 67 | 34.0% |
| Marietta, IL | 61 | 41 | 18.0% |
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