| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 22°F | 40°F | 31°F | 2.4" |
| February | 22°F | 41°F | 31°F | 2.2" |
| March | 31°F | 52°F | 41°F | 2.8" |
| April | 41°F | 65°F | 52°F | 3.6" |
| May | 51°F | 74°F | 62°F | 3.7" |
| June | 61°F | 82°F | 72°F | 5.4" |
| July | 65°F | 86°F | 75°F | 4.6" |
| August | 62°F | 84°F | 73°F | 3.4" |
| September | 56°F | 78°F | 67°F | 4.7" |
| October | 45°F | 66°F | 55°F | 4.5" |
| November | 35°F | 54°F | 44°F | 2.3" |
| December | 28°F | 45°F | 36°F | 3.3" |
| Index | Marietta | Pennsylvania | National |
| Air quality index | 41 | 45 | 42 |
| Pollution index | 11,091,200 | 4,584,805 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 299 |
| Days with moderate air quality | 66 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.140% |
| Carbon Tetrachloride | 0.030% |
| Lead | 0.310% |
| Mercury | 0.004% |
| City | Population | Air Quality Index | National Avg. |
| Winterstown, PA | 521 | 41 | 18.0% |
| Jacobus, PA | 2,510 | 41 | 18.0% |
| Red Lion, PA | 6,334 | 41 | 18.0% |
| Valley View, PA | 2,956 | 41 | 18.0% |
| East Prospect, PA | 756 | 41 | 18.0% |
| Marietta, PA | 2,616 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Division No. 22, Unorganized, MB | 2,616 | n/a | |
| Unionville, TN | 2,615 | 31 | 38.0% |
| Ruleville, MS | 2,617 | 43 | 14.0% |
| Freemansburg, PA | 2,617 | 42 | 16.0% |
| Cedaredge, CO | 2,614 | 44 | 12.0% |
| Marietta, PA | 2,616 | 41 | 18.0% |
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