| 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 | Cornwall | Pennsylvania | National |
| Air quality index | 42 | 45 | 42 |
| Pollution index | 11,091,200 | 4,584,805 | 2,726,193 |
| Measurement | Total |
| Days measured | 360 |
| Days with good air quality | 278 |
| Days with moderate air quality | 78 |
| Days w/ poor A.Q. for sensitive groups | 4 |
| 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. |
| Marietta, PA | 2,616 | 41 | 18.0% |
| Elizabethtown, PA | 11,445 | 41 | 18.0% |
| Rheems, PA | 2,836 | 41 | 18.0% |
| Bainbridge, PA | 1,399 | 41 | 18.0% |
| Maytown, PA | 4,130 | 41 | 18.0% |
| Cornwall, PA | 4,319 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Chester, MD | 4,319 | 46 | 8.0% |
| French Island, WI | 4,318 | 35 | 30.0% |
| Grantley, PA | 4,320 | 41 | 18.0% |
| Riva, MD | 4,321 | 47 | 6.0% |
| Barrow, AK | 4,316 | 38 | 24.0% |
| Cornwall, PA | 4,319 | 42 | 16.0% |
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