| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 20°F | 38°F | 29°F | 2.4" |
| February | 20°F | 40°F | 30°F | 1.9" |
| March | 29°F | 51°F | 39°F | 2.6" |
| April | 39°F | 65°F | 52°F | 3.4" |
| May | 49°F | 75°F | 61°F | 3.8" |
| June | 59°F | 83°F | 71°F | 3.9" |
| July | 62°F | 86°F | 74°F | 4.7" |
| August | 60°F | 84°F | 71°F | 4.2" |
| September | 53°F | 78°F | 64°F | 4.4" |
| October | 43°F | 66°F | 53°F | 4.5" |
| November | 32°F | 54°F | 42°F | 3.0" |
| December | 26°F | 43°F | 34°F | 3.2" |
| Index | Northumberland | Pennsylvania | National |
| Air quality index | 35 | 45 | 42 |
| Pollution index | 3,637,020 | 4,584,805 | 2,726,193 |
| Measurement | Total |
| Days measured | 361 |
| Days with good air quality | 340 |
| Days with moderate air quality | 21 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.020% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.010% |
| Mercury | 0.002% |
| City | Population | Air Quality Index | National Avg. |
| Linntown, PA | 1,799 | 35 | 30.0% |
| Vicksburg, PA | 283 | 35 | 30.0% |
| Kratzerville, PA | 336 | 35 | 30.0% |
| Penns Creek, PA | 507 | 35 | 30.0% |
| Freeburg, PA | 793 | 35 | 30.0% |
| Northumberland, PA | 3,732 | 35 | 30.0% |
| City | Population | Air Quality Index | National Avg. |
| Albia, IA | 3,732 | 39 | 22.0% |
| Larned, KS | 3,732 | 15 | 70.0% |
| Stanley, NC | 3,734 | 36 | 28.0% |
| San Andreas, CA | 3,729 | 39 | 22.0% |
| Collinsville, CT | 3,729 | 37 | 26.0% |
| Northumberland, PA | 3,732 | 35 | 30.0% |
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