| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 22°F | 38°F | 30°F | 5.3" |
| February | 22°F | 41°F | 31°F | 4.5" |
| March | 31°F | 52°F | 41°F | 6.9" |
| April | 41°F | 65°F | 53°F | 6.5" |
| May | 51°F | 74°F | 62°F | 7.6" |
| June | 60°F | 82°F | 71°F | 8.8" |
| July | 64°F | 87°F | 75°F | 7.6" |
| August | 62°F | 83°F | 72°F | 8.9" |
| September | 55°F | 77°F | 66°F | 10.8" |
| October | 44°F | 66°F | 55°F | 8.6" |
| November | 34°F | 54°F | 44°F | 6.0" |
| December | 28°F | 43°F | 35°F | 6.6" |
| Index | Williamstown | Pennsylvania | National |
| Air quality index | 42 | 45 | 42 |
| Pollution index | 3,637,020 | 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 | 0.020% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.010% |
| Mercury | 0.002% |
| City | Population | Air Quality Index | National Avg. |
| Reinerton, PA | 446 | 42 | 16.0% |
| Donaldson, PA | 313 | 42 | 16.0% |
| Locustdale, PA | 133 | 42 | 16.0% |
| Pine Grove, PA | 2,088 | 42 | 16.0% |
| Fredericksburg CDP (Crawford County), PA | 777 | 42 | 16.0% |
| Williamstown, PA | 1,172 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Jennerstown, PA | 1,172 | 36 | 28.0% |
| Saegertown, PA | 1,172 | 36 | 28.0% |
| Buchanan, VA | 1,172 | 38 | 24.0% |
| Challenge-Brownsville, CA | 1,171 | 44 | 12.0% |
| Silverado Resort, CA | 1,171 | 35 | 30.0% |
| Williamstown, PA | 1,172 | 42 | 16.0% |
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