Williamstown, Pennsylvania has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.7 °C (low) to 15.7 °C (high). Annual precipitation averages 1295.7 Millimeters. Williamstown is located in USDA Plant Hardiness Zone 6b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -7.2 °C | 1.9 °C | 89.66 mm |
| February | -6.4 °C | 3.3 °C | 71.88 mm |
| March | -2.1 °C | 8.2 °C | 103.63 mm |
| April | 3.8 °C | 15.7 °C | 108.46 mm |
| May | 9.3 °C | 21.3 °C | 110.24 mm |
| June | 14.2 °C | 25.9 °C | 123.44 mm |
| July | 16.3 °C | 28.5 °C | 122.68 mm |
| August | 15.3 °C | 27.7 °C | 116.59 mm |
| September | 11.4 °C | 23.9 °C | 132.08 mm |
| October | 5.3 °C | 17.3 °C | 118.36 mm |
| November | 0.2 °C | 10.3 °C | 97.79 mm |
| December | -3.9 °C | 4.5 °C | 100.84 mm |
| Annual | 4.7 °C | 15.7 °C | 1295.65 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 6b : -20.6°C to -17.8°C |
| Plant Hardiness Zone - 2012 | Zone 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |