Williamstown, Massachusetts has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 2.7 °C (low) to 13.9 °C (high). Annual precipitation averages 1110 Millimeters. Williamstown is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.2 °C | -0.4 °C | 74.17 mm |
| February | -9.3 °C | 1.1 °C | 54.86 mm |
| March | -4.8 °C | 5.8 °C | 70.87 mm |
| April | 1.4 °C | 13.4 °C | 87.12 mm |
| May | 7.3 °C | 19.9 °C | 88.65 mm |
| June | 12.2 °C | 24.6 °C | 113.28 mm |
| July | 14.8 °C | 26.9 °C | 128.27 mm |
| August | 13.9 °C | 25.9 °C | 108.97 mm |
| September | 10 °C | 22.2 °C | 110.49 mm |
| October | 3.9 °C | 15.4 °C | 106.93 mm |
| November | -1.3 °C | 8.8 °C | 85.09 mm |
| December | -6.1 °C | 2.7 °C | 81.28 mm |
| Annual | 2.7 °C | 13.9 °C | 1109.98 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |