Petersham, Massachusetts has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 2.3 °C (low) to 13.5 °C (high). Annual precipitation averages 1219.5 Millimeters. Petersham is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around May 11 - May 20 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.6 °C | -0.5 °C | 92.46 mm |
| February | -9.7 °C | 1.2 °C | 80.52 mm |
| March | -5.4 °C | 5.4 °C | 103.63 mm |
| April | 0.9 °C | 12.7 °C | 96.27 mm |
| May | 6.4 °C | 18.9 °C | 91.44 mm |
| June | 11.9 °C | 23.8 °C | 116.33 mm |
| July | 15.1 °C | 26.9 °C | 103.63 mm |
| August | 13.4 °C | 26 °C | 111 mm |
| September | 9.4 °C | 21.7 °C | 105.41 mm |
| October | 3.8 °C | 14.6 °C | 121.16 mm |
| November | -1.4 °C | 8.3 °C | 94.49 mm |
| December | -6.6 °C | 2.4 °C | 103.12 mm |
| Annual | 2.3 °C | 13.5 °C | 1219.46 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 11 - May 20 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 6a : -23.3°C to -20.6°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |