Petersham, Massachusetts has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 36.1 °F (low) to 56.2 °F (high). Annual precipitation averages 48 Inches. 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 | 13.0 °F | 31.1 °F | 3.64 in. |
| February | 14.5 °F | 34.1 °F | 3.17 in. |
| March | 22.2 °F | 41.7 °F | 4.08 in. |
| April | 33.6 °F | 54.9 °F | 3.79 in. |
| May | 43.6 °F | 66.1 °F | 3.6 in. |
| June | 53.5 °F | 74.9 °F | 4.58 in. |
| July | 59.2 °F | 80.5 °F | 4.08 in. |
| August | 56.1 °F | 78.8 °F | 4.37 in. |
| September | 48.9 °F | 71.0 °F | 4.15 in. |
| October | 38.8 °F | 58.3 °F | 4.77 in. |
| November | 29.5 °F | 46.9 °F | 3.72 in. |
| December | 20.2 °F | 36.4 °F | 4.06 in. |
| Annual | 36.1 °F | 56.2 °F | 48.01 in. |
| 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 : -10°F to -5°F |
| Plant Hardiness Zone - 2012 | Zone 5b : -15°F to -10°F |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |