Grantham, New Hampshire has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 33.3 °F (low) to 53.6 °F (high). Annual precipitation averages 45.6 Inches. Grantham is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around May 21 - May 31 through the first frost around Sep. 21 - Sep. 30.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 8.3 °F | 27.0 °F | 3.08 in. |
| February | 9.5 °F | 30.3 °F | 2.69 in. |
| March | 18.4 °F | 38.8 °F | 3.26 in. |
| April | 31.1 °F | 52.4 °F | 3.52 in. |
| May | 42.6 °F | 65.2 °F | 3.66 in. |
| June | 51.8 °F | 73.4 °F | 4.45 in. |
| July | 56.5 °F | 78.0 °F | 4.4 in. |
| August | 54.6 °F | 76.6 °F | 3.99 in. |
| September | 47.5 °F | 69.3 °F | 3.84 in. |
| October | 36.5 °F | 56.3 °F | 5.23 in. |
| November | 26.9 °F | 43.8 °F | 3.56 in. |
| December | 16.4 °F | 32.6 °F | 3.88 in. |
| Annual | 33.3 °F | 53.6 °F | 45.56 in. |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 21 - May 31 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 5a : -20°F to -15°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |