Grantham, New Hampshire has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 0.7 °C (low) to 12 °C (high). Annual precipitation averages 1157.2 Millimeters. 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 | -13.2 °C | -2.8 °C | 78.23 mm |
| February | -12.5 °C | -0.9 °C | 68.33 mm |
| March | -7.6 °C | 3.8 °C | 82.8 mm |
| April | -0.5 °C | 11.3 °C | 89.41 mm |
| May | 5.9 °C | 18.4 °C | 92.96 mm |
| June | 11 °C | 23 °C | 113.03 mm |
| July | 13.6 °C | 25.6 °C | 111.76 mm |
| August | 12.6 °C | 24.8 °C | 101.35 mm |
| September | 8.6 °C | 20.7 °C | 97.54 mm |
| October | 2.5 °C | 13.5 °C | 132.84 mm |
| November | -2.8 °C | 6.6 °C | 90.42 mm |
| December | -8.7 °C | 0.3 °C | 98.55 mm |
| Annual | 0.7 °C | 12 °C | 1157.22 mm |
| 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 : -28.9°C to -26.1°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |