Northumberland, New Hampshire has a Subarctic With Cool Summers And Year Around Rainfall climate (Köppen classification: Dfc). Average annual temperatures range from -0.9 °C (low) to 11.6 °C (high). Annual precipitation averages 1078.5 Millimeters. Northumberland is located in USDA Plant Hardiness Zone 4a. 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 | -15.5 °C | -3.5 °C | 65.28 mm |
| February | -14.9 °C | -1.7 °C | 59.18 mm |
| March | -9.2 °C | 3.3 °C | 66.29 mm |
| April | -1.8 °C | 10.7 °C | 84.07 mm |
| May | 4.4 °C | 18.4 °C | 101.09 mm |
| June | 9.4 °C | 22.9 °C | 115.32 mm |
| July | 12.1 °C | 25.4 °C | 112.52 mm |
| August | 11.2 °C | 24.7 °C | 107.7 mm |
| September | 6.8 °C | 20.6 °C | 90.17 mm |
| October | 1.1 °C | 13.1 °C | 112.27 mm |
| November | -4.2 °C | 5.8 °C | 78.49 mm |
| December | -10.7 °C | -0.5 °C | 86.11 mm |
| Annual | -0.9 °C | 11.6 °C | 1078.49 mm |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 21 - May 31 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 4a : -34.4°C to -31.7°C |
| Plant Hardiness Zone - 2012 | Zone 4a : -34.4°C to -31.7°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfc - Subarctic With Cool Summers And Year Around Rainfall |