Northumberland, New Hampshire has a Subarctic With Cool Summers And Year Around Rainfall climate (Köppen classification: Dfc). Average annual temperatures range from 30.3 °F (low) to 52.9 °F (high). Annual precipitation averages 42.5 Inches. 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 | 4.1 °F | 25.7 °F | 2.57 in. |
| February | 5.1 °F | 29.0 °F | 2.33 in. |
| March | 15.4 °F | 37.9 °F | 2.61 in. |
| April | 28.7 °F | 51.3 °F | 3.31 in. |
| May | 40.0 °F | 65.2 °F | 3.98 in. |
| June | 49.0 °F | 73.3 °F | 4.54 in. |
| July | 53.7 °F | 77.8 °F | 4.43 in. |
| August | 52.1 °F | 76.4 °F | 4.24 in. |
| September | 44.3 °F | 69.1 °F | 3.55 in. |
| October | 34.0 °F | 55.6 °F | 4.42 in. |
| November | 24.4 °F | 42.5 °F | 3.09 in. |
| December | 12.8 °F | 31.1 °F | 3.39 in. |
| Annual | 30.3 °F | 52.9 °F | 42.46 in. |
| 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 : -30°F to -25°F |
| Plant Hardiness Zone - 2012 | Zone 4a : -30°F to -25°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfc - Subarctic With Cool Summers And Year Around Rainfall |