Stewartstown, New Hampshire has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from -1.4 °C (low) to 9 °C (high). Annual precipitation averages 1273.3 Millimeters. Stewartstown 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.11 - Sep. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -16.8 °C | -6.3 °C | 91.19 mm |
| February | -15.9 °C | -4.9 °C | 77.47 mm |
| March | -10.6 °C | 0.1 °C | 92.46 mm |
| April | -1.7 °C | 8.2 °C | 92.71 mm |
| May | 3.9 °C | 15.7 °C | 114.55 mm |
| June | 9.3 °C | 20.7 °C | 132.08 mm |
| July | 12.2 °C | 23 °C | 122.17 mm |
| August | 11.1 °C | 22.3 °C | 115.82 mm |
| September | 7 °C | 18.3 °C | 102.11 mm |
| October | 0.9 °C | 10.7 °C | 123.95 mm |
| November | -4.9 °C | 3.3 °C | 99.06 mm |
| December | -11.8 °C | -2.7 °C | 109.73 mm |
| Annual | -1.4 °C | 9 °C | 1273.3 mm |
| Average First Frost Date | Sep.11 - Sep. 20 |
| 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 3b : -37.2°C to -34.4°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |