Thornton, New Hampshire has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 33.8 °F (low) to 53 °F (high). Annual precipitation averages 51.4 Inches. Thornton is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Sep. 21 - Sep. 30.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 9.1 °F | 27.3 °F | 3.48 in. |
| February | 10.4 °F | 29.8 °F | 3.12 in. |
| March | 19.4 °F | 37.9 °F | 3.47 in. |
| April | 31.3 °F | 51.4 °F | 4.21 in. |
| May | 42.9 °F | 64.7 °F | 4.4 in. |
| June | 51.8 °F | 72.6 °F | 4.62 in. |
| July | 56.6 °F | 77.2 °F | 4.98 in. |
| August | 55.3 °F | 75.7 °F | 4.61 in. |
| September | 47.8 °F | 68.2 °F | 4.35 in. |
| October | 36.8 °F | 55.2 °F | 5.61 in. |
| November | 27.4 °F | 43.0 °F | 3.97 in. |
| December | 17.1 °F | 32.7 °F | 4.62 in. |
| Annual | 33.8 °F | 53 °F | 51.44 in. |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | Over 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 | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |