Alton, New Hampshire has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 36.9 °F (low) to 56.6 °F (high). Annual precipitation averages 46.7 Inches. Alton is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 13.0 °F | 30.1 °F | 3.51 in. |
| February | 14.0 °F | 33.3 °F | 2.99 in. |
| March | 22.4 °F | 41.7 °F | 3.54 in. |
| April | 33.4 °F | 55.4 °F | 3.65 in. |
| May | 44.8 °F | 67.7 °F | 3.42 in. |
| June | 54.5 °F | 76.5 °F | 4.4 in. |
| July | 60.1 °F | 81.6 °F | 4.39 in. |
| August | 58.6 °F | 79.8 °F | 3.52 in. |
| September | 51.1 °F | 71.9 °F | 4.01 in. |
| October | 39.8 °F | 58.6 °F | 5.0 in. |
| November | 30.0 °F | 46.4 °F | 3.81 in. |
| December | 20.6 °F | 35.9 °F | 4.42 in. |
| Annual | 36.9 °F | 56.6 °F | 46.66 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 5b : -15°F to -10°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |