Alton, New Hampshire has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 2.7 °C (low) to 13.7 °C (high). Annual precipitation averages 1185.2 Millimeters. 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 | -10.6 °C | -1.1 °C | 89.15 mm |
| February | -10 °C | 0.7 °C | 75.95 mm |
| March | -5.3 °C | 5.4 °C | 89.92 mm |
| April | 0.8 °C | 13 °C | 92.71 mm |
| May | 7.1 °C | 19.8 °C | 86.87 mm |
| June | 12.5 °C | 24.7 °C | 111.76 mm |
| July | 15.6 °C | 27.6 °C | 111.51 mm |
| August | 14.8 °C | 26.6 °C | 89.41 mm |
| September | 10.6 °C | 22.2 °C | 101.85 mm |
| October | 4.3 °C | 14.8 °C | 127 mm |
| November | -1.1 °C | 8 °C | 96.77 mm |
| December | -6.3 °C | 2.2 °C | 112.27 mm |
| Annual | 2.7 °C | 13.7 °C | 1185.17 mm |
| 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 : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |