Alstead, New Hampshire has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 2.4 °C (low) to 12.7 °C (high). Annual precipitation averages 1262.6 Millimeters. Alstead is located in USDA Plant Hardiness Zone 5b. 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 | -10.8 °C | -1.7 °C | 87.12 mm |
| February | -10.1 °C | 0 °C | 79.25 mm |
| March | -5.7 °C | 4.4 °C | 93.47 mm |
| April | 0.8 °C | 12.1 °C | 102.11 mm |
| May | 6.7 °C | 18.8 °C | 103.38 mm |
| June | 12.4 °C | 23.3 °C | 118.36 mm |
| July | 15.4 °C | 25.9 °C | 121.16 mm |
| August | 13.8 °C | 25.2 °C | 114.05 mm |
| September | 9.7 °C | 21.3 °C | 109.47 mm |
| October | 4.3 °C | 14.2 °C | 129.29 mm |
| November | -1.4 °C | 7.5 °C | 95.76 mm |
| December | -6.3 °C | 1.5 °C | 109.22 mm |
| Annual | 2.4 °C | 12.7 °C | 1262.64 mm |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 21 - May 31 |
| 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 | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |