Alstead, New Hampshire has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 36.4 °F (low) to 54.9 °F (high). Annual precipitation averages 49.7 Inches. 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 | 12.5 °F | 29.0 °F | 3.43 in. |
| February | 13.9 °F | 32.0 °F | 3.12 in. |
| March | 21.8 °F | 39.9 °F | 3.68 in. |
| April | 33.4 °F | 53.8 °F | 4.02 in. |
| May | 44.1 °F | 65.8 °F | 4.07 in. |
| June | 54.4 °F | 73.9 °F | 4.66 in. |
| July | 59.8 °F | 78.7 °F | 4.77 in. |
| August | 56.9 °F | 77.4 °F | 4.49 in. |
| September | 49.5 °F | 70.3 °F | 4.31 in. |
| October | 39.8 °F | 57.6 °F | 5.09 in. |
| November | 29.4 °F | 45.5 °F | 3.77 in. |
| December | 20.7 °F | 34.7 °F | 4.3 in. |
| Annual | 36.4 °F | 54.9 °F | 49.71 in. |
| 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 : -15°F to -10°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |