Portsmouth, New Hampshire has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 38.5 °F (low) to 57.7 °F (high). Annual precipitation averages 48.4 Inches. Portsmouth is located in USDA Plant Hardiness Zone 6a. 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 | 16.3 °F | 33.8 °F | 3.61 in. |
| February | 17.5 °F | 36.4 °F | 3.59 in. |
| March | 25.3 °F | 43.7 °F | 4.41 in. |
| April | 35.1 °F | 55.5 °F | 4.55 in. |
| May | 45.1 °F | 65.6 °F | 3.88 in. |
| June | 54.9 °F | 75.0 °F | 4.24 in. |
| July | 60.8 °F | 81.0 °F | 3.66 in. |
| August | 59.5 °F | 80.0 °F | 3.25 in. |
| September | 52.3 °F | 72.5 °F | 3.8 in. |
| October | 40.9 °F | 60.3 °F | 4.98 in. |
| November | 31.6 °F | 49.6 °F | 3.95 in. |
| December | 22.8 °F | 39.2 °F | 4.46 in. |
| Annual | 38.5 °F | 57.7 °F | 48.38 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 6a : -10°F to -5°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Severe Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |