Portsmouth, New Hampshire has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 3.6 °C (low) to 14.3 °C (high). Annual precipitation averages 1228.8 Millimeters. 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 | -8.7 °C | 1 °C | 91.69 mm |
| February | -8.1 °C | 2.4 °C | 91.19 mm |
| March | -3.7 °C | 6.5 °C | 112.01 mm |
| April | 1.7 °C | 13.1 °C | 115.57 mm |
| May | 7.3 °C | 18.7 °C | 98.55 mm |
| June | 12.7 °C | 23.9 °C | 107.7 mm |
| July | 16 °C | 27.2 °C | 92.96 mm |
| August | 15.3 °C | 26.7 °C | 82.55 mm |
| September | 11.3 °C | 22.5 °C | 96.52 mm |
| October | 4.9 °C | 15.7 °C | 126.49 mm |
| November | -0.2 °C | 9.8 °C | 100.33 mm |
| December | -5.1 °C | 4 °C | 113.28 mm |
| Annual | 3.6 °C | 14.3 °C | 1228.84 mm |
| 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 : -23.3°C to -20.6°C |
| Plant Hardiness Zone - 2012 | Zone 6a : -23.3°C to -20.6°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |