Portland, Maine has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 4.4 °C (low) to 13.2 °C (high). Annual precipitation averages 1205.2 Millimeters. Portland 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. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -8.7 °C | 0 °C | 88.9 mm |
| February | -7.6 °C | 1.4 °C | 89.15 mm |
| March | -3.1 °C | 5.2 °C | 102.36 mm |
| April | 2.7 °C | 11.5 °C | 108.97 mm |
| May | 8.1 °C | 17.2 °C | 91.69 mm |
| June | 13.4 °C | 22.6 °C | 107.7 mm |
| July | 16.6 °C | 25.8 °C | 87.12 mm |
| August | 16.4 °C | 25.5 °C | 84.07 mm |
| September | 12.2 °C | 21.6 °C | 102.36 mm |
| October | 6.2 °C | 15 °C | 120.65 mm |
| November | 0.7 °C | 8.9 °C | 106.68 mm |
| December | -4.6 °C | 3.4 °C | 115.57 mm |
| Annual | 4.4 °C | 13.2 °C | 1205.22 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| 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 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |