Belfast, Maine has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 1.9 °C (low) to 12.5 °C (high). Annual precipitation averages 1238.3 Millimeters. Belfast is located in USDA Plant Hardiness Zone 5b. 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 | -11.5 °C | -1.3 °C | 101.09 mm |
| February | -10.7 °C | 0.3 °C | 82.55 mm |
| March | -5.8 °C | 4.4 °C | 98.3 mm |
| April | 0.3 °C | 11.1 °C | 112.27 mm |
| May | 5.9 °C | 17.4 °C | 97.03 mm |
| June | 11.2 °C | 22.3 °C | 109.98 mm |
| July | 14.4 °C | 25.5 °C | 79.76 mm |
| August | 13.9 °C | 25.2 °C | 82.55 mm |
| September | 9.7 °C | 21.2 °C | 100.33 mm |
| October | 4 °C | 14.4 °C | 139.45 mm |
| November | -1.2 °C | 7.9 °C | 118.36 mm |
| December | -7.2 °C | 2 °C | 116.59 mm |
| Annual | 1.9 °C | 12.5 °C | 1238.26 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°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 |