Sullivan, Maine has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 2.4 °C (low) to 12.3 °C (high). Annual precipitation averages 1286.8 Millimeters. Sullivan is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -9.8 °C | -1.3 °C | 108.97 mm |
| February | -9.2 °C | 0.1 °C | 91.95 mm |
| March | -5.4 °C | 4.2 °C | 112.78 mm |
| April | 0.2 °C | 10.8 °C | 109.73 mm |
| May | 5.8 °C | 16.9 °C | 104.14 mm |
| June | 10.7 °C | 21.9 °C | 101.35 mm |
| July | 14.2 °C | 25.2 °C | 82.3 mm |
| August | 13.7 °C | 24.9 °C | 74.93 mm |
| September | 9.7 °C | 20.9 °C | 99.82 mm |
| October | 4.6 °C | 14.2 °C | 138.68 mm |
| November | -0.2 °C | 7.8 °C | 130.05 mm |
| December | -5.9 °C | 2.1 °C | 132.08 mm |
| Annual | 2.4 °C | 12.3 °C | 1286.78 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 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 | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |