Southwest Harbor, Maine has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 3 °C (low) to 12 °C (high). Annual precipitation averages 1355.6 Millimeters. Southwest Harbor is located in USDA Plant Hardiness Zone 6a. 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 | -9.3 °C | -0.4 °C | 116.84 mm |
| February | -8.6 °C | 0.5 °C | 97.79 mm |
| March | -4.2 °C | 4.2 °C | 120.4 mm |
| April | 1.3 °C | 10 °C | 114.55 mm |
| May | 6.4 °C | 15.7 °C | 111 mm |
| June | 11.1 °C | 20.9 °C | 106.68 mm |
| July | 14.4 °C | 24.1 °C | 82.3 mm |
| August | 14.3 °C | 23.9 °C | 76.45 mm |
| September | 10.9 °C | 20.1 °C | 104.14 mm |
| October | 5.4 °C | 13.9 °C | 144.02 mm |
| November | 0.2 °C | 8.3 °C | 140.46 mm |
| December | -5.6 °C | 2.9 °C | 140.97 mm |
| Annual | 3 °C | 12 °C | 1355.6 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 6a : -23.3°C to -20.6°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 |