Grand Isle, Maine has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from -1.8 °C (low) to 9.6 °C (high). Annual precipitation averages 1083.8 Millimeters. Grand Isle is located in USDA Plant Hardiness Zone 4a. The growing season typically runs from the last frost around May 21 - May 31 through the first frost around Sep. 21 - Sep. 30.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -18.7 °C | -6.9 °C | 83.57 mm |
| February | -17.9 °C | -5.1 °C | 67.56 mm |
| March | -11.3 °C | 0.6 °C | 72.14 mm |
| April | -2.6 °C | 8.2 °C | 74.93 mm |
| May | 4.1 °C | 16.6 °C | 87.88 mm |
| June | 9.2 °C | 21.7 °C | 105.41 mm |
| July | 12.4 °C | 24.6 °C | 108.46 mm |
| August | 11.4 °C | 24 °C | 90.68 mm |
| September | 7 °C | 19.3 °C | 97.28 mm |
| October | 1.2 °C | 11.4 °C | 110.74 mm |
| November | -4.2 °C | 3.9 °C | 92.71 mm |
| December | -12.7 °C | -3.1 °C | 92.46 mm |
| Annual | -1.8 °C | 9.6 °C | 1083.82 mm |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 21 - May 31 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 4a : -34.4°C to -31.7°C |
| Plant Hardiness Zone - 2012 | Zone 3b : -37.2°C to -34.4°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |