Grand Isle, Maine has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 28.7 °F (low) to 49.3 °F (high). Annual precipitation averages 42.7 Inches. 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 | -1.7 °F | 19.5 °F | 3.29 in. |
| February | -0.2 °F | 22.9 °F | 2.66 in. |
| March | 11.6 °F | 33.1 °F | 2.84 in. |
| April | 27.4 °F | 46.8 °F | 2.95 in. |
| May | 39.3 °F | 61.9 °F | 3.46 in. |
| June | 48.6 °F | 71.0 °F | 4.15 in. |
| July | 54.3 °F | 76.3 °F | 4.27 in. |
| August | 52.6 °F | 75.2 °F | 3.57 in. |
| September | 44.6 °F | 66.7 °F | 3.83 in. |
| October | 34.1 °F | 52.6 °F | 4.36 in. |
| November | 24.5 °F | 39.0 °F | 3.65 in. |
| December | 9.2 °F | 26.5 °F | 3.64 in. |
| Annual | 28.7 °F | 49.3 °F | 42.67 in. |
| 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 : -30°F to -25°F |
| Plant Hardiness Zone - 2012 | Zone 3b : -35°F to -30°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |