Cornwall, Connecticut has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 3 °C (low) to 15.3 °C (high). Annual precipitation averages 1174.7 Millimeters. Cornwall 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. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -9.4 °C | 1.4 °C | 81.28 mm |
| February | -8.6 °C | 3.2 °C | 65.02 mm |
| March | -4.2 °C | 7.6 °C | 87.12 mm |
| April | 1.6 °C | 15.1 °C | 87.88 mm |
| May | 7.2 °C | 21.3 °C | 93.22 mm |
| June | 12.4 °C | 25.7 °C | 121.92 mm |
| July | 15.4 °C | 28.4 °C | 121.92 mm |
| August | 14.4 °C | 27.3 °C | 105.16 mm |
| September | 10.4 °C | 23.1 °C | 113.28 mm |
| October | 3.9 °C | 16.6 °C | 117.86 mm |
| November | -1.4 °C | 10.2 °C | 82.8 mm |
| December | -5.8 °C | 4 °C | 97.28 mm |
| Annual | 3 °C | 15.3 °C | 1174.74 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| 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 | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |