Cornwall, Ontario climate data is summarized below. Average annual temperatures range from 1.5 °C (low) to 12.5 °C (high). Annual precipitation averages 684 Millimeters. Cornwall 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. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -13.71 °C | -3.7 °C | 0 mm |
| February | -13.61 °C | -3.32 °C | 6 mm |
| March | -7.33 °C | 3.5 °C | 24 mm |
| April | 0.93 °C | 12.49 °C | 61 mm |
| May | 7.25 °C | 20.12 °C | 76 mm |
| June | 12.54 °C | 24.46 °C | 96 mm |
| July | 15.03 °C | 27.13 °C | 111 mm |
| August | 14.05 °C | 26.24 °C | 94 mm |
| September | 9.21 °C | 21.91 °C | 78 mm |
| October | 3.96 °C | 14.37 °C | 82 mm |
| November | -1.94 °C | 7.29 °C | 40 mm |
| December | -8.45 °C | -0.8 °C | 16 mm |
| Annual | 1.5 °C | 12.5 °C | 684 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 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Normal |