Cornwall, Ontario climate data is summarized below. Average annual temperatures range from 34.7 °F (low) to 54.4 °F (high). Annual precipitation averages 26.9 Inches. 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 | -25.4 °F | -19.8 °F | 0 in. |
| February | -25.3 °F | -19.6 °F | 152.4 in. |
| March | -21.9 °F | -15.8 °F | 609.6 in. |
| April | -17.3 °F | -10.8 °F | 1549.4 in. |
| May | -13.8 °F | -6.6 °F | 1930.4 in. |
| June | -10.8 °F | -4.2 °F | 2438.4 in. |
| July | -9.4 °F | -2.7 °F | 2819.4 in. |
| August | -10 °F | -3.2 °F | 2387.6 in. |
| September | -12.7 °F | -5.6 °F | 1981.2 in. |
| October | -15.6 °F | -9.8 °F | 2082.8 in. |
| November | -18.9 °F | -13.7 °F | 1016 in. |
| December | -22.5 °F | -18.2 °F | 406.4 in. |
| Annual | 34.7 °F | 54.4 °F | 26.92 in. |
| 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 : -15°F to -10°F |
| Current Drought Conditions | Normal |