Thornbury, Ontario climate data is summarized below. Average annual temperatures range from 39.4 °F (low) to 51.8 °F (high). Annual precipitation averages 9.1 Inches. Thornbury is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -21.9 °F | -18.6 °F | 0 in. |
| February | -23 °F | -19.3 °F | 0 in. |
| March | -20.3 °F | -16.3 °F | 177.8 in. |
| April | -17 °F | -12.8 °F | 431.8 in. |
| May | -13.9 °F | -9.4 °F | 685.8 in. |
| June | -10.9 °F | -6.6 °F | 939.8 in. |
| July | -8.9 °F | -4.7 °F | 1295.4 in. |
| August | -8.9 °F | -4.8 °F | 1117.6 in. |
| September | -10.8 °F | -6.7 °F | 558.8 in. |
| October | -13.7 °F | -10.1 °F | 431.8 in. |
| November | -16.9 °F | -13.6 °F | 228.6 in. |
| December | -19.7 °F | -17 °F | 25.4 in. |
| Annual | 39.4 °F | 51.8 °F | 9.14 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |