Thornbury, Ontario climate data is summarized below. Average annual temperatures range from 4.1 °C (low) to 11 °C (high). Annual precipitation averages 232 Millimeters. 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 | -7.33 °C | -1.51 °C | 0 mm |
| February | -9.31 °C | -2.71 °C | 0 mm |
| March | -4.47 °C | 2.59 °C | 7 mm |
| April | 1.4 °C | 8.94 °C | 17 mm |
| May | 6.93 °C | 15.16 °C | 27 mm |
| June | 12.39 °C | 20.19 °C | 37 mm |
| July | 15.95 °C | 23.48 °C | 51 mm |
| August | 15.94 °C | 23.37 °C | 44 mm |
| September | 12.51 °C | 19.95 °C | 22 mm |
| October | 7.39 °C | 13.86 °C | 17 mm |
| November | 1.64 °C | 7.5 °C | 9 mm |
| December | -3.53 °C | 1.32 °C | 1 mm |
| Annual | 4.1 °C | 11 °C | 232 mm |
| 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 : -23.3°C to -20.6°C |
| Current Drought Conditions | Normal |