St Thomas, Ontario climate data is summarized below. Average annual temperatures range from 40 °F (low) to 56.7 °F (high). Annual precipitation averages 20.1 Inches. St Thomas is located in USDA Plant Hardiness Zone 6b. 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 | -22.1 °F | -18.1 °F | 279.4 in. |
| February | -23 °F | -18.5 °F | 25.4 in. |
| March | -19.5 °F | -14.6 °F | 685.8 in. |
| April | -16.3 °F | -10.5 °F | 939.8 in. |
| May | -13 °F | -6.7 °F | 1422.4 in. |
| June | -10.1 °F | -4 °F | 1778 in. |
| July | -8.7 °F | -2.7 °F | 2184.4 in. |
| August | -9 °F | -3.1 °F | 2159 in. |
| September | -11.2 °F | -5.3 °F | 1574.8 in. |
| October | -14.1 °F | -9.1 °F | 990.6 in. |
| November | -17.1 °F | -12.9 °F | 584.2 in. |
| December | -19.6 °F | -16.4 °F | 355.6 in. |
| Annual | 40 °F | 56.7 °F | 20.13 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 6b : -5°F to 0°F |
| Current Drought Conditions | Normal |