Thunder Bay, Ontario climate data is summarized below. Average annual temperatures range from 29 °F (low) to 48 °F (high). Annual precipitation averages 21.5 Inches. Thunder Bay is located in USDA Plant Hardiness Zone 3b. The growing season typically runs from the last frost around May 21 - May 31 through the first frost around Sep. 21 - Sep. 30.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -27.6 °F | -22.2 °F | 0 in. |
| February | -27.6 °F | -21.3 °F | 0 in. |
| March | -23.1 °F | -16.9 °F | 0 in. |
| April | -19.3 °F | -13.2 °F | 685.8 in. |
| May | -15.8 °F | -9.5 °F | 2311.4 in. |
| June | -12.7 °F | -6.4 °F | 2311.4 in. |
| July | -10.8 °F | -4.3 °F | 2387.6 in. |
| August | -11.1 °F | -4.4 °F | 1879.6 in. |
| September | -13.8 °F | -7.3 °F | 1828.8 in. |
| October | -16.6 °F | -11.7 °F | 1701.8 in. |
| November | -20.4 °F | -15.9 °F | 762 in. |
| December | -25.6 °F | -20.9 °F | 0 in. |
| Annual | 29 °F | 48 °F | 21.48 in. |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 21 - May 31 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 3b : -35°F to -30°F |
| Current Drought Conditions | Normal |