Chelmsford, Ontario climate data is summarized below. Average annual temperatures range from 29.7 °F (low) to 50.4 °F (high). Annual precipitation averages 23.8 Inches. Chelmsford is located in USDA Plant Hardiness Zone 4b. The growing season typically runs from the last frost around May 11 - May 20 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -27.1 °F | -21.4 °F | 0 in. |
| February | -27.4 °F | -21 °F | 0 in. |
| March | -23.5 °F | -16.9 °F | 330.2 in. |
| April | -19 °F | -12.3 °F | 1524 in. |
| May | -15 °F | -7.5 °F | 1803.4 in. |
| June | -12 °F | -4.8 °F | 1752.6 in. |
| July | -11 °F | -3.7 °F | 1879.6 in. |
| August | -11.3 °F | -4.3 °F | 2260.6 in. |
| September | -14 °F | -7 °F | 2057.4 in. |
| October | -16.8 °F | -11.3 °F | 2565.4 in. |
| November | -20.1 °F | -15.4 °F | 1168.4 in. |
| December | -24.5 °F | -19.7 °F | 25.4 in. |
| Annual | 29.7 °F | 50.4 °F | 23.82 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 11 - May 20 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 4b : -25°F to -20°F |
| Current Drought Conditions | Normal |