Chelmsford, Massachusetts has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 37.8 °F (low) to 59.5 °F (high). Annual precipitation averages 48 Inches. Chelmsford is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 15.1 °F | 34.7 °F | 3.66 in. |
| February | 17.0 °F | 37.5 °F | 3.32 in. |
| March | 24.6 °F | 45.2 °F | 4.35 in. |
| April | 35.1 °F | 57.8 °F | 3.97 in. |
| May | 45.4 °F | 68.5 °F | 3.8 in. |
| June | 54.8 °F | 77.3 °F | 4.24 in. |
| July | 60.6 °F | 83.2 °F | 3.77 in. |
| August | 59.0 °F | 81.7 °F | 3.82 in. |
| September | 51.0 °F | 74.6 °F | 3.86 in. |
| October | 39.3 °F | 62.3 °F | 4.88 in. |
| November | 30.0 °F | 50.9 °F | 3.88 in. |
| December | 21.7 °F | 40.4 °F | 4.47 in. |
| Annual | 37.8 °F | 59.5 °F | 48.02 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 6a : -10°F to -5°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Severe Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |