Chelmsford, Massachusetts has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 3.2 °C (low) to 15.3 °C (high). Annual precipitation averages 1219.7 Millimeters. 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 | -9.4 °C | 1.5 °C | 92.96 mm |
| February | -8.3 °C | 3.1 °C | 84.33 mm |
| March | -4.1 °C | 7.3 °C | 110.49 mm |
| April | 1.7 °C | 14.3 °C | 100.84 mm |
| May | 7.4 °C | 20.3 °C | 96.52 mm |
| June | 12.7 °C | 25.2 °C | 107.7 mm |
| July | 15.9 °C | 28.4 °C | 95.76 mm |
| August | 15 °C | 27.6 °C | 97.03 mm |
| September | 10.6 °C | 23.7 °C | 98.04 mm |
| October | 4.1 °C | 16.8 °C | 123.95 mm |
| November | -1.1 °C | 10.5 °C | 98.55 mm |
| December | -5.7 °C | 4.7 °C | 113.54 mm |
| Annual | 3.2 °C | 15.3 °C | 1219.71 mm |
| 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 : -23.3°C to -20.6°C |
| Plant Hardiness Zone - 2012 | Zone 6a : -23.3°C to -20.6°C |
| Current Drought Conditions | Severe Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |