Onset, Massachusetts has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 6.1 °C (low) to 15.3 °C (high). Annual precipitation averages 1288.3 Millimeters. Onset is located in USDA Plant Hardiness Zone 7a. 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 | -5.2 °C | 3.8 °C | 107.44 mm |
| February | -4.8 °C | 4.4 °C | 92.96 mm |
| March | -1.6 °C | 7.7 °C | 137.41 mm |
| April | 3.5 °C | 13 °C | 113.79 mm |
| May | 8.9 °C | 18.7 °C | 92.71 mm |
| June | 14.3 °C | 23.7 °C | 95.5 mm |
| July | 18.1 °C | 27.1 °C | 80.26 mm |
| August | 17.4 °C | 26.4 °C | 91.69 mm |
| September | 13.5 °C | 22.9 °C | 110.24 mm |
| October | 7.7 °C | 17.1 °C | 124.97 mm |
| November | 2.7 °C | 11.8 °C | 110.49 mm |
| December | -1.8 °C | 6.9 °C | 130.81 mm |
| Annual | 6.1 °C | 15.3 °C | 1288.27 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 7a : -17.8°C to -15°C |
| Plant Hardiness Zone - 2012 | Zone 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Severe Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |