Colchester, Connecticut has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 4.4 °C (low) to 15.6 °C (high). Annual precipitation averages 1293.9 Millimeters. Colchester is located in USDA Plant Hardiness Zone 6b. 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 | -7.3 °C | 2.4 °C | 99.57 mm |
| February | -6.7 °C | 3.8 °C | 85.34 mm |
| March | -2.6 °C | 8.1 °C | 117.86 mm |
| April | 2.8 °C | 14.8 °C | 114.81 mm |
| May | 8.2 °C | 20.4 °C | 93.98 mm |
| June | 13.4 °C | 25.1 °C | 108.46 mm |
| July | 16.6 °C | 28 °C | 98.3 mm |
| August | 15.8 °C | 27.3 °C | 109.98 mm |
| September | 11.5 °C | 23.5 °C | 116.33 mm |
| October | 5.4 °C | 17.1 °C | 125.73 mm |
| November | 0.1 °C | 10.9 °C | 104.14 mm |
| December | -4 °C | 5.5 °C | 119.38 mm |
| Annual | 4.4 °C | 15.6 °C | 1293.88 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 6b : -20.6°C to -17.8°C |
| Plant Hardiness Zone - 2012 | Zone 6a : -23.3°C to -20.6°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |