Haddam, Connecticut has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 5 °C (low) to 15.7 °C (high). Annual precipitation averages 1341.6 Millimeters. Haddam is located in USDA Plant Hardiness Zone 6b. The growing season typically runs from the last frost around Apr. 11 - Apr. 20 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -6.6 °C | 2.6 °C | 103.12 mm |
| February | -5.9 °C | 4.1 °C | 88.14 mm |
| March | -2.2 °C | 8.3 °C | 120.65 mm |
| April | 3.3 °C | 14.8 °C | 116.59 mm |
| May | 8.5 °C | 20.5 °C | 96.77 mm |
| June | 13.8 °C | 25.3 °C | 125.48 mm |
| July | 16.8 °C | 28.2 °C | 99.31 mm |
| August | 16.2 °C | 27.4 °C | 113.28 mm |
| September | 12.1 °C | 23.5 °C | 113.03 mm |
| October | 6.1 °C | 17.1 °C | 135.38 mm |
| November | 0.9 °C | 11.1 °C | 108.71 mm |
| December | -3.2 °C | 5.7 °C | 121.16 mm |
| Annual | 5 °C | 15.7 °C | 1341.62 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 100-125 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 |