Old Lyme, Connecticut has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 5.9 °C (low) to 15.8 °C (high). Annual precipitation averages 1260.1 Millimeters. Old Lyme is located in USDA Plant Hardiness Zone 7a. The growing season typically runs from the last frost around Apr. 11 - Apr. 20 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -5.8 °C | 3.8 °C | 94.74 mm |
| February | -5 °C | 4.8 °C | 84.84 mm |
| March | -1.3 °C | 8.3 °C | 118.36 mm |
| April | 3.7 °C | 14.3 °C | 117.35 mm |
| May | 8.8 °C | 19.4 °C | 95.5 mm |
| June | 14.2 °C | 24.2 °C | 108.2 mm |
| July | 17.6 °C | 27.4 °C | 87.12 mm |
| August | 17 °C | 26.6 °C | 104.65 mm |
| September | 13.3 °C | 23.7 °C | 110.24 mm |
| October | 7.8 °C | 17.8 °C | 119.38 mm |
| November | 2.6 °C | 12.1 °C | 102.11 mm |
| December | -2.1 °C | 6.9 °C | 117.6 mm |
| Annual | 5.9 °C | 15.8 °C | 1260.09 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| 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 | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |