Topsham, Maine has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 2.3 °C (low) to 13 °C (high). Annual precipitation averages 1272.3 Millimeters. Topsham is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -11.5 °C | -0.9 °C | 93.98 mm |
| February | -10.4 °C | 0.8 °C | 89.66 mm |
| March | -5.2 °C | 5.1 °C | 105.16 mm |
| April | 0.7 °C | 11.8 °C | 113.28 mm |
| May | 6.5 °C | 17.9 °C | 92.2 mm |
| June | 11.9 °C | 22.8 °C | 121.41 mm |
| July | 15.2 °C | 26.1 °C | 93.22 mm |
| August | 14.4 °C | 25.7 °C | 87.63 mm |
| September | 10.3 °C | 21.6 °C | 102.62 mm |
| October | 4.2 °C | 14.7 °C | 138.68 mm |
| November | -1.2 °C | 8.2 °C | 115.06 mm |
| December | -7.1 °C | 2.4 °C | 119.38 mm |
| Annual | 2.3 °C | 13 °C | 1272.28 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |