Monmouth, Maine has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 2.2 °C (low) to 12.8 °C (high). Annual precipitation averages 1213.6 Millimeters. Monmouth 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 | -11.9 °C | -1.6 °C | 89.66 mm |
| February | -10.9 °C | 0.1 °C | 80.01 mm |
| March | -5.5 °C | 4.7 °C | 94.49 mm |
| April | 0.7 °C | 11.6 °C | 108.2 mm |
| May | 6.7 °C | 18.2 °C | 90.93 mm |
| June | 12 °C | 23.1 °C | 116.33 mm |
| July | 15.2 °C | 26.3 °C | 92.2 mm |
| August | 14.4 °C | 25.8 °C | 90.17 mm |
| September | 10.2 °C | 21.7 °C | 96.77 mm |
| October | 4 °C | 14.5 °C | 135.64 mm |
| November | -1.5 °C | 7.8 °C | 108.71 mm |
| December | -7.3 °C | 1.8 °C | 110.49 mm |
| Annual | 2.2 °C | 12.8 °C | 1213.6 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 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 | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |