Westbrook, Maine has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 2.9 °C (low) to 13.6 °C (high). Annual precipitation averages 1257.6 Millimeters. Westbrook 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 | -10.6 °C | -0.2 °C | 89.92 mm |
| February | -9.8 °C | 1.4 °C | 89.66 mm |
| March | -4.6 °C | 5.7 °C | 105.41 mm |
| April | 1.3 °C | 12.1 °C | 117.09 mm |
| May | 7.1 °C | 18.4 °C | 92.2 mm |
| June | 12.7 °C | 23.4 °C | 109.73 mm |
| July | 16 °C | 26.6 °C | 92.71 mm |
| August | 15.2 °C | 26.2 °C | 87.63 mm |
| September | 10.9 °C | 22.1 °C | 98.55 mm |
| October | 4.6 °C | 15.3 °C | 143.76 mm |
| November | -0.9 °C | 8.8 °C | 109.73 mm |
| December | -6.6 °C | 2.9 °C | 121.16 mm |
| Annual | 2.9 °C | 13.6 °C | 1257.55 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 | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |