Bethlehem, New Hampshire has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from -0.4 °C (low) to 11.8 °C (high). Annual precipitation averages 1051.8 Millimeters. Bethlehem is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Sep.11 - Sep. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -14.2 °C | -3.1 °C | 61.47 mm |
| February | -13.5 °C | -1.3 °C | 56.64 mm |
| March | -8.4 °C | 3.8 °C | 65.53 mm |
| April | -1.7 °C | 11 °C | 81.28 mm |
| May | 4.7 °C | 18.7 °C | 97.28 mm |
| June | 9.8 °C | 23.1 °C | 108.71 mm |
| July | 12.3 °C | 25.4 °C | 112.78 mm |
| August | 11.3 °C | 24.5 °C | 106.93 mm |
| September | 7.3 °C | 20.6 °C | 88.65 mm |
| October | 1.8 °C | 13.1 °C | 108.46 mm |
| November | -3.8 °C | 6.1 °C | 76.96 mm |
| December | -9.9 °C | -0.3 °C | 87.12 mm |
| Annual | -0.4 °C | 11.8 °C | 1051.81 mm |
| Average First Frost Date | Sep.11 - Sep. 20 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 5a : -28.9°C to -26.1°C |
| Plant Hardiness Zone - 2012 | Zone 4b : -31.7°C to -28.9°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |