Northumberland, New York has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 3.1 °C (low) to 14.6 °C (high). Annual precipitation averages 1144 Millimeters. Northumberland 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. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.3 °C | -0.3 °C | 84.33 mm |
| February | -9.7 °C | 1.4 °C | 63.5 mm |
| March | -4.7 °C | 6.4 °C | 83.57 mm |
| April | 1.9 °C | 14.4 °C | 94.49 mm |
| May | 8.2 °C | 21.2 °C | 92.2 mm |
| June | 13.2 °C | 25.7 °C | 115.82 mm |
| July | 15.6 °C | 28.3 °C | 115.06 mm |
| August | 14.9 °C | 27.2 °C | 107.95 mm |
| September | 10.7 °C | 23.2 °C | 97.54 mm |
| October | 4.6 °C | 16 °C | 109.47 mm |
| November | -0.9 °C | 8.9 °C | 85.09 mm |
| December | -5.8 °C | 2.6 °C | 95 mm |
| Annual | 3.1 °C | 14.6 °C | 1144.02 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| 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 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |