Darby, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from -0.2 °C (low) to 14.8 °C (high). Annual precipitation averages 390.7 Millimeters. Darby is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around May 11 - May 20 through the first frost around Sep.1 - Sep. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -7.9 °C | 2.2 °C | 28.7 mm |
| February | -6.9 °C | 4.9 °C | 27.18 mm |
| March | -3.7 °C | 9.8 °C | 25.15 mm |
| April | -0.8 °C | 13.9 °C | 30.48 mm |
| May | 2.9 °C | 19.2 °C | 43.94 mm |
| June | 6.4 °C | 23.4 °C | 55.63 mm |
| July | 8.9 °C | 29.5 °C | 23.62 mm |
| August | 8 °C | 28.9 °C | 23.88 mm |
| September | 4.2 °C | 23.4 °C | 27.69 mm |
| October | -0.4 °C | 15 °C | 34.54 mm |
| November | -4.7 °C | 6.6 °C | 34.54 mm |
| December | -8.2 °C | 1.3 °C | 35.31 mm |
| Annual | -0.2 °C | 14.8 °C | 390.66 mm |
| Average First Frost Date | Sep.1 - Sep. 10 |
| Average Last Frost Date | May 11 - May 20 |
| Average Days With Frost | Over 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 |