Darby, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 31.7 °F (low) to 58.7 °F (high). Annual precipitation averages 15.4 Inches. 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 | 17.8 °F | 35.9 °F | 1.13 in. |
| February | 19.5 °F | 40.8 °F | 1.07 in. |
| March | 25.3 °F | 49.7 °F | 0.99 in. |
| April | 30.5 °F | 57.0 °F | 1.2 in. |
| May | 37.3 °F | 66.5 °F | 1.73 in. |
| June | 43.5 °F | 74.1 °F | 2.19 in. |
| July | 48.1 °F | 85.1 °F | 0.93 in. |
| August | 46.4 °F | 84.1 °F | 0.94 in. |
| September | 39.6 °F | 74.1 °F | 1.09 in. |
| October | 31.3 °F | 59.0 °F | 1.36 in. |
| November | 23.6 °F | 43.8 °F | 1.36 in. |
| December | 17.3 °F | 34.4 °F | 1.39 in. |
| Annual | 31.7 °F | 58.7 °F | 15.38 in. |
| 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 : -15°F to -10°F |
| Plant Hardiness Zone - 2012 | Zone 5b : -15°F to -10°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |