Big Arm, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 34.1 °F (low) to 56.8 °F (high). Annual precipitation averages 15.7 Inches. Big Arm is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around May 11 - May 20 through the first frost around Sep. 21 - Sep. 30.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 20.1 °F | 33.9 °F | 1.15 in. |
| February | 20.8 °F | 37.3 °F | 0.93 in. |
| March | 26.1 °F | 46.4 °F | 1.02 in. |
| April | 32.1 °F | 55.9 °F | 1.44 in. |
| May | 39.6 °F | 65.4 °F | 1.63 in. |
| June | 46.1 °F | 71.9 °F | 2.55 in. |
| July | 51.0 °F | 83.2 °F | 1.31 in. |
| August | 49.9 °F | 83.3 °F | 0.62 in. |
| September | 42.6 °F | 72.4 °F | 1.15 in. |
| October | 33.5 °F | 56.4 °F | 1.39 in. |
| November | 26.4 °F | 41.8 °F | 1.44 in. |
| December | 20.8 °F | 33.6 °F | 1.02 in. |
| Annual | 34.1 °F | 56.8 °F | 15.65 in. |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 11 - May 20 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 6a : -10°F to -5°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |