Plains, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 34.8 °F (low) to 60.1 °F (high). Annual precipitation averages 15.7 Inches. Plains is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 21.8 °F | 36.0 °F | 1.38 in. |
| February | 22.9 °F | 40.8 °F | 1.19 in. |
| March | 27.0 °F | 50.2 °F | 1.13 in. |
| April | 32.5 °F | 59.1 °F | 1.6 in. |
| May | 39.4 °F | 69.1 °F | 1.55 in. |
| June | 46.2 °F | 76.2 °F | 1.93 in. |
| July | 51.0 °F | 87.7 °F | 0.67 in. |
| August | 50.1 °F | 87.7 °F | 0.65 in. |
| September | 43.0 °F | 76.3 °F | 1.05 in. |
| October | 33.9 °F | 59.2 °F | 1.61 in. |
| November | 27.1 °F | 43.5 °F | 1.57 in. |
| December | 22.4 °F | 35.1 °F | 1.37 in. |
| Annual | 34.8 °F | 60.1 °F | 15.7 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| 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 |