Plains, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 1.6 °C (low) to 15.6 °C (high). Annual precipitation averages 398.8 Millimeters. 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 | -5.7 °C | 2.2 °C | 35.05 mm |
| February | -5.1 °C | 4.9 °C | 30.23 mm |
| March | -2.8 °C | 10.1 °C | 28.7 mm |
| April | 0.3 °C | 15.1 °C | 40.64 mm |
| May | 4.1 °C | 20.6 °C | 39.37 mm |
| June | 7.9 °C | 24.6 °C | 49.02 mm |
| July | 10.6 °C | 30.9 °C | 17.02 mm |
| August | 10.1 °C | 30.9 °C | 16.51 mm |
| September | 6.1 °C | 24.6 °C | 26.67 mm |
| October | 1.1 °C | 15.1 °C | 40.89 mm |
| November | -2.7 °C | 6.4 °C | 39.88 mm |
| December | -5.3 °C | 1.7 °C | 34.8 mm |
| Annual | 1.6 °C | 15.6 °C | 398.78 mm |
| 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 : -23.3°C to -20.6°C |
| Plant Hardiness Zone - 2012 | Zone 6a : -23.3°C to -20.6°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |