Roundup, Montana has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from 0.8 °C (low) to 16.1 °C (high). Annual precipitation averages 378.2 Millimeters. Roundup is located in USDA Plant Hardiness Zone 4b. 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 | -9.9 °C | 2.8 °C | 11.18 mm |
| February | -8.8 °C | 4.5 °C | 11.18 mm |
| March | -4.6 °C | 10.1 °C | 19.3 mm |
| April | -0.1 °C | 15.2 °C | 43.69 mm |
| May | 5.1 °C | 20.3 °C | 66.04 mm |
| June | 9.7 °C | 25.4 °C | 72.64 mm |
| July | 13.2 °C | 31.7 °C | 44.96 mm |
| August | 12 °C | 30.6 °C | 27.18 mm |
| September | 6.7 °C | 24.8 °C | 28.7 mm |
| October | 0.9 °C | 16.6 °C | 29.46 mm |
| November | -4.9 °C | 8.6 °C | 12.19 mm |
| December | -9.4 °C | 3 °C | 11.68 mm |
| Annual | 0.8 °C | 16.1 °C | 378.2 mm |
| 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 4b : -31.7°C to -28.9°C |
| Plant Hardiness Zone - 2012 | Zone 4b : -31.7°C to -28.9°C |
| Current Drought Conditions | Severe Drought |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |