Roundup, Montana has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from 33.5 °F (low) to 61 °F (high). Annual precipitation averages 14.9 Inches. 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 | 14.1 °F | 37.0 °F | 0.44 in. |
| February | 16.1 °F | 40.1 °F | 0.44 in. |
| March | 23.7 °F | 50.2 °F | 0.76 in. |
| April | 31.8 °F | 59.3 °F | 1.72 in. |
| May | 41.1 °F | 68.6 °F | 2.6 in. |
| June | 49.5 °F | 77.8 °F | 2.86 in. |
| July | 55.7 °F | 89.0 °F | 1.77 in. |
| August | 53.6 °F | 87.1 °F | 1.07 in. |
| September | 44.1 °F | 76.6 °F | 1.13 in. |
| October | 33.6 °F | 61.9 °F | 1.16 in. |
| November | 23.1 °F | 47.4 °F | 0.48 in. |
| December | 15.1 °F | 37.4 °F | 0.46 in. |
| Annual | 33.5 °F | 61 °F | 14.89 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 4b : -25°F to -20°F |
| Plant Hardiness Zone - 2012 | Zone 4b : -25°F to -20°F |
| Current Drought Conditions | Severe Drought |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |