Sweet Grass, Montana has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from 30.3 °F (low) to 55 °F (high). Annual precipitation averages 15.3 Inches. Sweet Grass is located in USDA Plant Hardiness Zone 4a. The growing season typically runs from the last frost around May 21 - May 31 through the first frost around Sep.1 - Sep. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 10.6 °F | 31.3 °F | 0.52 in. |
| February | 12.6 °F | 34.0 °F | 0.4 in. |
| March | 20.1 °F | 42.7 °F | 0.8 in. |
| April | 29.2 °F | 54.2 °F | 1.25 in. |
| May | 38.2 °F | 64.0 °F | 2.17 in. |
| June | 46.2 °F | 71.3 °F | 4.03 in. |
| July | 51.1 °F | 80.8 °F | 1.28 in. |
| August | 49.9 °F | 80.4 °F | 1.42 in. |
| September | 42.0 °F | 70.0 °F | 1.43 in. |
| October | 31.1 °F | 56.4 °F | 0.96 in. |
| November | 20.5 °F | 42.0 °F | 0.54 in. |
| December | 12.0 °F | 32.4 °F | 0.53 in. |
| Annual | 30.3 °F | 55 °F | 15.33 in. |
| Average First Frost Date | Sep.1 - Sep. 10 |
| Average Last Frost Date | May 21 - May 31 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 4a : -30°F to -25°F |
| Plant Hardiness Zone - 2012 | Zone 4a : -30°F to -25°F |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |