Roy, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 32.9 °F (low) to 56.8 °F (high). Annual precipitation averages 15.6 Inches. Roy is located in USDA Plant Hardiness Zone 4b. The growing season typically runs from the last frost around May 21 - May 31 through the first frost around Sep. 21 - Sep. 30.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 13.5 °F | 34.1 °F | 0.51 in. |
| February | 15.2 °F | 36.0 °F | 0.44 in. |
| March | 22.5 °F | 45.2 °F | 0.71 in. |
| April | 30.9 °F | 54.2 °F | 1.45 in. |
| May | 39.9 °F | 63.6 °F | 2.97 in. |
| June | 48.5 °F | 72.6 °F | 3.02 in. |
| July | 54.7 °F | 83.7 °F | 1.72 in. |
| August | 53.3 °F | 83.2 °F | 1.33 in. |
| September | 44.4 °F | 72.0 °F | 1.22 in. |
| October | 33.6 °F | 57.5 °F | 1.12 in. |
| November | 23.0 °F | 44.6 °F | 0.56 in. |
| December | 15.3 °F | 35.4 °F | 0.5 in. |
| Annual | 32.9 °F | 56.8 °F | 15.55 in. |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| Average Last Frost Date | May 21 - May 31 |
| 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 | Dfb - Humid Continental Mild Summer, Wet All Year |