Roy, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 0.5 °C (low) to 13.8 °C (high). Annual precipitation averages 395 Millimeters. 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 | -10.3 °C | 1.2 °C | 12.95 mm |
| February | -9.3 °C | 2.2 °C | 11.18 mm |
| March | -5.3 °C | 7.3 °C | 18.03 mm |
| April | -0.6 °C | 12.3 °C | 36.83 mm |
| May | 4.4 °C | 17.6 °C | 75.44 mm |
| June | 9.2 °C | 22.6 °C | 76.71 mm |
| July | 12.6 °C | 28.7 °C | 43.69 mm |
| August | 11.8 °C | 28.4 °C | 33.78 mm |
| September | 6.9 °C | 22.2 °C | 30.99 mm |
| October | 0.9 °C | 14.2 °C | 28.45 mm |
| November | -5 °C | 7 °C | 14.22 mm |
| December | -9.3 °C | 1.9 °C | 12.7 mm |
| Annual | 0.5 °C | 13.8 °C | 394.97 mm |
| 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 : -31.7°C to -28.9°C |
| Plant Hardiness Zone - 2012 | Zone 4b : -31.7°C to -28.9°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |