Roy, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 5.7 °C (low) to 16 °C (high). Annual precipitation averages 1212.9 Millimeters. Roy is located in USDA Plant Hardiness Zone 8b. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 1.4 °C | 8.2 °C | 181.1 mm |
| February | 1.2 °C | 9.7 °C | 118.36 mm |
| March | 2.4 °C | 12.1 °C | 135.89 mm |
| April | 4.1 °C | 14.8 °C | 94.74 mm |
| May | 7.1 °C | 18.7 °C | 56.9 mm |
| June | 9.7 °C | 21.2 °C | 46.74 mm |
| July | 11.6 °C | 24.9 °C | 18.8 mm |
| August | 11.7 °C | 25.4 °C | 24.89 mm |
| September | 9.3 °C | 22.2 °C | 41.66 mm |
| October | 5.9 °C | 15.9 °C | 123.44 mm |
| November | 2.9 °C | 10.7 °C | 195.33 mm |
| December | 1.1 °C | 7.6 °C | 175.01 mm |
| Annual | 5.7 °C | 16 °C | 1212.86 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 50-75 Days |
| Plant Hardiness Zone | Zone 8b : -9.4°C to -6.7°C |
| Plant Hardiness Zone - 2012 | Zone 8b : -9.4°C to -6.7°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |