Rock Island, Washington has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from 41.7 °F (low) to 63.7 °F (high). Annual precipitation averages 9 Inches. Rock Island is located in USDA Plant Hardiness Zone 7b. The growing season typically runs from the last frost around Mar. 21 - Mar. 31 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 25.8 °F | 36.5 °F | 1.38 in. |
| February | 28.3 °F | 44.5 °F | 0.89 in. |
| March | 33.5 °F | 55.0 °F | 0.88 in. |
| April | 39.9 °F | 64.6 °F | 0.66 in. |
| May | 48.1 °F | 74.4 °F | 0.77 in. |
| June | 54.4 °F | 80.8 °F | 0.58 in. |
| July | 60.7 °F | 90.2 °F | 0.28 in. |
| August | 59.8 °F | 89.5 °F | 0.2 in. |
| September | 51.4 °F | 80.2 °F | 0.27 in. |
| October | 40.6 °F | 64.3 °F | 0.73 in. |
| November | 31.9 °F | 47.4 °F | 0.89 in. |
| December | 26.2 °F | 36.5 °F | 1.48 in. |
| Annual | 41.7 °F | 63.7 °F | 9.01 in. |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Mar. 21 - Mar. 31 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 7b : 5°F to 10°F |
| Plant Hardiness Zone - 2012 | Zone 7a : 0°F to 5°F |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |