Mercer Island, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 6.6 °C (low) to 15.7 °C (high). Annual precipitation averages 1051.8 Millimeters. Mercer Island is located in USDA Plant Hardiness Zone 9a. The growing season typically runs from the last frost around Mar. 11 - Mar. 20 through the first frost around Nov. 11 - Nov. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 2.1 °C | 8.2 °C | 141.73 mm |
| February | 2 °C | 9.6 °C | 99.06 mm |
| March | 3.3 °C | 11.8 °C | 111.76 mm |
| April | 5.1 °C | 14.7 °C | 82.8 mm |
| May | 8.2 °C | 18.6 °C | 53.09 mm |
| June | 10.6 °C | 20.9 °C | 46.99 mm |
| July | 12.5 °C | 24.6 °C | 20.07 mm |
| August | 12.6 °C | 24.9 °C | 24.64 mm |
| September | 10.5 °C | 21.7 °C | 41.66 mm |
| October | 7 °C | 15.6 °C | 105.16 mm |
| November | 3.9 °C | 10.6 °C | 167.89 mm |
| December | 1.8 °C | 7.6 °C | 156.97 mm |
| Annual | 6.6 °C | 15.7 °C | 1051.82 mm |
| Average First Frost Date | Nov. 11 - Nov. 20 |
| Average Last Frost Date | Mar. 11 - Mar. 20 |
| Average Days With Frost | 20-30 Days |
| Plant Hardiness Zone | Zone 9a : -6.7°C to -3.9°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 |