Index, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 5.4 °C (low) to 15.3 °C (high). Annual precipitation averages 2239.5 Millimeters. Index is located in USDA Plant Hardiness Zone 8b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 0.6 °C | 6.6 °C | 315.47 mm |
| February | 0.7 °C | 8.6 °C | 212.34 mm |
| March | 1.8 °C | 11.1 °C | 232.16 mm |
| April | 3.9 °C | 14.7 °C | 185.93 mm |
| May | 7.2 °C | 18.8 °C | 124.71 mm |
| June | 9.5 °C | 21 °C | 102.87 mm |
| July | 11.6 °C | 25 °C | 40.64 mm |
| August | 11.6 °C | 25.5 °C | 43.94 mm |
| September | 9.3 °C | 21.8 °C | 105.66 mm |
| October | 5.9 °C | 14.9 °C | 231.65 mm |
| November | 2.7 °C | 9.4 °C | 348.74 mm |
| December | 0.4 °C | 6 °C | 295.4 mm |
| Annual | 5.4 °C | 15.3 °C | 2239.51 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 50-75 Days |
| Plant Hardiness Zone | Zone 8b : -9.4°C to -6.7°C |
| Plant Hardiness Zone - 2012 | Zone 8a : -12.2°C to -9.4°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |