Hamilton, Washington has a Temperate Oceanic Climate climate (Köppen classification: Cfb). Average annual temperatures range from 42.2 °F (low) to 59.3 °F (high). Annual precipitation averages 52.3 Inches. Hamilton 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 Nov. 1 - Nov. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 33.6 °F | 44.1 °F | 6.21 in. |
| February | 33.9 °F | 47.5 °F | 4.36 in. |
| March | 36.3 °F | 52.4 °F | 5.25 in. |
| April | 40.2 °F | 58.4 °F | 3.92 in. |
| May | 45.4 °F | 65.3 °F | 3.13 in. |
| June | 49.7 °F | 69.1 °F | 2.36 in. |
| July | 52.3 °F | 75.3 °F | 1.14 in. |
| August | 52.4 °F | 76.5 °F | 1.37 in. |
| September | 48.7 °F | 70.6 °F | 2.64 in. |
| October | 42.7 °F | 59.9 °F | 5.54 in. |
| November | 37.7 °F | 49.4 °F | 10.08 in. |
| December | 33.7 °F | 43.4 °F | 6.26 in. |
| Annual | 42.2 °F | 59.3 °F | 52.26 in. |
| Average First Frost Date | Nov. 1 - Nov. 10 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 50-75 Days |
| Plant Hardiness Zone | Zone 8b : 15°F to 20°F |
| Plant Hardiness Zone - 2012 | Zone 8a : 10°F to 15°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfb - Temperate Oceanic Climate |