Cosmopolis, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 43.4 °F (low) to 59.2 °F (high). Annual precipitation averages 85.3 Inches. Cosmopolis is located in USDA Plant Hardiness Zone 9a. The growing season typically runs from the last frost around Mar. 21 - Mar. 31 through the first frost around Nov. 11 - Nov. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 36.9 °F | 47.6 °F | 13.96 in. |
| February | 36.2 °F | 50.2 °F | 8.14 in. |
| March | 37.9 °F | 53.2 °F | 9.84 in. |
| April | 40.4 °F | 57.4 °F | 6.43 in. |
| May | 45.4 °F | 63.0 °F | 3.48 in. |
| June | 49.5 °F | 66.2 °F | 2.49 in. |
| July | 52.8 °F | 70.9 °F | 1.0 in. |
| August | 52.8 °F | 72.5 °F | 1.57 in. |
| September | 49.7 °F | 69.6 °F | 2.85 in. |
| October | 44.1 °F | 60.5 °F | 8.46 in. |
| November | 39.1 °F | 52.1 °F | 13.64 in. |
| December | 36.1 °F | 46.8 °F | 13.41 in. |
| Annual | 43.4 °F | 59.2 °F | 85.27 in. |
| Average First Frost Date | Nov. 11 - Nov. 20 |
| Average Last Frost Date | Mar. 21 - Mar. 31 |
| Average Days With Frost | 30-40 Days |
| Plant Hardiness Zone | Zone 9a : 20°F to 25°F |
| Plant Hardiness Zone - 2012 | Zone 9a : 20°F to 25°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |