Cosmopolis, Washington has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 6.3 °C (low) to 15.1 °C (high). Annual precipitation averages 2165.9 Millimeters. 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 | 2.7 °C | 8.7 °C | 354.58 mm |
| February | 2.3 °C | 10.1 °C | 206.76 mm |
| March | 3.3 °C | 11.8 °C | 249.94 mm |
| April | 4.7 °C | 14.1 °C | 163.32 mm |
| May | 7.4 °C | 17.2 °C | 88.39 mm |
| June | 9.7 °C | 19 °C | 63.25 mm |
| July | 11.6 °C | 21.6 °C | 25.4 mm |
| August | 11.6 °C | 22.5 °C | 39.88 mm |
| September | 9.8 °C | 20.9 °C | 72.39 mm |
| October | 6.7 °C | 15.8 °C | 214.88 mm |
| November | 3.9 °C | 11.2 °C | 346.46 mm |
| December | 2.3 °C | 8.2 °C | 340.61 mm |
| Annual | 6.3 °C | 15.1 °C | 2165.86 mm |
| 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 : -6.7°C to -3.9°C |
| Plant Hardiness Zone - 2012 | Zone 9a : -6.7°C to -3.9°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |