Washington, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.5 °C (low) to 15.9 °C (high). Annual precipitation averages 962.7 Millimeters. Washington is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.1 °C | -0.7 °C | 33.78 mm |
| February | -8.1 °C | 1.9 °C | 39.37 mm |
| March | -2.2 °C | 9.1 °C | 58.42 mm |
| April | 3.9 °C | 16.8 °C | 92.46 mm |
| May | 10.7 °C | 22.7 °C | 130.56 mm |
| June | 16.3 °C | 27.7 °C | 130.3 mm |
| July | 18.1 °C | 29.5 °C | 104.9 mm |
| August | 16.7 °C | 28.7 °C | 107.44 mm |
| September | 11.7 °C | 25.2 °C | 89.41 mm |
| October | 5.2 °C | 18.2 °C | 75.95 mm |
| November | -1.2 °C | 9.4 °C | 56.64 mm |
| December | -6.8 °C | 2.3 °C | 43.43 mm |
| Annual | 4.5 °C | 15.9 °C | 962.66 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |