Washington, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 41.1 °F (low) to 61.6 °F (high). Annual precipitation averages 38.4 Inches. Washington is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around Apr. 11 - Apr. 20 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 15.8 °F | 32.5 °F | 2.18 in. |
| February | 19.3 °F | 37.4 °F | 2.09 in. |
| March | 29.0 °F | 49.6 °F | 2.56 in. |
| April | 39.8 °F | 62.7 °F | 3.81 in. |
| May | 51.4 °F | 73.3 °F | 4.67 in. |
| June | 61.1 °F | 82.3 °F | 4.39 in. |
| July | 64.3 °F | 85.4 °F | 3.73 in. |
| August | 62.2 °F | 83.7 °F | 3.5 in. |
| September | 54.2 °F | 78.6 °F | 3.46 in. |
| October | 42.6 °F | 65.5 °F | 3.15 in. |
| November | 31.4 °F | 50.5 °F | 2.77 in. |
| December | 21.6 °F | 37.8 °F | 2.11 in. |
| Annual | 41.1 °F | 61.6 °F | 38.42 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 6a : -10°F to -5°F |
| Plant Hardiness Zone - 2012 | Zone 5b : -15°F to -10°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |