Dewey, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 41.4 °F (low) to 61.9 °F (high). Annual precipitation averages 40.9 Inches. Dewey 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 | 17.1 °F | 33.2 °F | 2.34 in. |
| February | 20.2 °F | 37.8 °F | 2.1 in. |
| March | 29.4 °F | 49.5 °F | 2.77 in. |
| April | 40.0 °F | 63.1 °F | 4.05 in. |
| May | 52.0 °F | 74.1 °F | 4.5 in. |
| June | 61.2 °F | 82.6 °F | 4.72 in. |
| July | 64.0 °F | 85.1 °F | 4.4 in. |
| August | 61.5 °F | 83.6 °F | 3.79 in. |
| September | 54.2 °F | 78.9 °F | 3.34 in. |
| October | 42.3 °F | 66.0 °F | 3.33 in. |
| November | 31.8 °F | 50.8 °F | 3.13 in. |
| December | 22.7 °F | 38.0 °F | 2.39 in. |
| Annual | 41.4 °F | 61.9 °F | 40.86 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 |