Chillicothe, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 5.2 °C (low) to 16.5 °C (high). Annual precipitation averages 956.1 Millimeters. Chillicothe 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 | -8.8 °C | 0.2 °C | 52.07 mm |
| February | -6.9 °C | 2.7 °C | 47.75 mm |
| March | -1.4 °C | 9.7 °C | 63.75 mm |
| April | 4.5 °C | 17.2 °C | 96.27 mm |
| May | 10.8 °C | 23.2 °C | 126.75 mm |
| June | 16.1 °C | 28.1 °C | 103.89 mm |
| July | 18.2 °C | 29.8 °C | 90.93 mm |
| August | 17 °C | 29.1 °C | 93.47 mm |
| September | 12.4 °C | 25.8 °C | 87.12 mm |
| October | 5.9 °C | 18.8 °C | 75.44 mm |
| November | -0.2 °C | 10.3 °C | 66.55 mm |
| December | -5.6 °C | 3.2 °C | 52.07 mm |
| Annual | 5.2 °C | 16.5 °C | 956.06 mm |
| 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 : -23.3°C to -20.6°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 |