Illiopolis, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 5.9 °C (low) to 17.3 °C (high). Annual precipitation averages 990.9 Millimeters. Illiopolis 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 | -7.4 °C | 1.8 °C | 55.63 mm |
| February | -5.7 °C | 4.4 °C | 50.8 mm |
| March | -0.6 °C | 10.8 °C | 69.34 mm |
| April | 5.4 °C | 18 °C | 103.89 mm |
| May | 11.7 °C | 23.6 °C | 115.57 mm |
| June | 16.8 °C | 28.4 °C | 116.84 mm |
| July | 18.4 °C | 30.1 °C | 98.3 mm |
| August | 17.3 °C | 29.2 °C | 84.07 mm |
| September | 12.7 °C | 26.4 °C | 79.25 mm |
| October | 6.3 °C | 19.5 °C | 83.06 mm |
| November | 0.4 °C | 11.1 °C | 76.45 mm |
| December | -4.4 °C | 4.5 °C | 57.66 mm |
| Annual | 5.9 °C | 17.3 °C | 990.86 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 6a : -23.3°C to -20.6°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |