Illiopolis, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 42.7 °F (low) to 63.2 °F (high). Annual precipitation averages 39 Inches. 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 | 18.7 °F | 35.2 °F | 2.19 in. |
| February | 21.8 °F | 39.9 °F | 2.0 in. |
| March | 31.0 °F | 51.5 °F | 2.73 in. |
| April | 41.8 °F | 64.4 °F | 4.09 in. |
| May | 53.1 °F | 74.5 °F | 4.55 in. |
| June | 62.3 °F | 83.2 °F | 4.6 in. |
| July | 65.2 °F | 86.1 °F | 3.87 in. |
| August | 63.1 °F | 84.6 °F | 3.31 in. |
| September | 54.8 °F | 79.5 °F | 3.12 in. |
| October | 43.3 °F | 67.1 °F | 3.27 in. |
| November | 32.7 °F | 52.0 °F | 3.01 in. |
| December | 24.1 °F | 40.1 °F | 2.27 in. |
| Annual | 42.7 °F | 63.2 °F | 39.01 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 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |