Illinois City, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.7 °C (low) to 15.3 °C (high). Annual precipitation averages 955.1 Millimeters. Illinois City is located in USDA Plant Hardiness Zone 5b. 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 | -10.1 °C | -1.1 °C | 35.05 mm |
| February | -8.1 °C | 1.4 °C | 42.67 mm |
| March | -2.2 °C | 8.6 °C | 60.96 mm |
| April | 4.1 °C | 16 °C | 92.2 mm |
| May | 10.6 °C | 22.1 °C | 124.21 mm |
| June | 16.1 °C | 27.3 °C | 128.02 mm |
| July | 18.1 °C | 29 °C | 103.89 mm |
| August | 16.9 °C | 27.9 °C | 101.35 mm |
| September | 12.3 °C | 24.6 °C | 91.44 mm |
| October | 5.7 °C | 17.4 °C | 72.39 mm |
| November | -0.9 °C | 9.1 °C | 55.63 mm |
| December | -6.7 °C | 1.8 °C | 47.24 mm |
| Annual | 4.7 °C | 15.3 °C | 955.05 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 5b : -26.1°C to -23.3°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 |