Sullivan, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 6.2 °C (low) to 17.3 °C (high). Annual precipitation averages 1030.2 Millimeters. Sullivan 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 | -7.1 °C | 1.8 °C | 60.45 mm |
| February | -5.3 °C | 4.5 °C | 55.37 mm |
| March | -0.3 °C | 10.7 °C | 73.91 mm |
| April | 5.6 °C | 17.8 °C | 114.05 mm |
| May | 12 °C | 23.6 °C | 121.67 mm |
| June | 16.9 °C | 28.4 °C | 119.38 mm |
| July | 18.6 °C | 30 °C | 102.62 mm |
| August | 17.2 °C | 29.3 °C | 72.9 mm |
| September | 12.9 °C | 26.6 °C | 76.2 mm |
| October | 6.7 °C | 19.6 °C | 85.34 mm |
| November | 0.7 °C | 11.2 °C | 84.07 mm |
| December | -3.8 °C | 4.5 °C | 64.26 mm |
| Annual | 6.2 °C | 17.3 °C | 1030.22 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 |