Decatur, 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 17.2 °C (high). Annual precipitation averages 1003.8 Millimeters. Decatur 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 | -7.3 °C | 1.7 °C | 59.18 mm |
| February | -5.8 °C | 4.4 °C | 51.31 mm |
| March | -0.7 °C | 10.6 °C | 69.09 mm |
| April | 5.5 °C | 17.9 °C | 106.43 mm |
| May | 11.9 °C | 23.4 °C | 117.86 mm |
| June | 16.6 °C | 28.4 °C | 119.89 mm |
| July | 18.3 °C | 30 °C | 92.46 mm |
| August | 17.3 °C | 29.1 °C | 83.57 mm |
| September | 12.7 °C | 26.3 °C | 79.5 mm |
| October | 6.2 °C | 19.3 °C | 83.82 mm |
| November | 0.2 °C | 11 °C | 79.25 mm |
| December | -4.1 °C | 4.3 °C | 61.47 mm |
| Annual | 5.9 °C | 17.2 °C | 1003.83 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 |