Casey, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 6.5 °C (low) to 17.8 °C (high). Annual precipitation averages 1114.1 Millimeters. Casey is located in USDA Plant Hardiness Zone 6b. 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 | -6.2 °C | 2.7 °C | 76.96 mm |
| February | -4.7 °C | 5.4 °C | 64.52 mm |
| March | 0.3 °C | 11.4 °C | 80.01 mm |
| April | 6.1 °C | 18.4 °C | 124.97 mm |
| May | 12.2 °C | 23.8 °C | 117.6 mm |
| June | 17.1 °C | 28.6 °C | 124.97 mm |
| July | 18.7 °C | 30.3 °C | 107.7 mm |
| August | 17.5 °C | 29.8 °C | 76.45 mm |
| September | 13.2 °C | 26.8 °C | 76.71 mm |
| October | 6.8 °C | 19.8 °C | 93.98 mm |
| November | 1.1 °C | 11.8 °C | 96.77 mm |
| December | -3.6 °C | 5.2 °C | 73.41 mm |
| Annual | 6.5 °C | 17.8 °C | 1114.05 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 6b : -20.6°C to -17.8°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 |