Olney, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 7.2 °C (low) to 18.5 °C (high). Annual precipitation averages 1191.3 Millimeters. Olney is located in USDA Plant Hardiness Zone 6b. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -5.3 °C | 3.6 °C | 86.87 mm |
| February | -3.7 °C | 6.3 °C | 64.52 mm |
| March | 1.1 °C | 12.2 °C | 101.09 mm |
| April | 6.8 °C | 18.9 °C | 125.73 mm |
| May | 12.8 °C | 24.3 °C | 134.11 mm |
| June | 17.7 °C | 29.1 °C | 124.46 mm |
| July | 19.3 °C | 30.7 °C | 116.84 mm |
| August | 17.9 °C | 30.2 °C | 71.37 mm |
| September | 13.7 °C | 27.3 °C | 84.33 mm |
| October | 7.3 °C | 20.6 °C | 92.96 mm |
| November | 1.6 °C | 12.5 °C | 102.62 mm |
| December | -2.8 °C | 6.1 °C | 86.36 mm |
| Annual | 7.2 °C | 18.5 °C | 1191.26 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| 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 |