Onarga, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.8 °C (low) to 16.4 °C (high). Annual precipitation averages 989.6 Millimeters. Onarga is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -8.8 °C | 0.2 °C | 57.15 mm |
| February | -7.1 °C | 2.8 °C | 49.02 mm |
| March | -1.8 °C | 9.4 °C | 66.04 mm |
| April | 3.8 °C | 16.9 °C | 97.03 mm |
| May | 10.3 °C | 23.1 °C | 113.79 mm |
| June | 15.7 °C | 27.9 °C | 116.84 mm |
| July | 17.5 °C | 29.4 °C | 111.76 mm |
| August | 16.2 °C | 28.5 °C | 91.69 mm |
| September | 11.7 °C | 25.9 °C | 77.22 mm |
| October | 5.5 °C | 18.8 °C | 84.07 mm |
| November | -0.3 °C | 10.2 °C | 70.36 mm |
| December | -5.7 °C | 3.1 °C | 54.61 mm |
| Annual | 4.8 °C | 16.4 °C | 989.58 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| 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 |