O'Fallon, Illinois has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 8.2 °C (low) to 19 °C (high). Annual precipitation averages 1150.1 Millimeters. O'Fallon is located in USDA Plant Hardiness Zone 7a. 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 | -4.9 °C | 4.3 °C | 73.15 mm |
| February | -3.1 °C | 7.3 °C | 62.48 mm |
| March | 1.8 °C | 13.1 °C | 90.68 mm |
| April | 7.8 °C | 19.6 °C | 129.79 mm |
| May | 13.8 °C | 24.6 °C | 134.62 mm |
| June | 18.7 °C | 29.6 °C | 127.51 mm |
| July | 20.8 °C | 31.3 °C | 103.12 mm |
| August | 19.7 °C | 30.7 °C | 88.39 mm |
| September | 15.1 °C | 27.2 °C | 88.14 mm |
| October | 8.8 °C | 20.7 °C | 82.3 mm |
| November | 2.3 °C | 12.8 °C | 97.28 mm |
| December | -2.3 °C | 6.7 °C | 72.64 mm |
| Annual | 8.2 °C | 19 °C | 1150.1 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 75-100 Days |
| Plant Hardiness Zone | Zone 7a : -17.8°C to -15°C |
| Plant Hardiness Zone - 2012 | Zone 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |