O'Fallon, Illinois has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 46.8 °F (low) to 66.2 °F (high). Annual precipitation averages 45.3 Inches. 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 | 23.2 °F | 39.8 °F | 2.88 in. |
| February | 26.5 °F | 45.1 °F | 2.46 in. |
| March | 35.3 °F | 55.5 °F | 3.57 in. |
| April | 46.0 °F | 67.2 °F | 5.11 in. |
| May | 56.9 °F | 76.2 °F | 5.3 in. |
| June | 65.7 °F | 85.2 °F | 5.02 in. |
| July | 69.5 °F | 88.3 °F | 4.06 in. |
| August | 67.4 °F | 87.2 °F | 3.48 in. |
| September | 59.1 °F | 80.9 °F | 3.47 in. |
| October | 47.8 °F | 69.3 °F | 3.24 in. |
| November | 36.2 °F | 55.1 °F | 3.83 in. |
| December | 27.8 °F | 44.1 °F | 2.86 in. |
| Annual | 46.8 °F | 66.2 °F | 45.28 in. |
| 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 : 0°F to 5°F |
| Plant Hardiness Zone - 2012 | Zone 6b : -5°F to 0°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |