O'Fallon, Missouri has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 7.6 °C (low) to 18.9 °C (high). Annual precipitation averages 1096 Millimeters. O'Fallon 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. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -5.4 °C | 4.2 °C | 64.26 mm |
| February | -3.7 °C | 7.3 °C | 58.67 mm |
| March | 1.3 °C | 13 °C | 87.63 mm |
| April | 7.2 °C | 19.6 °C | 119.63 mm |
| May | 13.1 °C | 24.5 °C | 129.79 mm |
| June | 18.2 °C | 29.2 °C | 115.82 mm |
| July | 20.3 °C | 31.2 °C | 102.11 mm |
| August | 19.1 °C | 30.6 °C | 92.71 mm |
| September | 14.5 °C | 27 °C | 83.57 mm |
| October | 7.8 °C | 20.4 °C | 83.82 mm |
| November | 1.9 °C | 12.9 °C | 88.9 mm |
| December | -2.9 °C | 6.6 °C | 69.09 mm |
| Annual | 7.6 °C | 18.9 °C | 1096 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 75-100 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 |