O'Fallon, Missouri has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 45.7 °F (low) to 66 °F (high). Annual precipitation averages 43.2 Inches. 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 | 22.2 °F | 39.5 °F | 2.53 in. |
| February | 25.4 °F | 45.2 °F | 2.31 in. |
| March | 34.3 °F | 55.4 °F | 3.45 in. |
| April | 45.0 °F | 67.2 °F | 4.71 in. |
| May | 55.5 °F | 76.1 °F | 5.11 in. |
| June | 64.7 °F | 84.5 °F | 4.56 in. |
| July | 68.5 °F | 88.2 °F | 4.02 in. |
| August | 66.4 °F | 87.1 °F | 3.65 in. |
| September | 58.1 °F | 80.6 °F | 3.29 in. |
| October | 46.1 °F | 68.8 °F | 3.3 in. |
| November | 35.4 °F | 55.3 °F | 3.5 in. |
| December | 26.7 °F | 43.9 °F | 2.72 in. |
| Annual | 45.7 °F | 66 °F | 43.15 in. |
| 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 : -5°F to 0°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |