| 2023 Hardiness Zone: | Zone 6b: -5°F to 0°F Zone 7a: 0°F to 5°F |
| 2012 Hardiness Zone: | Zone 6b: -5°F to 0°F |
| 1990 Hardiness Zone: | Zone 6a: -10F to -5F |
| Average First Frost Date: | Oct. 21 - Oct. 31 |
| Average Last Frost Date: | Apr. 11 - Apr. 20 |
| Koppen-Geiger Climate Zone: | Cfa - Humid Subtropical Climate |
| Ecoregion: | 72j - Southern Illinoian Till Plain |
| Heat Zone Days: | 61 - 90 days Over 86°F |
Annual Climate Data for ZIP Code 62269 - O Fallon, IL | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg Min Temp (°F) | 21 | 26 | 35 | 44 | 54 | 63 | 67 | 64 | 57 | 45 | 36 | 26 | 45 |
| Avg Max Temp (°F) | 39 | 45 | 56 | 67 | 77 | 85 | 89 | 87 | 81 | 70 | 55 | 43 | 66 |
| Avg Precip (In.) | 2.1 | 2.25 | 3.56 | 3.91 | 4.21 | 3.94 | 3.6 | 3.32 | 3.07 | 2.9 | 3.9 | 3.02 | 39.78 |