| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 23°F | 44°F | 32°F | 2.1" |
| February | 25°F | 46°F | 35°F | 1.8" |
| March | 36°F | 60°F | 47°F | 3.7" |
| April | 46°F | 71°F | 58°F | 5.3" |
| May | 55°F | 78°F | 66°F | 4.5" |
| June | 65°F | 87°F | 76°F | 4.8" |
| July | 68°F | 90°F | 79°F | 2.9" |
| August | 66°F | 89°F | 77°F | 3.4" |
| September | 58°F | 82°F | 69°F | 3.9" |
| October | 46°F | 70°F | 57°F | 3.0" |
| November | 36°F | 59°F | 46°F | 2.8" |
| December | 27°F | 45°F | 35°F | 3.1" |
| Index | O'Fallon | Missouri | National |
| Air quality index | 54 | 97 | 42 |
| Pollution index | 3,042,530 | 1,630,255 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 134 |
| Days with moderate air quality | 221 |
| Days w/ poor A.Q. for sensitive groups | 8 |
| Days with unhealthy air quality | 2 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.040% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.010% |
| Mercury | 0.001% |
| City | Population | Air Quality Index | National Avg. |
| Brussels, IL | 113 | 54 | 8.0% |
| Josephville, MO | 632 | 54 | 8.0% |
| Country Life Acres, MO | 52 | 54 | 8.0% |
| St. Charles, MO | 70,650 | 54 | 8.0% |
| Chesterfield, MO | 47,603 | 54 | 8.0% |
| O'Fallon, MO | 88,132 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Palm Coast, FL | 88,222 | 31 | 38.0% |
| Champaign, IL | 88,288 | 42 | 16.0% |
| Newton, MA | 88,322 | 43 | 14.0% |
| Reading, PA | 88,333 | 42 | 16.0% |
| Conroe, TX | 88,369 | 52 | 4.0% |
| O'Fallon, MO | 88,132 | 54 | 8.0% |
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