| 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 | Defiance | 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. |
| Innsbrook, MO | 843 | 54 | 8.0% |
| Wright City, MO | 4,012 | 54 | 8.0% |
| St. Peters, MO | 57,536 | 54 | 8.0% |
| Byrnes Mill, MO | 2,983 | 54 | 8.0% |
| Grayhawk, MO | 785 | 54 | 8.0% |
| Defiance, MO | 68 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Ekwok, AK | 68 | 31 | 38.0% |
| Union Grove, AL | 68 | 44 | 12.0% |
| Anthonyville, AR | 68 | 45 | 10.0% |
| Marie, AR | 68 | 45 | 10.0% |
| Keddie, CA | 68 | 46 | 8.0% |
| Defiance, MO | 68 | 54 | 8.0% |
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