| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 24°F | 45°F | 33°F | 2.4" |
| February | 25°F | 48°F | 35°F | 2.2" |
| March | 37°F | 61°F | 48°F | 3.6" |
| April | 47°F | 72°F | 59°F | 5.4" |
| May | 57°F | 80°F | 68°F | 4.6" |
| June | 67°F | 89°F | 78°F | 5.2" |
| July | 70°F | 92°F | 81°F | 4.2" |
| August | 69°F | 91°F | 79°F | 3.6" |
| September | 61°F | 84°F | 72°F | 4.3" |
| October | 48°F | 72°F | 60°F | 3.4" |
| November | 37°F | 60°F | 48°F | 3.4" |
| December | 27°F | 47°F | 37°F | 3.5" |
| Index | Clayton | Missouri | National |
| Air quality index | 54 | 97 | 42 |
| Pollution index | 8,473,050 | 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 | 0.020% |
| Benzene | 0.070% |
| Carbon Tetrachloride | 0.020% |
| Lead | 0.010% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Hartford, IL | 1,769 | 54 | 8.0% |
| Brooklyn, IL | 428 | 54 | 8.0% |
| Concord, MO | 18,521 | 54 | 8.0% |
| Frontenac, MO | 3,538 | 54 | 8.0% |
| Lemay, MO | 16,565 | 54 | 8.0% |
| Clayton, MO | 16,763 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Bull Run, VA | 16,769 | 47 | 6.0% |
| Seven Oaks, SC | 16,743 | 43 | 14.0% |
| Defiance, OH | 16,741 | 39 | 22.0% |
| Knightdale, NC | 16,798 | 46 | 8.0% |
| Seagoville, TX | 16,726 | 51 | 2.0% |
| Clayton, MO | 16,763 | 54 | 8.0% |
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