| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 20°F | 37°F | 28°F | 2.1" |
| February | 21°F | 38°F | 29°F | 2.0" |
| March | 34°F | 54°F | 43°F | 3.0" |
| April | 44°F | 66°F | 55°F | 4.3" |
| May | 54°F | 74°F | 63°F | 3.7" |
| June | 64°F | 84°F | 74°F | 6.0" |
| July | 66°F | 86°F | 76°F | 4.5" |
| August | 66°F | 86°F | 75°F | 4.2" |
| September | 57°F | 79°F | 67°F | 4.3" |
| October | 45°F | 64°F | 54°F | 3.9" |
| November | 36°F | 54°F | 44°F | 2.7" |
| December | 23°F | 38°F | 29°F | 2.4" |
| Index | Clayton | Illinois | National |
| Air quality index | 41 | 41 | 50 |
| Pollution index | 222,633 | 3,347,295 | 5,066,862 |
| Measurement | Total |
| Days measured | 241 |
| Days with good air quality | 208 |
| Days with moderate air quality | 33 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.003% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.002% |
| Mercury | 0.000% |
| City, State | Population | Air Quality Index | National Avg. |
| Coatsburg, IL | 155 | 41 | 18% |
| Mount Sterling, IL | 2,184 | 41 | 18% |
| Camden, IL | 100 | 41 | 18% |
| Loraine, IL | 306 | 41 | 18% |
| Liberty, IL | 504 | 41 | 18% |
| Clayton, IL | 607 | 41 | 18% |
| City, State | Population | Air Quality Index | National Avg. |
| Keota, OK | 607 | 36 | 28% |
| Arivaca, AZ | 607 | 31 | 38% |
| Poplar, WI | 607 | 35 | 30% |
| Walsh, CO | 607 | 15 | 70% |
| South Williamson, KY | 607 | 42 | 16% |
| Clayton, IL | 607 | 41 | 18% |
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