| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 22°F | 41°F | 31°F | 1.5" |
| February | 23°F | 44°F | 33°F | 1.7" |
| March | 36°F | 59°F | 46°F | 3.1" |
| April | 46°F | 70°F | 57°F | 5.1" |
| May | 56°F | 78°F | 66°F | 4.4" |
| June | 66°F | 87°F | 76°F | 4.8" |
| July | 68°F | 90°F | 78°F | 4.4" |
| August | 66°F | 88°F | 77°F | 3.9" |
| September | 58°F | 81°F | 69°F | 4.0" |
| October | 46°F | 70°F | 57°F | 3.0" |
| November | 36°F | 57°F | 45°F | 2.6" |
| December | 27°F | 44°F | 35°F | 3.2" |
| Index | Portage Des Sioux | Missouri | National |
| Air quality index | 54 | 97 | 42 |
| Pollution index | 8,870,230 | 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 | 0.180% |
| Benzene | 0.060% |
| Carbon Tetrachloride | 0.020% |
| Lead | 0.030% |
| Mercury | 0.120% |
| City | Population | Air Quality Index | National Avg. |
| Jerseyville, IL | 7,860 | 54 | 8.0% |
| Grafton, IL | 672 | 54 | 8.0% |
| Florissant, MO | 51,220 | 54 | 8.0% |
| Hartford, IL | 1,769 | 54 | 8.0% |
| Wood River, IL | 10,149 | 54 | 8.0% |
| Portage Des Sioux, MO | 305 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Iron City, GA | 305 | 35 | 30.0% |
| Deloit, IA | 305 | 35 | 30.0% |
| Center Point, IN | 305 | 45 | 10.0% |
| Newtown, IN | 305 | 42 | 16.0% |
| Staunton, IN | 305 | 45 | 10.0% |
| Portage Des Sioux, MO | 305 | 54 | 8.0% |
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