| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 28°F | 43°F | 35°F | 11.3" |
| February | 26°F | 43°F | 33°F | 3.1" |
| March | 39°F | 61°F | 49°F | 5.3" |
| April | 45°F | 68°F | 56°F | 5.9" |
| May | 56°F | 80°F | 68°F | 4.4" |
| June | 65°F | 88°F | 76°F | 4.0" |
| July | 67°F | 88°F | 77°F | 9.1" |
| August | 69°F | 91°F | 79°F | 7.4" |
| September | 60°F | 84°F | 71°F | 8.8" |
| October | 48°F | 72°F | 59°F | 4.0" |
| November | 37°F | 59°F | 47°F | 8.5" |
| December | 27°F | 42°F | 34°F | 3.1" |
| Index | Millstadt | Illinois | National |
| Air quality index | 54 | 41 | 42 |
| Pollution index | 6,055,640 | 3,148,694 | 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 | 0.070% |
| Carbon Tetrachloride | 0.040% |
| Lead | 0.100% |
| Mercury | 0.230% |
| City | Population | Air Quality Index | National Avg. |
| Caseyville, IL | 4,203 | 54 | 8.0% |
| Clayton, MO | 16,763 | 54 | 8.0% |
| Lemay, MO | 16,565 | 54 | 8.0% |
| Cahokia, IL | 14,035 | 54 | 8.0% |
| Shiloh, IL | 13,465 | 54 | 8.0% |
| Millstadt, IL | 3,882 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Green Oaks, IL | 3,882 | 39 | 22.0% |
| Orange, MA | 3,882 | 34 | 32.0% |
| Weston, WV | 3,882 | 31 | 38.0% |
| Wallace, NC | 3,883 | 34 | 32.0% |
| Swanton, OH | 3,884 | 40 | 20.0% |
| Millstadt, IL | 3,882 | 54 | 8.0% |
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