| 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 | Festus | Missouri | National |
| Air quality index | 54 | 97 | 42 |
| Pollution index | 753,473 | 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 | 0.001% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.020% |
| Mercury | 0.020% |
| City | Population | Air Quality Index | National Avg. |
| Valmeyer, IL | 1,208 | 54 | 8.0% |
| Imperial, MO | 4,947 | 54 | 8.0% |
| Cedar Hill Lakes, MO | 345 | 54 | 8.0% |
| Hillsboro, MO | 3,251 | 54 | 8.0% |
| Scotsdale, MO | 123 | 54 | 8.0% |
| Festus, MO | 12,054 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Londonderry, NH | 12,053 | 37 | 26.0% |
| Scotts Valley, CA | 12,055 | 35 | 30.0% |
| Cheval, FL | 12,052 | 43 | 14.0% |
| Enumclaw, WA | 12,062 | 45 | 10.0% |
| Brookhaven, MS | 12,045 | 44 | 12.0% |
| Festus, MO | 12,054 | 54 | 8.0% |
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