| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 21°F | 39°F | 29°F | 3.5" |
| February | 23°F | 41°F | 31°F | 3.2" |
| March | 36°F | 56°F | 44°F | 3.6" |
| April | 46°F | 68°F | 56°F | 7.4" |
| May | 56°F | 76°F | 65°F | 7.6" |
| June | 64°F | 84°F | 74°F | 10.4" |
| July | 67°F | 87°F | 76°F | 8.6" |
| August | 65°F | 87°F | 75°F | 6.1" |
| September | 58°F | 81°F | 68°F | 8.5" |
| October | 46°F | 68°F | 56°F | 5.6" |
| November | 36°F | 55°F | 45°F | 5.5" |
| December | 27°F | 42°F | 33°F | 6.1" |
| Index | Vandalia | Missouri | National |
| Air quality index | 41 | 97 | 42 |
| Pollution index | 964,937 | 1,630,255 | 2,726,193 |
| 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 | 0.002% |
| Lead | 0.130% |
| Mercury | 0.120% |
| City | Population | Air Quality Index | National Avg. |
| Middletown, MO | 216 | 38 | 24.0% |
| Benton City, MO | 139 | 38 | 24.0% |
| Rush Hill, MO | 183 | 38 | 24.0% |
| Laddonia, MO | 579 | 38 | 24.0% |
| Farber, MO | 406 | 38 | 24.0% |
| Vandalia, MO | 3,903 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Greendale, IN | 3,903 | 54 | 8.0% |
| Bunkie, LA | 3,903 | 30 | 40.0% |
| Ridgeland, SC | 3,903 | 29 | 42.0% |
| Vulcan County, AB | 3,903 | n/a | |
| Malmstrom AFB, MT | 3,902 | 21 | 58.0% |
| Vandalia, MO | 3,903 | 41 | 18.0% |
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