| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 25°F | 41°F | 32°F | 3.5" |
| February | 27°F | 44°F | 34°F | 2.0" |
| March | 39°F | 58°F | 47°F | 3.7" |
| April | 48°F | 69°F | 58°F | 7.8" |
| May | 57°F | 77°F | 66°F | 5.4" |
| June | 66°F | 86°F | 76°F | 8.3" |
| July | 68°F | 88°F | 78°F | 7.4" |
| August | 66°F | 87°F | 76°F | 7.2" |
| September | 59°F | 81°F | 69°F | 8.9" |
| October | 48°F | 70°F | 58°F | 5.6" |
| November | 38°F | 57°F | 47°F | 4.2" |
| December | 30°F | 45°F | 37°F | 4.9" |
| Index | Vandalia | Illinois | National |
| Air quality index | 40 | 41 | 42 |
| Pollution index | 647,280 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 245 |
| Days with good air quality | 224 |
| Days with moderate air quality | 21 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Coffeen, IL | 640 | 40 | 20.0% |
| St. Elmo, IL | 1,466 | 40 | 20.0% |
| Fillmore, IL | 296 | 40 | 20.0% |
| Mulberry Grove, IL | 847 | 40 | 20.0% |
| St. Peter, IL | 438 | 40 | 20.0% |
| Vandalia, IL | 6,615 | 40 | 20.0% |
| City | Population | Air Quality Index | National Avg. |
| Southgate, FL | 6,617 | 38 | 24.0% |
| Scenic Oaks, TX | 6,611 | 49 | 2.0% |
| Red Chute, LA | 6,619 | 39 | 22.0% |
| Trinity, NC | 6,619 | 40 | 20.0% |
| Kentville, NS | 6,619 | n/a | |
| Vandalia, IL | 6,615 | 40 | 20.0% |
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