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| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 23°F | 42°F | 32°F | 3.7" |
| February | 25°F | 44°F | 34°F | 3.5" |
| March | 37°F | 58°F | 46°F | 5.2" |
| April | 46°F | 70°F | 57°F | 8.7" |
| May | 54°F | 77°F | 65°F | 6.1" |
| June | 63°F | 87°F | 74°F | 8.3" |
| July | 66°F | 89°F | 77°F | 8.1" |
| August | 64°F | 88°F | 75°F | 11.0" |
| September | 57°F | 81°F | 68°F | 5.5" |
| October | 45°F | 70°F | 56°F | 5.6" |
| November | 34°F | 56°F | 44°F | 7.5" |
| December | 28°F | 45°F | 36°F | 6.9" |
| Index | Mount Vernon | Illinois | National |
| Air quality index | 43 | 41 | 42 |
| Pollution index | 569,178 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 344 |
| Days with good air quality | 269 |
| Days with moderate air quality | 74 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| 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. |
| Centralia, IL | 12,354 | 43 | 14.0% |
| Walnut Hill, IL | 112 | 43 | 14.0% |
| Opdyke, IL | 303 | 43 | 14.0% |
| Dahlgren, IL | 699 | 43 | 14.0% |
| Richview, IL | 306 | 43 | 14.0% |
| Mount Vernon, IL | 14,769 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Gloversville, NY | 14,771 | 37 | 26.0% |
| South Farmingdale, NY | 14,766 | 51 | 2.0% |
| Hobe Sound, FL | 14,764 | 36 | 28.0% |
| Sterling, IL | 14,774 | 42 | 16.0% |
| Whitewater, WI | 14,762 | 37 | 26.0% |
| Mount Vernon, IL | 14,769 | 43 | 14.0% |
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