| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 24°F | 43°F | 32°F | 1.8" |
| February | 26°F | 45°F | 34°F | 1.6" |
| March | 37°F | 59°F | 47°F | 3.7" |
| April | 47°F | 70°F | 58°F | 5.3" |
| May | 56°F | 78°F | 67°F | 4.8" |
| June | 65°F | 88°F | 76°F | 4.4" |
| July | 68°F | 90°F | 79°F | 3.9" |
| August | 66°F | 89°F | 77°F | 3.1" |
| September | 58°F | 82°F | 69°F | 4.2" |
| October | 47°F | 71°F | 58°F | 3.1" |
| November | 36°F | 58°F | 46°F | 2.8" |
| December | 29°F | 45°F | 36°F | 3.1" |
| Index | Madison | Illinois | National |
| Air quality index | 54 | 41 | 42 |
| Pollution index | 8,473,050 | 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 | 0.020% |
| Benzene | 0.070% |
| Carbon Tetrachloride | 0.020% |
| Lead | 0.010% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Mitchell, IL | 1,233 | 54 | 8.0% |
| Troy, IL | 10,278 | 54 | 8.0% |
| Brooklyn, IL | 428 | 54 | 8.0% |
| Clayton, MO | 16,763 | 54 | 8.0% |
| Green Park, MO | 2,626 | 54 | 8.0% |
| Madison, IL | 2,941 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Port Ludlow, WA | 2,940 | 45 | 10.0% |
| Percé, QC | 2,942 | n/a | |
| Pine Canyon, CA | 2,939 | 37 | 26.0% |
| East Syracuse, NY | 2,938 | 37 | 26.0% |
| Boutte, LA | 2,944 | 41 | 18.0% |
| Madison, IL | 2,941 | 54 | 8.0% |
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