| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 24°F | 41°F | 31°F | 3.4" |
| February | 25°F | 43°F | 33°F | 3.7" |
| March | 38°F | 57°F | 46°F | 3.1" |
| April | 47°F | 68°F | 56°F | 6.6" |
| May | 56°F | 76°F | 65°F | 6.1" |
| June | 65°F | 85°F | 75°F | 3.5" |
| July | 67°F | 87°F | 77°F | 7.3" |
| August | 66°F | 86°F | 75°F | 6.8" |
| September | 58°F | 80°F | 68°F | 4.4" |
| October | 47°F | 68°F | 57°F | 6.7" |
| November | 36°F | 56°F | 45°F | 4.9" |
| December | 28°F | 43°F | 35°F | 6.3" |
| Index | Alton | 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. |
| Dorchester, IL | 151 | 54 | 8.0% |
| Brighton, IL | 2,315 | 54 | 8.0% |
| Grafton, IL | 672 | 54 | 8.0% |
| Florissant, MO | 51,220 | 54 | 8.0% |
| Edwardsville, IL | 25,332 | 54 | 8.0% |
| Alton, IL | 26,490 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Conception Bay South, NL | 26,497 | n/a | |
| Batavia, IL | 26,479 | 55 | 10.0% |
| Pearl, MS | 26,462 | 44 | 12.0% |
| Wooster, OH | 26,557 | 43 | 14.0% |
| Brownsburg, IN | 26,560 | 54 | 8.0% |
| Alton, IL | 26,490 | 54 | 8.0% |
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