| 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 | Brighton | Illinois | National |
| Air quality index | 54 | 41 | 42 |
| Pollution index | 8,870,230 | 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 | 0.180% |
| Benzene | 0.060% |
| Carbon Tetrachloride | 0.020% |
| Lead | 0.030% |
| Mercury | 0.120% |
| City | Population | Air Quality Index | National Avg. |
| Chesterfield, IL | 208 | 40 | 20.0% |
| Dorchester, IL | 151 | 54 | 8.0% |
| Medora, IL | 323 | 54 | 8.0% |
| Elsah, IL | 701 | 54 | 8.0% |
| Old Jamestown, MO | 19,602 | 54 | 8.0% |
| Brighton, IL | 2,315 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Petersburg, IN | 2,315 | 1 | 98.0% |
| Custer, SD | 2,314 | 44 | 12.0% |
| Mayer, MN | 2,313 | 46 | 8.0% |
| Brownsville, PA | 2,313 | 36 | 28.0% |
| Victor, ID | 2,317 | 43 | 14.0% |
| Brighton, IL | 2,315 | 54 | 8.0% |
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