| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 34°F | 26°F | 3.3" |
| February | 19°F | 35°F | 26°F | 3.0" |
| March | 32°F | 51°F | 40°F | 4.3" |
| April | 42°F | 64°F | 52°F | 7.2" |
| May | 53°F | 75°F | 63°F | 8.0" |
| June | 62°F | 83°F | 72°F | 7.5" |
| July | 66°F | 85°F | 75°F | 7.1" |
| August | 64°F | 83°F | 73°F | 8.2" |
| September | 56°F | 78°F | 66°F | 6.3" |
| October | 45°F | 65°F | 54°F | 6.6" |
| November | 34°F | 51°F | 42°F | 4.3" |
| December | 23°F | 37°F | 30°F | 3.5" |
| Index | Dayton | Illinois | National |
| Air quality index | 1 | 41 | 50 |
| Pollution index | 1,874,490 | 3,347,295 | 5,066,862 |
| Measurement | Total |
| Days measured | 351 |
| Days with good air quality | 351 |
| Days with moderate air quality | |
| Days w/ poor A.Q. for sensitive groups | |
| Days with unhealthy air quality |
| Pollutant | Total |
| Arsenic | 0.002% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.010% |
| Mercury | 0.010% |
| City, State | Population | Air Quality Index | National Avg. |
| Newark, IL | 1,139 | 1 | 98% |
| Seneca, IL | 2,565 | 1 | 98% |
| Grand Ridge, IL | 588 | 1 | 98% |
| Marseilles, IL | 4,892 | 1 | 98% |
| Millbrook, IL | 341 | 1 | 98% |
| Dayton, IL | 520 | 1 | 98% |
| City, State | Population | Air Quality Index | National Avg. |
| Sidell, IL | 520 | 36 | 28% |
| Peterstown, WV | 520 | 35 | 30% |
| Rocky Ridge, OH | 520 | 36 | 28% |
| Troutville, VA | 520 | 36 | 28% |
| Michigantown, IN | 520 | 52 | 4% |
| Dayton, IL | 520 | 1 | 98% |