| 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 | 0 | 41 | 42 |
| Pollution index | 1,874,490 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 335 |
| Days with good air quality | 334 |
| Days with moderate air quality | 1 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.002% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.010% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| North Utica, IL | 1,136 | 0 | 100.0% |
| Leonore, IL | 135 | 0 | 100.0% |
| Seneca, IL | 2,534 | 0 | 100.0% |
| Earlville, IL | 2,057 | 0 | 100.0% |
| Ottawa, IL | 18,742 | 0 | 100.0% |
| Dayton, IL | 475 | 0 | 100.0% |
| City | Population | Air Quality Index | National Avg. |
| Ualapue, HI | 475 | 15 | 70.0% |
| Diagonal, IA | 475 | 39 | 22.0% |
| Garden Valley, ID | 475 | 36 | 28.0% |
| Grantfork, IL | 475 | 54 | 8.0% |
| Matthews, IN | 475 | 35 | 30.0% |
| Dayton, IL | 475 | 0 | 100.0% |
Here are the best places to live in Illinois
Safest States In The US
Check out the 10 most dangerous cities in Illinois
Most Dangerous States In The US