| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 25°F | 39°F | 31°F | 6.1" |
| February | 23°F | 37°F | 29°F | 2.4" |
| March | 37°F | 55°F | 45°F | 4.1" |
| April | 44°F | 65°F | 53°F | 3.9" |
| May | 55°F | 77°F | 65°F | 2.6" |
| June | 64°F | 85°F | 75°F | 4.7" |
| July | 65°F | 86°F | 75°F | 8.2" |
| August | 68°F | 88°F | 77°F | 7.2" |
| September | 59°F | 83°F | 71°F | 8.1" |
| October | 49°F | 69°F | 58°F | 8.2" |
| November | 36°F | 54°F | 44°F | 3.6" |
| December | 25°F | 38°F | 31°F | 2.6" |
| Index | Sullivan | Illinois | National |
| Air quality index | 44 | 41 | 42 |
| Pollution index | 703,218 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 255 |
| Days with moderate air quality | 110 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Mattoon, IL | 17,273 | 40 | 20.0% |
| Gays, IL | 209 | 40 | 20.0% |
| Shelbyville, IL | 4,198 | 40 | 20.0% |
| Windsor village, IL | 965 | 40 | 20.0% |
| Windsor, IL | 1,150 | 40 | 20.0% |
| Sullivan, IL | 4,450 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| Tri-City, OR | 4,449 | 24 | 52.0% |
| Millsboro, DE | 4,448 | 36 | 28.0% |
| Saukville, WI | 4,448 | 44 | 12.0% |
| Montoursville, PA | 4,452 | 35 | 30.0% |
| Wolfville, NS | 4,453 | n/a | |
| Sullivan, IL | 4,450 | 44 | 12.0% |
Here are the best places to live in Illinois
These are the 10 most affordable cities in Illinois
Top 10 best public schools in Sullivan
10 best cities to buy a new house in Illinois