| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 24°F | 38°F | 31°F | 6.8" |
| February | 23°F | 37°F | 29°F | 3.4" |
| March | 35°F | 56°F | 45°F | 3.0" |
| April | 43°F | 65°F | 54°F | 4.2" |
| May | 54°F | 78°F | 66°F | 3.1" |
| June | 64°F | 86°F | 75°F | 8.1" |
| July | 65°F | 86°F | 76°F | 4.0" |
| August | 68°F | 89°F | 78°F | 3.9" |
| September | 59°F | 85°F | 72°F | 7.7" |
| October | 49°F | 71°F | 60°F | 7.1" |
| November | 37°F | 56°F | 45°F | 6.1" |
| December | 24°F | 37°F | 30°F | 2.5" |
| Index | Decatur | 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. |
| Blue Mound, IL | 1,135 | 44 | 12.0% |
| Bethany, IL | 1,311 | 44 | 12.0% |
| Harristown, IL | 1,294 | 44 | 12.0% |
| Cisco, IL | 284 | 44 | 12.0% |
| Warrensburg, IL | 1,261 | 44 | 12.0% |
| Decatur, IL | 71,857 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| Frederick, MD | 71,843 | 40 | 20.0% |
| Lauderhill, FL | 71,827 | 41 | 18.0% |
| Pawtucket, RI | 71,821 | 44 | 12.0% |
| Pittsburg, CA | 71,723 | 40 | 20.0% |
| Redlands, CA | 71,680 | 89 | 78.0% |
| Decatur, IL | 71,857 | 44 | 12.0% |
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