| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 17°F | 35°F | 26°F | 2.9" |
| February | 18°F | 37°F | 27°F | 2.4" |
| March | 28°F | 51°F | 39°F | 3.1" |
| April | 38°F | 65°F | 51°F | 4.0" |
| May | 49°F | 75°F | 62°F | 4.9" |
| June | 59°F | 84°F | 71°F | 5.1" |
| July | 62°F | 86°F | 74°F | 4.1" |
| August | 60°F | 84°F | 72°F | 4.4" |
| September | 53°F | 79°F | 65°F | 3.3" |
| October | 42°F | 67°F | 53°F | 2.9" |
| November | 31°F | 53°F | 42°F | 3.2" |
| December | 23°F | 39°F | 31°F | 3.1" |
| Index | Decatur | Indiana | National |
| Air quality index | 45 | 56 | 42 |
| Pollution index | 951,494 | 2,719,453 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 236 |
| Days with moderate air quality | 128 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.002% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.002% |
| Lead | 0.010% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Bluffton, IN | 10,298 | 31 | 38.0% |
| Vera Cruz, IN | 67 | 31 | 38.0% |
| Rockford, OH | 883 | 39 | 22.0% |
| Monroeville, IN | 1,315 | 45 | 10.0% |
| Willshire, OH | 347 | 45 | 10.0% |
| Decatur, IN | 9,700 | 45 | 10.0% |
| City | Population | Air Quality Index | National Avg. |
| Rockcreek, OR | 9,698 | 38 | 24.0% |
| Keansburg, NJ | 9,697 | 51 | 2.0% |
| Villas, NJ | 9,697 | 40 | 20.0% |
| Marysville, MI | 9,695 | 56 | 12.0% |
| Croydon, PA | 9,693 | 41 | 18.0% |
| Decatur, IN | 9,700 | 45 | 10.0% |
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