| 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 | Harlan | Indiana | National |
| Air quality index | 47 | 56 | 50 |
| Pollution index | 2,775,870 | 3,773,618 | 5,066,862 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 219 |
| Days with moderate air quality | 145 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.002% |
| Lead | 0.380% |
| Mercury | 0.030% |
| City, State | Population | Air Quality Index | National Avg. |
| Garrett, IN | 6,620 | 47 | 6% |
| New Haven, IN | 15,677 | 47 | 6% |
| Hicksville, OH | 3,110 | 47 | 6% |
| Altona, IN | 226 | 47 | 6% |
| Grabill, IN | 1,448 | 47 | 6% |
| Harlan, IN | 1,486 | 47 | 6% |
| City, State | Population | Air Quality Index | National Avg. |
| Culver, OR | 1,486 | 35 | 30% |
| Osseo, WI | 1,486 | 355 | 610% |
| Sibley, LA | 1,487 | 43 | 14% |
| Linn, MO | 1,485 | 37 | 26% |
| Fredericksburg, PA | 1,485 | 43 | 14% |
| Harlan, IN | 1,486 | 47 | 6% |