| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 17°F | 34°F | 24°F | 1.9" |
| February | 17°F | 34°F | 25°F | 2.4" |
| March | 31°F | 52°F | 40°F | 3.0" |
| April | 41°F | 65°F | 52°F | 4.3" |
| May | 53°F | 75°F | 63°F | 6.0" |
| June | 62°F | 84°F | 72°F | 4.4" |
| July | 65°F | 86°F | 75°F | 7.8" |
| August | 63°F | 83°F | 72°F | 4.4" |
| September | 54°F | 78°F | 65°F | 5.3" |
| October | 44°F | 66°F | 53°F | 5.8" |
| November | 32°F | 52°F | 41°F | 3.9" |
| December | 21°F | 37°F | 28°F | 4.0" |
| Index | Harper | Iowa | National |
| Air quality index | 35 | 39 | 42 |
| Pollution index | 577,838 | 1,154,535 | 2,726,193 |
| Measurement | Total |
| Days measured | 364 |
| Days with good air quality | 278 |
| Days with moderate air quality | 86 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.090% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Millersburg, IA | 181 | 35 | 30.0% |
| Ollie, IA | 173 | 35 | 30.0% |
| Brighton, IA | 688 | 35 | 30.0% |
| Keswick, IA | 295 | 35 | 30.0% |
| Webster, IA | 170 | 35 | 30.0% |
| Harper, IA | 96 | 35 | 30.0% |
| City | Population | Air Quality Index | National Avg. |
| Campo, CO | 96 | 20 | 60.0% |
| Belleair Shore, FL | 96 | 43 | 14.0% |
| Ellisville, IL | 96 | 41 | 18.0% |
| Kinsman, IL | 96 | 0 | 100.0% |
| Hickory Hill, KY | 96 | 49 | 2.0% |
| Harper, IA | 96 | 35 | 30.0% |
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