| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 13°F | 33°F | 22°F | 0.4" |
| February | 15°F | 37°F | 25°F | 0.7" |
| March | 29°F | 54°F | 40°F | 1.4" |
| April | 39°F | 66°F | 51°F | 3.5" |
| May | 51°F | 77°F | 63°F | 4.0" |
| June | 62°F | 86°F | 73°F | 5.1" |
| July | 65°F | 89°F | 76°F | 3.0" |
| August | 62°F | 85°F | 73°F | 4.9" |
| September | 53°F | 79°F | 65°F | 3.9" |
| October | 41°F | 67°F | 52°F | 2.4" |
| November | 28°F | 52°F | 39°F | 0.9" |
| December | 16°F | 36°F | 25°F | 1.2" |
| Index | Defiance | Iowa | National |
| Air quality index | 40 | 39 | 50 |
| Pollution index | 517,691 | 1,643,836 | 5,066,862 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 283 |
| Days with moderate air quality | 82 |
| 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 | 0.002% |
| Mercury | 0.000% |
| City, State | Population | Air Quality Index | National Avg. |
| Denison, IA | 8,326 | 35 | 30% |
| Arion, IA | 126 | 40 | 20% |
| Earling, IA | 443 | 40 | 20% |
| Harlan, IA | 5,008 | 40 | 20% |
| Dow City, IA | 463 | 40 | 20% |
| Defiance, IA | 240 | 40 | 20% |
| City, State | Population | Air Quality Index | National Avg. |
| East Shore, CA | 240 | 46 | 8% |
| Roy, NM | 240 | 15 | 70% |
| Cuylerville, NY | 240 | 37 | 26% |
| Cherokee Pass, MO | 240 | 54 | 8% |
| Garden City, CO | 240 | 45 | 10% |
| Defiance, IA | 240 | 40 | 20% |
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