| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 9°F | 25°F | 17°F | 0.7" |
| February | 11°F | 29°F | 20°F | 1.1" |
| March | 25°F | 44°F | 34°F | 2.2" |
| April | 36°F | 60°F | 47°F | 5.1" |
| May | 48°F | 71°F | 59°F | 5.0" |
| June | 58°F | 81°F | 69°F | 5.4" |
| July | 61°F | 84°F | 72°F | 4.2" |
| August | 59°F | 82°F | 70°F | 4.0" |
| September | 51°F | 76°F | 62°F | 4.3" |
| October | 38°F | 62°F | 49°F | 2.7" |
| November | 26°F | 47°F | 36°F | 1.5" |
| December | 13°F | 29°F | 21°F | 1.4" |
| Index | Duncan | Iowa | National |
| Air quality index | 15 | 39 | 42 |
| Pollution index | 784,924 | 1,154,535 | 2,726,193 |
| Measurement | Total |
| Days measured | 118 |
| Days with good air quality | 118 |
| Days with moderate air quality | 0 |
| 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 | Population | Air Quality Index | National Avg. |
| Fertile, IA | 295 | 15 | 70.0% |
| Hayfield, IA | 56 | 15 | 70.0% |
| Miller, IA | 30 | 15 | 70.0% |
| Woden, IA | 177 | 15 | 70.0% |
| Forest City, IA | 4,117 | 15 | 70.0% |
| Duncan, IA | 102 | 15 | 70.0% |
| City | Population | Air Quality Index | National Avg. |
| St. George, AK | 102 | 31 | 38.0% |
| Kingvale, CA | 102 | 44 | 12.0% |
| Mount Laguna, CA | 102 | 64 | 28.0% |
| Phillipsville, CA | 102 | 27 | 46.0% |
| Hull, GA | 102 | 44 | 12.0% |
| Duncan, IA | 102 | 15 | 70.0% |
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