| 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% |
Here are the best places to live in Iowa
Check out the 10 safest cities in Iowa
10 Best cities in Iowa for employment
Top 10 cities in Iowa with the best education system