| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 6°F | 28°F | 17°F | 0.5" |
| February | 9°F | 34°F | 20°F | 0.7" |
| March | 22°F | 48°F | 34°F | 1.0" |
| April | 33°F | 62°F | 47°F | 2.2" |
| May | 44°F | 73°F | 58°F | 3.2" |
| June | 55°F | 83°F | 69°F | 4.5" |
| July | 61°F | 89°F | 75°F | 2.7" |
| August | 58°F | 85°F | 71°F | 3.3" |
| September | 49°F | 79°F | 63°F | 2.8" |
| October | 36°F | 64°F | 49°F | 2.2" |
| November | 22°F | 49°F | 34°F | 0.8" |
| December | 9°F | 31°F | 19°F | 1.3" |
| Index | Iroquois | South Dakota | National |
| Air quality index | 25 | 36 | 42 |
| Pollution index | 290,986 | 345,623 | 2,726,193 |
| Measurement | Total |
| Days measured | 361 |
| Days with good air quality | 325 |
| Days with moderate air quality | 36 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.005% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.010% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Cavour, SD | 222 | 25 | 50.0% |
| Yale, SD | 162 | 25 | 50.0% |
| Bancroft, SD | 50 | 25 | 50.0% |
| De Smet, SD | 1,041 | 42 | 16.0% |
| Carthage, SD | 181 | 42 | 16.0% |
| Iroquois, SD | 378 | 25 | 50.0% |
| City | Population | Air Quality Index | National Avg. |
| Charlotte, IA | 378 | 42 | 16.0% |
| Belle Rive, IL | 378 | 43 | 14.0% |
| Hazel, KY | 378 | 44 | 12.0% |
| Akron, MI | 378 | 26 | 48.0% |
| Annapolis, MO | 378 | 54 | 8.0% |
| Iroquois, SD | 378 | 25 | 50.0% |
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