| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 11°F | 30°F | 20°F | 0.2" |
| February | 13°F | 34°F | 22°F | 0.4" |
| March | 26°F | 49°F | 36°F | 0.8" |
| April | 37°F | 62°F | 48°F | 2.2" |
| May | 48°F | 73°F | 60°F | 3.6" |
| June | 59°F | 83°F | 71°F | 3.3" |
| July | 64°F | 89°F | 76°F | 2.9" |
| August | 61°F | 84°F | 72°F | 3.2" |
| September | 52°F | 78°F | 64°F | 1.9" |
| October | 40°F | 64°F | 50°F | 2.2" |
| November | 26°F | 49°F | 36°F | 0.4" |
| December | 13°F | 32°F | 22°F | 0.5" |
| Index | Milltown | South Dakota | National |
| Air quality index | 36 | 36 | 42 |
| Pollution index | 145,854 | 345,623 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 321 |
| Days with moderate air quality | 43 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.002% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Ethan, SD | 328 | 36 | 28.0% |
| Olivet, SD | 82 | 36 | 28.0% |
| Parkston, SD | 1,756 | 36 | 28.0% |
| Emery, SD | 510 | 36 | 28.0% |
| Tripp, SD | 572 | 36 | 28.0% |
| Milltown, SD | 25 | 36 | 28.0% |
| City | Population | Air Quality Index | National Avg. |
| Mendeltna, AK | 25 | 31 | 38.0% |
| Beyerville, AZ | 25 | 31 | 38.0% |
| Butte Meadows, CA | 25 | 46 | 8.0% |
| Lazy Lake, FL | 25 | 41 | 18.0% |
| Blanchard, IA | 25 | 43 | 14.0% |
| Milltown, SD | 25 | 36 | 28.0% |
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