| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 5°F | 23°F | 14°F | 0.4" |
| February | 7°F | 27°F | 16°F | 0.6" |
| March | 22°F | 41°F | 30°F | 1.1" |
| April | 33°F | 57°F | 44°F | 1.9" |
| May | 45°F | 69°F | 56°F | 3.0" |
| June | 56°F | 78°F | 67°F | 3.7" |
| July | 61°F | 84°F | 72°F | 2.6" |
| August | 58°F | 81°F | 69°F | 3.6" |
| September | 50°F | 74°F | 61°F | 2.8" |
| October | 37°F | 58°F | 47°F | 2.3" |
| November | 23°F | 43°F | 32°F | 0.4" |
| December | 9°F | 26°F | 17°F | 0.7" |
| Index | Castlewood | South Dakota | National |
| Air quality index | 25 | 36 | 42 |
| Pollution index | 199,107 | 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 | 0.003% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Lake Poinsett, SD | 480 | 25 | 50.0% |
| Goodwin, SD | 102 | 25 | 50.0% |
| Watertown, SD | 22,249 | 25 | 50.0% |
| Hayti, SD | 391 | 25 | 50.0% |
| Lake Norden, SD | 531 | 25 | 50.0% |
| Castlewood, SD | 582 | 25 | 50.0% |
| City | Population | Air Quality Index | National Avg. |
| Sulphur Rock, AR | 582 | 45 | 10.0% |
| Laurel Hill, FL | 582 | 31 | 38.0% |
| Murrayville, IL | 582 | 40 | 20.0% |
| Tovey, IL | 582 | 40 | 20.0% |
| Bay Port, MI | 582 | 26 | 48.0% |
| Castlewood, SD | 582 | 25 | 50.0% |
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