| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 11°F | 28°F | 18°F | 0.8" |
| February | 15°F | 33°F | 23°F | 1.6" |
| March | 28°F | 50°F | 37°F | 3.2" |
| April | 37°F | 62°F | 48°F | 4.7" |
| May | 50°F | 74°F | 61°F | 5.6" |
| June | 62°F | 84°F | 72°F | 10.1" |
| July | 65°F | 91°F | 77°F | 0.0" |
| August | 62°F | 86°F | 73°F | 1.8" |
| September | 55°F | 83°F | 67°F | 4.7" |
| October | 42°F | 66°F | 52°F | 6.7" |
| November | 27°F | 51°F | 37°F | 1.5" |
| December | 11°F | 29°F | 20°F | 0.6" |
| Index | Letcher | South Dakota | National |
| Air quality index | 42 | 36 | 42 |
| Pollution index | 105,773 | 345,623 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 284 |
| Days with moderate air quality | 76 |
| Days w/ poor A.Q. for sensitive groups | 5 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.001% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Mount Vernon, SD | 500 | 36 | 28.0% |
| Loomis, SD | 0 | 36 | 28.0% |
| Mitchell, SD | 15,599 | 36 | 28.0% |
| Forestburg, SD | 78 | 42 | 16.0% |
| Storla, SD | 21 | 42 | 16.0% |
| Letcher, SD | 191 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Coffee Springs, AL | 191 | 35 | 30.0% |
| Vredenburgh, AL | 191 | 52 | 4.0% |
| Hackberry, AZ | 191 | 13 | 74.0% |
| Dixon, IA | 191 | 46 | 8.0% |
| Hanlontown, IA | 191 | 15 | 70.0% |
| Letcher, SD | 191 | 42 | 16.0% |
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