| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 15°F | 37°F | 25°F | 0.4" |
| February | 14°F | 36°F | 24°F | 0.6" |
| March | 23°F | 48°F | 34°F | 1.0" |
| April | 28°F | 53°F | 39°F | 2.4" |
| May | 37°F | 61°F | 48°F | 3.3" |
| June | 47°F | 74°F | 60°F | 2.9" |
| July | 53°F | 82°F | 67°F | 2.7" |
| August | 51°F | 80°F | 65°F | 2.4" |
| September | 43°F | 72°F | 56°F | 1.7" |
| October | 32°F | 57°F | 43°F | 1.3" |
| November | 23°F | 46°F | 34°F | 0.6" |
| December | 13°F | 35°F | 23°F | 0.7" |
| Index | Custer | South Dakota | National |
| Air quality index | 44 | 36 | 42 |
| Pollution index | 1,968,790 | 345,623 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 274 |
| Days with moderate air quality | 87 |
| Days w/ poor A.Q. for sensitive groups | 4 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.003% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.020% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Pringle, SD | 226 | 44 | 12.0% |
| Hill City, SD | 587 | 44 | 12.0% |
| Keystone, SD | 542 | 44 | 12.0% |
| Custer, SD | 2,314 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| Mayer, MN | 2,313 | 46 | 8.0% |
| Brownsville, PA | 2,313 | 36 | 28.0% |
| Brighton, IL | 2,315 | 54 | 8.0% |
| Petersburg, IN | 2,315 | 1 | 98.0% |
| Collinsville, AL | 2,312 | 39 | 22.0% |
| Custer, SD | 2,314 | 44 | 12.0% |
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