| 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 | Keystone | South Dakota | National |
| Air quality index | 43 | 36 | 50 |
| Pollution index | 1,968,790 | 463,983 | 5,066,862 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 264 |
| Days with moderate air quality | 101 |
| Days w/ poor A.Q. for sensitive groups | |
| Days with unhealthy air quality |
| Pollutant | Total |
| Arsenic | 0.003% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.020% |
| Mercury | 0.010% |
| City, State | Population | Air Quality Index | National Avg. |
| Custer, SD | 1,770 | 43 | 14% |
| Johnson Siding, SD | 674 | 43 | 14% |
| Fairburn, SD | 79 | 43 | 14% |
| Rapid City, SD | 72,441 | 43 | 14% |
| Hermosa, SD | 459 | 43 | 14% |
| Keystone, SD | 318 | 43 | 14% |
| City, State | Population | Air Quality Index | National Avg. |
| Cumberland City, TN | 318 | 41 | 18% |
| Rutledge, AL | 318 | 37 | 26% |
| McVille, ND | 318 | 35 | 30% |
| Indianola, IL | 318 | 36 | 28% |
| Pentress, WV | 318 | 30 | 40% |
| Keystone, SD | 318 | 43 | 14% |
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