| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 15°F | 38°F | 26°F | 0.8" |
| February | 16°F | 40°F | 27°F | 1.2" |
| March | 26°F | 53°F | 38°F | 2.5" |
| April | 33°F | 59°F | 45°F | 4.6" |
| May | 43°F | 68°F | 55°F | 6.2" |
| June | 54°F | 81°F | 67°F | 5.1" |
| July | 61°F | 89°F | 75°F | 3.4" |
| August | 59°F | 87°F | 72°F | 6.4" |
| September | 50°F | 79°F | 63°F | 4.3" |
| October | 37°F | 63°F | 49°F | 5.6" |
| November | 25°F | 51°F | 37°F | 3.2" |
| December | 14°F | 36°F | 24°F | 1.6" |
| Index | Spearfish | South Dakota | National |
| Air quality index | 44 | 36 | 42 |
| Pollution index | 486,607 | 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 | 0.010% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.080% |
| Mercury | 0.001% |
| City | Population | Air Quality Index | National Avg. |
| Fruitdale, SD | 97 | 44 | 12.0% |
| Deadwood, SD | 1,584 | 44 | 12.0% |
| Sturgis, SD | 6,914 | 44 | 12.0% |
| Nisland, SD | 209 | 44 | 12.0% |
| Lead, SD | 2,979 | 44 | 12.0% |
| Spearfish, SD | 11,702 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| Crossville, TN | 11,704 | 53 | 6.0% |
| Little Chute, WI | 11,704 | 37 | 26.0% |
| El Campo, TX | 11,695 | 31 | 38.0% |
| Smyrna, DE | 11,694 | 42 | 16.0% |
| Salem, OH | 11,692 | 42 | 16.0% |
| Spearfish, SD | 11,702 | 44 | 12.0% |
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