| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 11°F | 35°F | 22°F | 0.3" |
| February | 15°F | 39°F | 27°F | 0.4" |
| March | 24°F | 51°F | 36°F | 0.9" |
| April | 29°F | 59°F | 43°F | 1.7" |
| May | 37°F | 67°F | 52°F | 2.2" |
| June | 44°F | 78°F | 60°F | 2.7" |
| July | 50°F | 90°F | 70°F | 0.8" |
| August | 47°F | 87°F | 66°F | 1.2" |
| September | 40°F | 77°F | 57°F | 1.0" |
| October | 31°F | 61°F | 44°F | 1.3" |
| November | 20°F | 44°F | 31°F | 1.0" |
| December | 9°F | 32°F | 20°F | 0.5" |
| Index | Harrison | Montana | National |
| Air quality index | 25 | 34 | 42 |
| Pollution index | 293,782 | 834,473 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 337 |
| Days with moderate air quality | 28 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.004% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Cardwell, MT | 107 | 20 | 60.0% |
| Whitehall, MT | 1,000 | 20 | 60.0% |
| Pony, MT | 138 | 25 | 50.0% |
| Willow Creek, MT | 216 | 25 | 50.0% |
| Three Forks, MT | 1,977 | 25 | 50.0% |
| Harrison, MT | 64 | 25 | 50.0% |
| City | Population | Air Quality Index | National Avg. |
| Theba, AZ | 64 | 74 | 48.0% |
| Sandyville, IA | 64 | 39 | 22.0% |
| La Prairie, IL | 64 | 41 | 18.0% |
| Sidney, IN | 64 | 31 | 38.0% |
| Reserve, KS | 64 | 43 | 14.0% |
| Harrison, MT | 64 | 25 | 50.0% |
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