| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 17°F | 39°F | 27°F | 0.5" |
| February | 19°F | 43°F | 29°F | 0.5" |
| March | 27°F | 54°F | 39°F | 1.1" |
| April | 34°F | 61°F | 46°F | 1.7" |
| May | 42°F | 70°F | 55°F | 3.0" |
| June | 51°F | 81°F | 65°F | 1.7" |
| July | 59°F | 92°F | 75°F | 0.9" |
| August | 56°F | 89°F | 72°F | 1.0" |
| September | 48°F | 79°F | 62°F | 1.6" |
| October | 38°F | 63°F | 49°F | 1.7" |
| November | 27°F | 50°F | 37°F | 0.8" |
| December | 16°F | 36°F | 25°F | 0.9" |
| Index | Roundup | Montana | National |
| Air quality index | 21 | 34 | 42 |
| Pollution index | 143,915 | 834,473 | 2,726,193 |
| Measurement | Total |
| Days measured | 362 |
| Days with good air quality | 357 |
| Days with moderate air quality | 4 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 1 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.003% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Camp Three, MT | 150 | 21 | 58.0% |
| Klein, MT | 156 | 21 | 58.0% |
| Roundup, MT | 1,723 | 21 | 58.0% |
| City | Population | Air Quality Index | National Avg. |
| Duxbury, MA | 1,723 | 43 | 14.0% |
| Lakeland, MN | 1,723 | 46 | 8.0% |
| Bay Springs, MS | 1,723 | 36 | 28.0% |
| Lyman, WY | 1,723 | 44 | 12.0% |
| Hazen, AR | 1,724 | 45 | 10.0% |
| Roundup, MT | 1,723 | 21 | 58.0% |
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