| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 34°F | 46°F | 39°F | 9.0" |
| February | 34°F | 49°F | 40°F | 4.0" |
| March | 36°F | 52°F | 43°F | 4.2" |
| April | 38°F | 58°F | 47°F | 2.9" |
| May | 43°F | 65°F | 53°F | 4.4" |
| June | 49°F | 70°F | 59°F | 3.3" |
| July | 54°F | 79°F | 65°F | 1.9" |
| August | 53°F | 77°F | 64°F | 3.2" |
| September | 49°F | 72°F | 59°F | 3.7" |
| October | 42°F | 60°F | 50°F | 5.2" |
| November | 39°F | 51°F | 44°F | 8.1" |
| December | 32°F | 44°F | 38°F | 6.1" |
| Index | Roy | Washington | National |
| Air quality index | 30 | 36 | 42 |
| Pollution index | 6,915,950 | 4,541,850 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 323 |
| Days with moderate air quality | 42 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.110% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.000% |
| Mercury | 0.001% |
| City | Population | Air Quality Index | National Avg. |
| Alder, WA | 134 | 20 | 60.0% |
| Tenino, WA | 1,744 | 20 | 60.0% |
| Midland, WA | 9,518 | 30 | 40.0% |
| Summit View, WA | 9,078 | 30 | 40.0% |
| McMillin, WA | 1,236 | 30 | 40.0% |
| Roy, WA | 814 | 30 | 40.0% |
| City | Population | Air Quality Index | National Avg. |
| Adair, IA | 814 | 39 | 22.0% |
| Science Hill, KY | 814 | 40 | 20.0% |
| Cleveland, NY | 814 | 37 | 26.0% |
| Poestenkill, NY | 814 | 37 | 26.0% |
| Eastview, TN | 814 | 27 | 46.0% |
| Roy, WA | 814 | 30 | 40.0% |
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