| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 37°F | 49°F | 42°F | 10.5" |
| February | 38°F | 51°F | 44°F | 6.3" |
| March | 40°F | 53°F | 46°F | 9.8" |
| April | 42°F | 57°F | 49°F | 5.4" |
| May | 47°F | 62°F | 53°F | 3.4" |
| June | 51°F | 65°F | 57°F | 1.8" |
| July | 53°F | 69°F | 60°F | 0.9" |
| August | 54°F | 70°F | 60°F | 1.0" |
| September | 51°F | 69°F | 59°F | 2.7" |
| October | 46°F | 61°F | 53°F | 7.6" |
| November | 41°F | 53°F | 46°F | 12.6" |
| December | 36°F | 47°F | 41°F | 10.1" |
| Index | Montesano | Washington | National |
| Air quality index | 20 | 36 | 42 |
| Pollution index | 2,873,390 | 4,541,850 | 2,726,193 |
| Measurement | Total |
| Days measured | 352 |
| Days with good air quality | 348 |
| Days with moderate air quality | 4 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.050% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.010% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Junction City, WA | 9 | 20 | 60.0% |
| Satsop, WA | 638 | 20 | 60.0% |
| Aberdeen, WA | 16,571 | 20 | 60.0% |
| Aberdeen Gardens, WA | 305 | 20 | 60.0% |
| Cosmopolis, WA | 1,968 | 20 | 60.0% |
| Montesano, WA | 4,002 | 20 | 60.0% |
| City | Population | Air Quality Index | National Avg. |
| La Pocatière, QC | 4,001 | n/a | |
| Eldorado, IL | 4,000 | 43 | 14.0% |
| Sparwood, BC | 3,999 | n/a | |
| Deer Park, IL | 4,005 | 55 | 10.0% |
| Pearl River, MS | 4,005 | 36 | 28.0% |
| Montesano, WA | 4,002 | 20 | 60.0% |
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