| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 36°F | 51°F | 43°F | 10.5" |
| February | 37°F | 53°F | 44°F | 6.7" |
| March | 39°F | 55°F | 46°F | 9.5" |
| April | 40°F | 57°F | 49°F | 6.0" |
| May | 45°F | 63°F | 53°F | 4.1" |
| June | 50°F | 66°F | 57°F | 2.4" |
| July | 53°F | 69°F | 60°F | 1.0" |
| August | 53°F | 71°F | 61°F | 0.8" |
| September | 50°F | 70°F | 59°F | 3.3" |
| October | 45°F | 63°F | 53°F | 7.4" |
| November | 40°F | 55°F | 47°F | 12.7" |
| December | 36°F | 49°F | 42°F | 11.3" |
| Index | Cathlamet | Washington | National |
| Air quality index | 18 | 36 | 42 |
| Pollution index | 4,215,410 | 4,541,850 | 2,726,193 |
| Measurement | Total |
| Days measured | 360 |
| Days with good air quality | 338 |
| Days with moderate air quality | 22 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.005% |
| Benzene | 0.050% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.030% |
| Mercury | 0.250% |
| City | Population | Air Quality Index | National Avg. |
| Westport, OR | 329 | 18 | 64.0% |
| Lower Elochoman, WA | 475 | 18 | 64.0% |
| Upper Elochoman, WA | 119 | 18 | 64.0% |
| Altoona, WA | 35 | 18 | 64.0% |
| Puget Island, WA | 1,227 | 18 | 64.0% |
| Cathlamet, WA | 605 | 18 | 64.0% |
| City | Population | Air Quality Index | National Avg. |
| Collins, GA | 605 | 39 | 22.0% |
| Portal, GA | 605 | 47 | 6.0% |
| Keystone, IA | 605 | 39 | 22.0% |
| Augusta, IL | 605 | 41 | 18.0% |
| Maple Hill, KS | 605 | 41 | 18.0% |
| Cathlamet, WA | 605 | 18 | 64.0% |
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