| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 36°F | 47°F | 41°F | 7.4" |
| February | 37°F | 50°F | 42°F | 3.8" |
| March | 40°F | 54°F | 46°F | 5.8" |
| April | 43°F | 58°F | 50°F | 2.8" |
| May | 48°F | 65°F | 56°F | 2.9" |
| June | 53°F | 68°F | 60°F | 1.8" |
| July | 56°F | 73°F | 64°F | 1.0" |
| August | 56°F | 73°F | 64°F | 1.4" |
| September | 52°F | 68°F | 59°F | 3.2" |
| October | 46°F | 60°F | 52°F | 5.3" |
| November | 40°F | 51°F | 45°F | 6.7" |
| December | 34°F | 45°F | 39°F | 6.3" |
| Index | Custer | Washington | National |
| Air quality index | 34 | 36 | 42 |
| Pollution index | 480,925 | 4,541,850 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 277 |
| Days with moderate air quality | 78 |
| Days w/ poor A.Q. for sensitive groups | 10 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.040% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Bellingham, WA | 90,821 | 34 | 32.0% |
| Birch Bay, WA | 9,917 | 34 | 32.0% |
| Nooksack, WA | 1,594 | 34 | 32.0% |
| Ferndale, WA | 14,591 | 34 | 32.0% |
| Blaine, WA | 5,427 | 34 | 32.0% |
| Custer, WA | 298 | 34 | 32.0% |
| City | Population | Air Quality Index | National Avg. |
| Akiak, AK | 298 | 31 | 38.0% |
| Goose Lake, IA | 298 | 42 | 16.0% |
| Weippe, ID | 298 | 29 | 42.0% |
| Taylor, MS | 298 | 33 | 34.0% |
| Toccopola, MS | 298 | 39 | 22.0% |
| Custer, WA | 298 | 34 | 32.0% |
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