| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 32°F | 45°F | 38°F | 9.1" |
| February | 35°F | 50°F | 41°F | 4.8" |
| March | 37°F | 54°F | 45°F | 7.6" |
| April | 41°F | 60°F | 49°F | 4.4" |
| May | 47°F | 68°F | 56°F | 3.5" |
| June | 51°F | 72°F | 61°F | 2.2" |
| July | 55°F | 79°F | 66°F | 1.0" |
| August | 55°F | 79°F | 66°F | 1.5" |
| September | 50°F | 72°F | 59°F | 3.9" |
| October | 44°F | 61°F | 51°F | 6.4" |
| November | 37°F | 50°F | 43°F | 9.5" |
| December | 32°F | 43°F | 37°F | 7.3" |
| Index | Sumas | 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. |
| Lynden, WA | 14,700 | 34 | 32.0% |
| Deming, WA | 174 | 34 | 32.0% |
| Marietta-Alderwood, WA | 3,463 | 34 | 32.0% |
| Kendall, WA | 884 | 34 | 32.0% |
| Everson, WA | 2,773 | 34 | 32.0% |
| Sumas, WA | 1,405 | 34 | 32.0% |
| City | Population | Air Quality Index | National Avg. |
| Fort Dick, CA | 1,405 | 20 | 60.0% |
| Ravenna, MI | 1,405 | 38 | 24.0% |
| Moroni, UT | 1,405 | 46 | 8.0% |
| Shell Lake, WI | 1,405 | 34 | 32.0% |
| Marlinton, WV | 1,405 | 38 | 24.0% |
| Sumas, WA | 1,405 | 34 | 32.0% |
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