| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 23°F | 37°F | 30°F | 0.7" |
| February | 28°F | 49°F | 38°F | 1.2" |
| March | 37°F | 61°F | 49°F | 1.8" |
| April | 40°F | 67°F | 54°F | 0.9" |
| May | 50°F | 78°F | 65°F | 1.7" |
| June | 55°F | 83°F | 69°F | 1.4" |
| July | 64°F | 95°F | 80°F | 1.1" |
| August | 60°F | 92°F | 76°F | 1.4" |
| September | 51°F | 80°F | 66°F | 2.3" |
| October | 42°F | 64°F | 53°F | 1.8" |
| November | 32°F | 47°F | 40°F | 1.5" |
| December | 25°F | 37°F | 31°F | 2.1" |
| Index | Mattawa | Washington | National |
| Air quality index | 38 | 36 | 42 |
| Pollution index | 2,683,410 | 4,541,850 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 227 |
| Days with moderate air quality | 128 |
| Days w/ poor A.Q. for sensitive groups | 10 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.040% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.030% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Royal City, WA | 2,384 | 18 | 64.0% |
| Vantage, WA | 0 | 23 | 54.0% |
| Desert Aire, WA | 2,716 | 38 | 24.0% |
| Mattawa, WA | 5,040 | 38 | 24.0% |
| City | Population | Air Quality Index | National Avg. |
| Gretna, NE | 5,039 | 40 | 20.0% |
| Delhi Hills, OH | 5,041 | 54 | 8.0% |
| Rainsville, AL | 5,038 | 39 | 22.0% |
| Hudson, TX | 5,038 | 36 | 28.0% |
| Coverdale, NB | 5,038 | n/a | |
| Mattawa, WA | 5,040 | 38 | 24.0% |
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