| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 27°F | 43°F | 34°F | 1.0" |
| February | 28°F | 50°F | 38°F | 0.7" |
| March | 33°F | 60°F | 46°F | 1.0" |
| April | 37°F | 67°F | 52°F | 0.8" |
| May | 44°F | 77°F | 61°F | 1.0" |
| June | 52°F | 84°F | 68°F | 0.8" |
| July | 58°F | 94°F | 76°F | 0.1" |
| August | 56°F | 92°F | 74°F | 0.2" |
| September | 47°F | 82°F | 64°F | 0.4" |
| October | 39°F | 67°F | 52°F | 1.0" |
| November | 32°F | 52°F | 41°F | 1.0" |
| December | 25°F | 41°F | 32°F | 1.2" |
| Index | Hermiston | Oregon | National |
| Air quality index | 38 | 32 | 42 |
| Pollution index | 4,405,170 | 7,156,356 | 2,726,193 |
| Measurement | Total |
| Days measured | 364 |
| Days with good air quality | 286 |
| Days with moderate air quality | 72 |
| Days w/ poor A.Q. for sensitive groups | 6 |
| Days with unhealthy air quality |
| Pollutant | Total |
| Arsenic | 0.002% |
| Benzene | 0.130% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.010% |
| Mercury | 0.002% |
| City | Population | Air Quality Index | National Avg. |
| Umatilla, OR | 7,215 | 37 | 26.0% |
| Irrigon, OR | 1,904 | 37 | 26.0% |
| Echo, OR | 577 | 38 | 24.0% |
| Stanfield, OR | 2,521 | 38 | 24.0% |
| Hermiston, OR | 17,512 | 38 | 24.0% |
| City | Population | Air Quality Index | National Avg. |
| Anoka, MN | 17,512 | 46 | 8.0% |
| Martinsburg, WV | 17,517 | 40 | 20.0% |
| Parole, MD | 17,505 | 46 | 8.0% |
| South Houston, TX | 17,522 | 52 | 4.0% |
| Kingsland, GA | 17,523 | 36 | 28.0% |
| Hermiston, OR | 17,512 | 38 | 24.0% |
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