| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 34°F | 49°F | 41°F | 5.6" |
| February | 35°F | 53°F | 43°F | 4.1" |
| March | 37°F | 58°F | 47°F | 5.4" |
| April | 39°F | 63°F | 50°F | 2.9" |
| May | 45°F | 71°F | 57°F | 2.3" |
| June | 50°F | 76°F | 62°F | 1.3" |
| July | 54°F | 86°F | 69°F | 0.3" |
| August | 54°F | 86°F | 69°F | 0.4" |
| September | 50°F | 80°F | 63°F | 1.8" |
| October | 43°F | 66°F | 54°F | 4.1" |
| November | 38°F | 55°F | 46°F | 6.7" |
| December | 33°F | 48°F | 40°F | 7.4" |
| Index | Monmouth | Oregon | National |
| Air quality index | 32 | 32 | 42 |
| Pollution index | 3,182,180 | 7,156,356 | 2,726,193 |
| Measurement | Total |
| Days measured | 356 |
| Days with good air quality | 302 |
| Days with moderate air quality | 54 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.040% |
| Carbon Tetrachloride | 0.030% |
| Lead | 0.190% |
| Mercury | 0.110% |
| City | Population | Air Quality Index | National Avg. |
| Kings Valley, OR | 194 | 17 | 66.0% |
| Millersburg, OR | 2,234 | 28 | 44.0% |
| Jefferson, OR | 3,348 | 28 | 44.0% |
| Adair Village, OR | 1,107 | 28 | 44.0% |
| Marion, OR | 416 | 28 | 44.0% |
| Monmouth, OR | 10,411 | 32 | 36.0% |
| City | Population | Air Quality Index | National Avg. |
| Cape Canaveral, FL | 10,411 | 36 | 28.0% |
| Portage, WI | 10,412 | 35 | 30.0% |
| Mono, ON | 10,413 | n/a | |
| Frederick, CO | 10,414 | 47 | 6.0% |
| Fort Madison, IA | 10,407 | 29 | 42.0% |
| Monmouth, OR | 10,411 | 32 | 36.0% |
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