| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 10°F | 29°F | 19°F | 1.4" |
| February | 12°F | 32°F | 22°F | 1.9" |
| March | 25°F | 47°F | 36°F | 2.3" |
| April | 36°F | 62°F | 49°F | 4.9" |
| May | 47°F | 72°F | 59°F | 4.5" |
| June | 58°F | 81°F | 69°F | 5.3" |
| July | 61°F | 84°F | 72°F | 6.9" |
| August | 59°F | 81°F | 70°F | 4.2" |
| September | 51°F | 76°F | 63°F | 3.2" |
| October | 40°F | 63°F | 51°F | 3.4" |
| November | 28°F | 49°F | 38°F | 2.2" |
| December | 15°F | 33°F | 24°F | 2.5" |
| Index | Monmouth | Iowa | National |
| Air quality index | 42 | 39 | 42 |
| Pollution index | 3,556,270 | 1,154,535 | 2,726,193 |
| Measurement | Total |
| Days measured | 364 |
| Days with good air quality | 268 |
| Days with moderate air quality | 96 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.060% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.002% |
| Lead | 0.020% |
| Mercury | 0.040% |
| City | Population | Air Quality Index | National Avg. |
| Clarence, IA | 1,151 | 35 | 30.0% |
| Stanwood, IA | 673 | 35 | 30.0% |
| Onslow, IA | 304 | 39 | 22.0% |
| Olin, IA | 966 | 39 | 22.0% |
| Wyoming, IA | 452 | 39 | 22.0% |
| Monmouth, IA | 131 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Lester, AL | 131 | 27 | 46.0% |
| St. Paul, AR | 131 | 44 | 12.0% |
| Genoa, CO | 131 | 45 | 10.0% |
| Hartwick, IA | 131 | 39 | 22.0% |
| Marne, IA | 131 | 40 | 20.0% |
| Monmouth, IA | 131 | 42 | 16.0% |
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