| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 17°F | 34°F | 24°F | 3.4" |
| February | 18°F | 36°F | 26°F | 3.6" |
| March | 32°F | 52°F | 41°F | 3.5" |
| April | 41°F | 65°F | 52°F | 6.9" |
| May | 52°F | 74°F | 63°F | 7.1" |
| June | 62°F | 83°F | 72°F | 7.9" |
| July | 65°F | 86°F | 75°F | 8.4" |
| August | 63°F | 84°F | 73°F | 7.4" |
| September | 54°F | 78°F | 65°F | 8.9" |
| October | 44°F | 66°F | 54°F | 5.8" |
| November | 32°F | 52°F | 41°F | 4.0" |
| December | 22°F | 37°F | 29°F | 4.5" |
| Index | Monmouth | Illinois | National |
| Air quality index | 32 | 41 | 42 |
| Pollution index | 1,709,230 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 300 |
| Days with moderate air quality | 65 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.050% |
| Carbon Tetrachloride | 0.040% |
| Lead | 0.001% |
| Mercury | 0.001% |
| City | Population | Air Quality Index | National Avg. |
| Stronghurst, IL | 1,066 | 29 | 42.0% |
| Kirkwood, IL | 742 | 29 | 42.0% |
| Media, IL | 110 | 29 | 42.0% |
| Biggsville, IL | 438 | 29 | 42.0% |
| Roseville, IL | 852 | 29 | 42.0% |
| Monmouth, IL | 8,837 | 32 | 36.0% |
| City | Population | Air Quality Index | National Avg. |
| Glencoe, IL | 8,836 | 55 | 10.0% |
| Clarendon Hills, IL | 8,834 | 55 | 10.0% |
| Whitestown, IN | 8,842 | 54 | 8.0% |
| Frankfort Square, IL | 8,831 | 55 | 10.0% |
| Sans Souci, SC | 8,844 | 46 | 8.0% |
| Monmouth, IL | 8,837 | 32 | 36.0% |
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