| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 36°F | 27°F | 1.7" |
| February | 19°F | 38°F | 28°F | 1.8" |
| March | 33°F | 54°F | 42°F | 2.3" |
| April | 42°F | 67°F | 54°F | 4.2" |
| May | 53°F | 77°F | 64°F | 4.2" |
| June | 63°F | 85°F | 73°F | 5.0" |
| July | 64°F | 85°F | 74°F | 4.5" |
| August | 62°F | 84°F | 73°F | 2.4" |
| September | 56°F | 80°F | 67°F | 3.4" |
| October | 44°F | 67°F | 55°F | 3.7" |
| November | 33°F | 53°F | 42°F | 2.8" |
| December | 24°F | 39°F | 31°F | 2.7" |
| Index | Monticello | Illinois | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 703,218 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 281 |
| Days with moderate air quality | 84 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Mansfield, IL | 1,006 | 42 | 16.0% |
| Bondville, IL | 455 | 42 | 16.0% |
| White Heath, IL | 153 | 42 | 16.0% |
| De Land, IL | 468 | 42 | 16.0% |
| Lake of the Woods, IL | 2,515 | 42 | 16.0% |
| Monticello, IL | 5,815 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| St. Johnsbury, VT | 5,815 | 40 | 20.0% |
| Elsmere, DE | 5,814 | 51 | 2.0% |
| Alma, AR | 5,816 | 36 | 28.0% |
| Little Silver, NJ | 5,817 | 51 | 2.0% |
| Monroeville, AL | 5,812 | 37 | 26.0% |
| Monticello, IL | 5,815 | 42 | 16.0% |
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