| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 20°F | 37°F | 28°F | 2.1" |
| February | 21°F | 38°F | 29°F | 2.0" |
| March | 34°F | 54°F | 43°F | 3.0" |
| April | 44°F | 66°F | 55°F | 4.3" |
| May | 54°F | 74°F | 63°F | 3.7" |
| June | 64°F | 84°F | 74°F | 6.0" |
| July | 66°F | 86°F | 76°F | 4.5" |
| August | 66°F | 86°F | 75°F | 4.2" |
| September | 57°F | 79°F | 67°F | 4.3" |
| October | 45°F | 64°F | 54°F | 3.9" |
| November | 36°F | 54°F | 44°F | 2.7" |
| December | 23°F | 38°F | 29°F | 2.4" |
| Index | Mendon | Illinois | National |
| Air quality index | 41 | 41 | 42 |
| Pollution index | 3,988,710 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 241 |
| Days with good air quality | 208 |
| Days with moderate air quality | 33 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.050% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.030% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Coatsburg, IL | 180 | 41 | 18.0% |
| West Point, IL | 219 | 41 | 18.0% |
| Liberty, IL | 398 | 41 | 18.0% |
| Quincy, IL | 40,111 | 41 | 18.0% |
| Lima, IL | 105 | 41 | 18.0% |
| Mendon, IL | 1,092 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Gun Club Estates, FL | 1,092 | 36 | 28.0% |
| Jamestown, IN | 1,092 | 54 | 8.0% |
| Silver City, NC | 1,092 | 41 | 18.0% |
| Poquott, NY | 1,092 | 43 | 14.0% |
| Joseph, OR | 1,092 | 20 | 60.0% |
| Mendon, IL | 1,092 | 41 | 18.0% |
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