| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 27°F | 43°F | 34°F | 5.0" |
| February | 28°F | 46°F | 36°F | 4.8" |
| March | 39°F | 59°F | 48°F | 4.5" |
| April | 48°F | 70°F | 58°F | 10.6" |
| May | 57°F | 78°F | 67°F | 6.0" |
| June | 66°F | 87°F | 76°F | 7.6" |
| July | 67°F | 88°F | 77°F | 6.8" |
| August | 65°F | 87°F | 76°F | 5.4" |
| September | 58°F | 82°F | 69°F | 6.5" |
| October | 47°F | 71°F | 58°F | 3.5" |
| November | 37°F | 58°F | 47°F | 6.2" |
| December | 31°F | 47°F | 38°F | 8.9" |
| Index | McLeansboro | Illinois | National |
| Air quality index | 43 | 41 | 42 |
| Pollution index | 4,489,340 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 344 |
| Days with good air quality | 269 |
| Days with moderate air quality | 74 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.080% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.002% |
| Lead | 0.020% |
| Mercury | 0.050% |
| City | Population | Air Quality Index | National Avg. |
| Thompsonville, IL | 527 | 43 | 14.0% |
| Opdyke, IL | 303 | 43 | 14.0% |
| Springerton, IL | 67 | 43 | 14.0% |
| Broughton, IL | 116 | 43 | 14.0% |
| Belle Prairie City, IL | 75 | 43 | 14.0% |
| McLeansboro, IL | 2,539 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Galva, IL | 2,539 | 41 | 18.0% |
| Madelia, MN | 2,539 | 32 | 36.0% |
| Sisseton, SD | 2,539 | 25 | 50.0% |
| Colby, WI | 2,539 | 34 | 32.0% |
| Clinton, AR | 2,538 | 36 | 28.0% |
| McLeansboro, IL | 2,539 | 43 | 14.0% |
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