| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 21°F | 37°F | 28°F | 1.5" |
| February | 22°F | 39°F | 30°F | 1.7" |
| March | 35°F | 54°F | 44°F | 2.1" |
| April | 45°F | 66°F | 55°F | 4.9" |
| May | 55°F | 75°F | 64°F | 3.8" |
| June | 64°F | 84°F | 74°F | 4.7" |
| July | 66°F | 86°F | 75°F | 4.2" |
| August | 64°F | 85°F | 74°F | 2.1" |
| September | 57°F | 79°F | 67°F | 3.0" |
| October | 46°F | 67°F | 56°F | 2.9" |
| November | 35°F | 53°F | 44°F | 2.4" |
| December | 27°F | 41°F | 33°F | 2.4" |
| Index | Lerna | Illinois | National |
| Air quality index | 40 | 41 | 42 |
| Pollution index | 1,664,370 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 245 |
| Days with good air quality | 224 |
| Days with moderate air quality | 21 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.040% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.030% |
| Mercury | 0.003% |
| City | Population | Air Quality Index | National Avg. |
| Charleston, IL | 20,141 | 40 | 20.0% |
| Gays, IL | 209 | 40 | 20.0% |
| Mattoon, IL | 17,273 | 40 | 20.0% |
| Neoga, IL | 1,579 | 40 | 20.0% |
| Greenup, IL | 1,754 | 40 | 20.0% |
| Lerna, IL | 251 | 40 | 20.0% |
| City | Population | Air Quality Index | National Avg. |
| Anaktuvuk Pass, AK | 251 | 38 | 24.0% |
| Clam Gulch, AK | 251 | 31 | 38.0% |
| Oxbow Estates, AZ | 251 | 60 | 20.0% |
| Caryville, FL | 251 | 35 | 30.0% |
| College Springs, IA | 251 | 43 | 14.0% |
| Lerna, IL | 251 | 40 | 20.0% |
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