| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 35°F | 26°F | 3.6" |
| February | 19°F | 38°F | 27°F | 3.0" |
| March | 33°F | 53°F | 42°F | 3.4" |
| April | 43°F | 67°F | 54°F | 5.4" |
| May | 53°F | 76°F | 64°F | 7.4" |
| June | 63°F | 84°F | 73°F | 10.8" |
| July | 65°F | 85°F | 75°F | 8.6" |
| August | 63°F | 84°F | 73°F | 8.8" |
| September | 55°F | 80°F | 66°F | 8.0" |
| October | 44°F | 68°F | 55°F | 8.3" |
| November | 33°F | 54°F | 42°F | 6.4" |
| December | 23°F | 38°F | 30°F | 5.1" |
| Index | Lewistown | Illinois | National |
| Air quality index | 41 | 41 | 42 |
| Pollution index | 1,001,710 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 285 |
| Days with moderate air quality | 78 |
| Days w/ poor A.Q. for sensitive groups | 2 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.004% |
| Benzene | 0.040% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.010% |
| Mercury | 0.020% |
| City | Population | Air Quality Index | National Avg. |
| Astoria, IL | 1,154 | 40 | 20.0% |
| Bath, IL | 300 | 40 | 20.0% |
| Havana, IL | 3,178 | 41 | 18.0% |
| Smithfield, IL | 181 | 41 | 18.0% |
| Table Grove, IL | 363 | 41 | 18.0% |
| Lewistown, IL | 2,021 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Gotha, FL | 2,021 | 39 | 22.0% |
| Suffield Depot, CT | 2,020 | 40 | 20.0% |
| Mountain Lake, MN | 2,022 | 32 | 36.0% |
| Creston, OH | 2,020 | 43 | 14.0% |
| Maple Creek, SK | 2,020 | n/a | |
| Lewistown, IL | 2,021 | 41 | 18.0% |
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