| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 26°F | 41°F | 33°F | 9.0" |
| February | 25°F | 43°F | 33°F | 4.8" |
| March | 36°F | 60°F | 47°F | 3.9" |
| April | 44°F | 68°F | 55°F | 4.3" |
| May | 56°F | 80°F | 67°F | 4.3" |
| June | 65°F | 87°F | 76°F | 5.8" |
| July | 66°F | 89°F | 77°F | 3.9" |
| August | 69°F | 91°F | 79°F | 4.6" |
| September | 60°F | 85°F | 71°F | 4.9" |
| October | 48°F | 71°F | 59°F | 3.7" |
| November | 36°F | 58°F | 46°F | 7.6" |
| December | 25°F | 40°F | 32°F | 2.5" |
| Index | Hardin | Illinois | National |
| Air quality index | 54 | 41 | 42 |
| Pollution index | 2,458,180 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 134 |
| Days with moderate air quality | 221 |
| Days w/ poor A.Q. for sensitive groups | 8 |
| Days with unhealthy air quality | 2 |
| Pollutant | Total |
| Arsenic | 0.002% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.120% |
| Mercury | 0.330% |
| City | Population | Air Quality Index | National Avg. |
| Paynesville, MO | 28 | 41 | 18.0% |
| Hamburg, IL | 153 | 54 | 8.0% |
| Elsah, IL | 701 | 54 | 8.0% |
| Fountain N' Lakes, MO | 232 | 54 | 8.0% |
| Kampsville, IL | 321 | 54 | 8.0% |
| Hardin, IL | 831 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Pearl City, IL | 831 | 42 | 16.0% |
| Ridgway, IL | 831 | 45 | 10.0% |
| Annville, KY | 831 | 33 | 34.0% |
| Libertytown, MD | 831 | 42 | 16.0% |
| Tilghman Island, MD | 831 | 41 | 18.0% |
| Hardin, IL | 831 | 54 | 8.0% |
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