| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 17°F | 34°F | 24°F | 3.4" |
| February | 18°F | 36°F | 26°F | 3.6" |
| March | 32°F | 52°F | 41°F | 3.5" |
| April | 41°F | 65°F | 52°F | 6.9" |
| May | 52°F | 74°F | 63°F | 7.1" |
| June | 62°F | 83°F | 72°F | 7.9" |
| July | 65°F | 86°F | 75°F | 8.4" |
| August | 63°F | 84°F | 73°F | 7.4" |
| September | 54°F | 78°F | 65°F | 8.9" |
| October | 44°F | 66°F | 54°F | 5.8" |
| November | 32°F | 52°F | 41°F | 4.0" |
| December | 22°F | 37°F | 29°F | 4.5" |
| Index | Henderson | Illinois | National |
| Air quality index | 46 | 41 | 42 |
| Pollution index | 1,709,230 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 217 |
| Days with moderate air quality | 145 |
| Days w/ poor A.Q. for sensitive groups | 3 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.050% |
| Carbon Tetrachloride | 0.040% |
| Lead | 0.001% |
| Mercury | 0.001% |
| City | Population | Air Quality Index | National Avg. |
| Monmouth, IL | 8,837 | 32 | 36.0% |
| East Galesburg, IL | 579 | 41 | 18.0% |
| Galesburg, IL | 30,405 | 41 | 18.0% |
| Gilson, IL | 194 | 41 | 18.0% |
| Oak Run, IL | 468 | 41 | 18.0% |
| Henderson, IL | 280 | 46 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Chuichu, AZ | 280 | 74 | 48.0% |
| Deep River, IA | 280 | 35 | 30.0% |
| South English, IA | 280 | 35 | 30.0% |
| Arlington, IN | 280 | 28 | 44.0% |
| Burns, KS | 280 | 40 | 20.0% |
| Henderson, IL | 280 | 46 | 8.0% |
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