| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 17°F | 37°F | 26°F | 2.3" |
| February | 18°F | 39°F | 28°F | 2.1" |
| March | 31°F | 54°F | 42°F | 3.1" |
| April | 41°F | 67°F | 54°F | 4.7" |
| May | 51°F | 77°F | 64°F | 4.4" |
| June | 62°F | 85°F | 73°F | 5.4" |
| July | 65°F | 88°F | 76°F | 3.4" |
| August | 64°F | 86°F | 74°F | 3.5" |
| September | 56°F | 81°F | 68°F | 4.1" |
| October | 44°F | 68°F | 55°F | 3.4" |
| November | 33°F | 55°F | 43°F | 2.9" |
| December | 22°F | 39°F | 30°F | 2.7" |
| Index | Eureka | Illinois | National |
| Air quality index | 41 | 41 | 42 |
| Pollution index | 672,031 | 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 | 0.001% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.001% |
| Mercury | 0.020% |
| City | Population | Air Quality Index | National Avg. |
| Germantown Hills, IL | 3,421 | 41 | 18.0% |
| Roanoke, IL | 1,880 | 41 | 18.0% |
| Washburn, IL | 1,109 | 41 | 18.0% |
| Deer Creek, IL | 641 | 41 | 18.0% |
| Mackinaw, IL | 1,970 | 41 | 18.0% |
| Eureka, IL | 5,485 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Anamosa, IA | 5,484 | 39 | 22.0% |
| Saint-Roch-de-l'Achigan, QC | 5,486 | n/a | |
| Chadron, NE | 5,482 | 19 | 62.0% |
| Glencoe, MN | 5,488 | 46 | 8.0% |
| Orchard Homes, MT | 5,480 | 40 | 20.0% |
| Eureka, IL | 5,485 | 41 | 18.0% |
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