| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 33°F | 25°F | 3.9" |
| February | 18°F | 35°F | 26°F | 2.3" |
| March | 31°F | 50°F | 39°F | 4.3" |
| April | 40°F | 63°F | 50°F | 8.4" |
| May | 51°F | 74°F | 62°F | 6.0" |
| June | 61°F | 82°F | 71°F | 5.7" |
| July | 63°F | 84°F | 73°F | 9.0" |
| August | 62°F | 82°F | 71°F | 5.4" |
| September | 54°F | 77°F | 65°F | 3.7" |
| October | 44°F | 65°F | 53°F | 7.1" |
| November | 33°F | 51°F | 41°F | 4.0" |
| December | 22°F | 37°F | 29°F | 3.0" |
| Index | Marseilles | Illinois | National |
| Air quality index | 0 | 41 | 42 |
| Pollution index | 1,736,930 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 335 |
| Days with good air quality | 334 |
| Days with moderate air quality | 1 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.004% |
| Benzene | 0.040% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.001% |
| Mercury | 0.001% |
| City | Population | Air Quality Index | National Avg. |
| Mazon, IL | 1,222 | 0 | 100.0% |
| Kangley, IL | 244 | 0 | 100.0% |
| Ransom, IL | 355 | 0 | 100.0% |
| Morris, IL | 14,420 | 0 | 100.0% |
| Newark, IL | 1,325 | 0 | 100.0% |
| Marseilles, IL | 4,579 | 0 | 100.0% |
| City | Population | Air Quality Index | National Avg. |
| Boiling Springs, NC | 4,580 | 36 | 28.0% |
| Donora, PA | 4,580 | 56 | 12.0% |
| Friendship Heights Village, MD | 4,577 | 47 | 6.0% |
| Blood 148, AB | 4,581 | n/a | |
| Bridgeport, PA | 4,576 | 51 | 2.0% |
| Marseilles, IL | 4,579 | 0 | 100.0% |
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