| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 32°F | 25°F | 4.1" |
| February | 16°F | 31°F | 23°F | 3.2" |
| March | 29°F | 48°F | 38°F | 3.7" |
| April | 38°F | 62°F | 49°F | 5.9" |
| May | 49°F | 74°F | 61°F | 4.0" |
| June | 60°F | 84°F | 72°F | 4.6" |
| July | 60°F | 85°F | 73°F | 9.3" |
| August | 62°F | 83°F | 72°F | 8.2" |
| September | 54°F | 81°F | 67°F | 2.4" |
| October | 45°F | 68°F | 55°F | 5.0" |
| November | 30°F | 49°F | 39°F | 3.4" |
| December | 16°F | 31°F | 23°F | 2.7" |
| Index | Oswego | Illinois | National |
| Air quality index | 0 | 41 | 42 |
| Pollution index | 1,874,490 | 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 | 0.002% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.010% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Millbrook, IL | 357 | 0 | 100.0% |
| Lisbon, IL | 281 | 0 | 100.0% |
| Plattville, IL | 190 | 0 | 100.0% |
| Big Rock, IL | 1,271 | 0 | 100.0% |
| Millington, IL | 662 | 0 | 100.0% |
| Oswego, IL | 35,470 | 0 | 100.0% |
| City | Population | Air Quality Index | National Avg. |
| Richmond, IN | 35,467 | 6 | 88.0% |
| Wildwood, MO | 35,476 | 54 | 8.0% |
| Inver Grove Heights, MN | 35,492 | 46 | 8.0% |
| Gahanna, OH | 35,442 | 46 | 8.0% |
| Mount Vernon, WA | 35,502 | 24 | 52.0% |
| Oswego, IL | 35,470 | 0 | 100.0% |
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