| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 20°F | 33°F | 26°F | 3.6" |
| February | 18°F | 32°F | 24°F | 3.0" |
| March | 30°F | 49°F | 39°F | 3.1" |
| April | 39°F | 62°F | 49°F | 5.2" |
| May | 49°F | 72°F | 60°F | 3.7" |
| June | 60°F | 83°F | 72°F | 4.9" |
| July | 62°F | 84°F | 73°F | 7.8" |
| August | 64°F | 84°F | 74°F | 8.3" |
| September | 56°F | 80°F | 68°F | 2.9" |
| October | 47°F | 67°F | 56°F | 4.3" |
| November | 32°F | 50°F | 40°F | 3.1" |
| December | 18°F | 32°F | 25°F | 2.2" |
| Index | Winfield | Illinois | National |
| Air quality index | 55 | 41 | 42 |
| Pollution index | 14,981,300 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 121 |
| Days with moderate air quality | 227 |
| Days w/ poor A.Q. for sensitive groups | 16 |
| Days with unhealthy air quality | 1 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.140% |
| Carbon Tetrachloride | 0.030% |
| Lead | 0.020% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Oswego, IL | 35,470 | 0 | 100.0% |
| Prestbury, IL | 1,978 | 0 | 100.0% |
| Montgomery, IL | 19,515 | 0 | 100.0% |
| Pingree Grove, IL | 8,768 | 42 | 16.0% |
| Sleepy Hollow, IL | 3,276 | 55 | 10.0% |
| Winfield, IL | 9,683 | 55 | 10.0% |
| City | Population | Air Quality Index | National Avg. |
| Notre-Dame-des-Prairies, QC | 9,684 | n/a | |
| Kensington, CT | 9,685 | 41 | 18.0% |
| Oakville, CT | 9,679 | 37 | 26.0% |
| Little River, SC | 9,692 | 34 | 32.0% |
| Loudoun Valley Estates, VA | 9,673 | 47 | 6.0% |
| Winfield, IL | 9,683 | 55 | 10.0% |
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