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| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 23°F | 42°F | 32°F | 5.6" |
| February | 25°F | 45°F | 34°F | 3.8" |
| March | 37°F | 59°F | 47°F | 7.1" |
| April | 47°F | 70°F | 57°F | 10.2" |
| May | 56°F | 77°F | 66°F | 8.9" |
| June | 65°F | 87°F | 76°F | 9.2" |
| July | 68°F | 89°F | 78°F | 8.0" |
| August | 66°F | 88°F | 76°F | 7.8" |
| September | 58°F | 82°F | 69°F | 8.4" |
| October | 47°F | 70°F | 57°F | 8.6" |
| November | 36°F | 57°F | 46°F | 6.9" |
| December | 28°F | 45°F | 36°F | 8.2" |
| Index | Swansea | Illinois | National |
| Air quality index | 54 | 41 | 42 |
| Pollution index | 6,055,640 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 134 |
| Days with moderate air quality | 221 |
| Days w/ poor A.Q. for sensitive groups | 8 |
| Days with unhealthy air quality | 2 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.070% |
| Carbon Tetrachloride | 0.040% |
| Lead | 0.100% |
| Mercury | 0.230% |
| City | Population | Air Quality Index | National Avg. |
| Rentchler, IL | 4 | 54 | 8.0% |
| Summerfield, IL | 324 | 54 | 8.0% |
| Scott AFB, IL | 3,845 | 54 | 8.0% |
| Venice, IL | 2,040 | 54 | 8.0% |
| Riverview, MO | 2,848 | 54 | 8.0% |
| Swansea, IL | 14,100 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Vail, AZ | 14,099 | 47 | 6.0% |
| Willowick, OH | 14,103 | 50 | = 100 |
| Auburn, CA | 14,104 | 52 | 4.0% |
| Red Bluff, CA | 14,091 | 41 | 18.0% |
| Lake Wylie, SC | 14,091 | 49 | 2.0% |
| Swansea, IL | 14,100 | 54 | 8.0% |
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