| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 23°F | 38°F | 29°F | 3.3" |
| February | 24°F | 40°F | 31°F | 3.5" |
| March | 36°F | 55°F | 44°F | 3.9" |
| April | 45°F | 67°F | 55°F | 9.3" |
| May | 55°F | 75°F | 64°F | 5.6" |
| June | 64°F | 84°F | 73°F | 7.9" |
| July | 66°F | 86°F | 76°F | 5.0" |
| August | 65°F | 86°F | 74°F | 5.1" |
| September | 57°F | 80°F | 67°F | 6.0" |
| October | 46°F | 68°F | 56°F | 5.6" |
| November | 36°F | 54°F | 44°F | 3.6" |
| December | 28°F | 42°F | 34°F | 3.6" |
| Index | Williamson | Illinois | National |
| Air quality index | 54 | 41 | 42 |
| Pollution index | 647,280 | 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 | 0.001% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Butler, IL | 269 | 40 | 20.0% |
| Litchfield, IL | 6,626 | 40 | 20.0% |
| Taylor Springs, IL | 587 | 40 | 20.0% |
| Dorchester, IL | 151 | 54 | 8.0% |
| New Douglas, IL | 321 | 54 | 8.0% |
| Williamson, IL | 202 | 54 | 8.0% |
| City | Population | Air Quality Index | National Avg. |
| Fairford, AL | 202 | 42 | 16.0% |
| Ratcliff, AR | 202 | 36 | 28.0% |
| Carrizo, AZ | 202 | 44 | 12.0% |
| Greenhorn, CA | 202 | 44 | 12.0% |
| Alpine, CO | 202 | 44 | 12.0% |
| Williamson, IL | 202 | 54 | 8.0% |
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