| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 15°F | 36°F | 24°F | 1.5" |
| February | 15°F | 35°F | 24°F | 2.3" |
| March | 30°F | 51°F | 39°F | 3.5" |
| April | 41°F | 65°F | 52°F | 8.7" |
| May | 52°F | 74°F | 62°F | 7.1" |
| June | 62°F | 83°F | 72°F | 5.9" |
| July | 66°F | 87°F | 76°F | 7.3" |
| August | 63°F | 84°F | 72°F | 7.4" |
| September | 54°F | 77°F | 64°F | 10.8" |
| October | 43°F | 66°F | 53°F | 4.9" |
| November | 30°F | 50°F | 39°F | 4.4" |
| December | 22°F | 39°F | 30°F | 4.0" |
| Index | Williamson | Iowa | National |
| Air quality index | 39 | 39 | 42 |
| Pollution index | 1,478,700 | 1,154,535 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 306 |
| Days with moderate air quality | 59 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.002% |
| Lead | 0.010% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Lacona, IA | 339 | 39 | 22.0% |
| Lucas, IA | 113 | 39 | 22.0% |
| Derby, IA | 113 | 39 | 22.0% |
| Melcher-Dallas, IA | 1,313 | 39 | 22.0% |
| Melrose, IA | 179 | 39 | 22.0% |
| Williamson, IA | 184 | 39 | 22.0% |
| City | Population | Air Quality Index | National Avg. |
| Deer Creek, AZ | 184 | 74 | 48.0% |
| Clio, CA | 184 | 44 | 12.0% |
| Lodoga, CA | 184 | 31 | 38.0% |
| Tooleville, CA | 184 | 60 | 20.0% |
| Farmington, DE | 184 | 36 | 28.0% |
| Williamson, IA | 184 | 39 | 22.0% |
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