| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 10°F | 27°F | 18°F | 2.1" |
| February | 14°F | 32°F | 22°F | 3.7" |
| March | 28°F | 46°F | 35°F | 3.9" |
| April | 38°F | 61°F | 48°F | 4.8" |
| May | 49°F | 72°F | 60°F | 6.4" |
| June | 60°F | 81°F | 70°F | 7.8" |
| July | 63°F | 84°F | 73°F | 7.3" |
| August | 60°F | 81°F | 70°F | 6.7" |
| September | 52°F | 77°F | 63°F | 4.7" |
| October | 40°F | 63°F | 50°F | 7.2" |
| November | 28°F | 48°F | 37°F | 4.5" |
| December | 16°F | 31°F | 23°F | 2.5" |
| Index | Williams | Iowa | National |
| Air quality index | 37 | 39 | 42 |
| Pollution index | 1,013,960 | 1,154,535 | 2,726,193 |
| Measurement | Total |
| Days measured | 214 |
| Days with good air quality | 210 |
| Days with moderate air quality | 4 |
| Days w/ poor A.Q. for sensitive groups | |
| Days with unhealthy air quality |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.001% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Dows, IA | 529 | 15 | 70.0% |
| Galt, IA | 35 | 15 | 70.0% |
| Bradford, IA | 61 | 15 | 70.0% |
| Popejoy, IA | 106 | 15 | 70.0% |
| Blairsburg, IA | 149 | 37 | 26.0% |
| Williams, IA | 327 | 37 | 26.0% |
| City | Population | Air Quality Index | National Avg. |
| King Salmon, AK | 327 | 31 | 38.0% |
| Fulton, AL | 327 | 36 | 28.0% |
| Mentone, AL | 327 | 39 | 22.0% |
| Chauncey, GA | 327 | 26 | 48.0% |
| Irwin, IA | 327 | 40 | 20.0% |
| Williams, IA | 327 | 37 | 26.0% |
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