Williamson, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.3 °C (low) to 15.8 °C (high). Annual precipitation averages 972.8 Millimeters. Williamson is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.4 °C | -0.3 °C | 25.91 mm |
| February | -8.2 °C | 2.3 °C | 35.05 mm |
| March | -2.3 °C | 9.5 °C | 53.85 mm |
| April | 3.7 °C | 16.3 °C | 102.36 mm |
| May | 10.3 °C | 21.8 °C | 138.94 mm |
| June | 15.9 °C | 27.1 °C | 134.62 mm |
| July | 18.2 °C | 29.5 °C | 117.09 mm |
| August | 16.7 °C | 28.5 °C | 112.52 mm |
| September | 11.8 °C | 24.8 °C | 94.23 mm |
| October | 5.1 °C | 17.8 °C | 71.63 mm |
| November | -1.6 °C | 9.4 °C | 51.05 mm |
| December | -7.2 °C | 2.7 °C | 35.56 mm |
| Annual | 4.3 °C | 15.8 °C | 972.81 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |