Williamson, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 39.8 °F (low) to 60.4 °F (high). Annual precipitation averages 38.3 Inches. 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 | 13.3 °F | 31.5 °F | 1.02 in. |
| February | 17.2 °F | 36.1 °F | 1.38 in. |
| March | 27.9 °F | 49.1 °F | 2.12 in. |
| April | 38.7 °F | 61.4 °F | 4.03 in. |
| May | 50.6 °F | 71.2 °F | 5.47 in. |
| June | 60.6 °F | 80.8 °F | 5.3 in. |
| July | 64.8 °F | 85.1 °F | 4.61 in. |
| August | 62.1 °F | 83.3 °F | 4.43 in. |
| September | 53.2 °F | 76.7 °F | 3.71 in. |
| October | 41.1 °F | 64.0 °F | 2.82 in. |
| November | 29.1 °F | 49.0 °F | 2.01 in. |
| December | 19.1 °F | 36.8 °F | 1.4 in. |
| Annual | 39.8 °F | 60.4 °F | 38.3 in. |
| 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 : -15°F to -10°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |