Williamson, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 7.2 °C (low) to 18.1 °C (high). Annual precipitation averages 1087.4 Millimeters. Williamson is located in USDA Plant Hardiness Zone 6b. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -5.9 °C | 3.1 °C | 66.29 mm |
| February | -3.9 °C | 5.9 °C | 58.42 mm |
| March | 0.9 °C | 11.9 °C | 81.28 mm |
| April | 6.8 °C | 18.6 °C | 122.43 mm |
| May | 12.9 °C | 23.8 °C | 128.78 mm |
| June | 17.8 °C | 28.8 °C | 112.27 mm |
| July | 19.7 °C | 30.4 °C | 103.38 mm |
| August | 18.5 °C | 29.9 °C | 89.15 mm |
| September | 13.8 °C | 26.6 °C | 84.58 mm |
| October | 7.4 °C | 20 °C | 82.55 mm |
| November | 1.6 °C | 11.9 °C | 90.42 mm |
| December | -3.2 °C | 5.7 °C | 67.82 mm |
| Annual | 7.2 °C | 18.1 °C | 1087.37 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 75-100 Days |
| Plant Hardiness Zone | Zone 6b : -20.6°C to -17.8°C |
| Plant Hardiness Zone - 2012 | Zone 6a : -23.3°C to -20.6°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |