Williamson, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 45 °F (low) to 64.5 °F (high). Annual precipitation averages 42.8 Inches. 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 | 21.4 °F | 37.5 °F | 2.61 in. |
| February | 24.9 °F | 42.7 °F | 2.3 in. |
| March | 33.7 °F | 53.5 °F | 3.2 in. |
| April | 44.3 °F | 65.4 °F | 4.82 in. |
| May | 55.2 °F | 74.9 °F | 5.07 in. |
| June | 64.1 °F | 83.8 °F | 4.42 in. |
| July | 67.4 °F | 86.8 °F | 4.07 in. |
| August | 65.3 °F | 85.8 °F | 3.51 in. |
| September | 56.9 °F | 79.9 °F | 3.33 in. |
| October | 45.4 °F | 68.0 °F | 3.25 in. |
| November | 34.9 °F | 53.5 °F | 3.56 in. |
| December | 26.2 °F | 42.2 °F | 2.67 in. |
| Annual | 45 °F | 64.5 °F | 42.81 in. |
| 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 : -5°F to 0°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |