Swansea, Illinois has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 47.3 °F (low) to 66.3 °F (high). Annual precipitation averages 45.8 Inches. Swansea is located in USDA Plant Hardiness Zone 7a. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 23.7 °F | 39.9 °F | 2.84 in. |
| February | 26.9 °F | 45.3 °F | 2.45 in. |
| March | 35.8 °F | 55.5 °F | 3.61 in. |
| April | 46.5 °F | 67.4 °F | 5.0 in. |
| May | 57.4 °F | 76.4 °F | 5.23 in. |
| June | 66.2 °F | 85.3 °F | 5.16 in. |
| July | 69.7 °F | 88.5 °F | 4.4 in. |
| August | 68.0 °F | 87.3 °F | 3.81 in. |
| September | 59.5 °F | 80.8 °F | 3.39 in. |
| October | 48.4 °F | 69.3 °F | 3.24 in. |
| November | 36.9 °F | 55.1 °F | 3.75 in. |
| December | 28.3 °F | 44.3 °F | 2.89 in. |
| Annual | 47.3 °F | 66.3 °F | 45.77 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 75-100 Days |
| Plant Hardiness Zone | Zone 7a : 0°F to 5°F |
| Plant Hardiness Zone - 2012 | Zone 6b : -5°F to 0°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |