Swansea, Illinois has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 8.5 °C (low) to 19 °C (high). Annual precipitation averages 1162.6 Millimeters. 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 | -4.6 °C | 4.4 °C | 72.14 mm |
| February | -2.8 °C | 7.4 °C | 62.23 mm |
| March | 2.1 °C | 13.1 °C | 91.69 mm |
| April | 8.1 °C | 19.7 °C | 127 mm |
| May | 14.1 °C | 24.7 °C | 132.84 mm |
| June | 19 °C | 29.6 °C | 131.06 mm |
| July | 20.9 °C | 31.4 °C | 111.76 mm |
| August | 20 °C | 30.7 °C | 96.77 mm |
| September | 15.3 °C | 27.1 °C | 86.11 mm |
| October | 9.1 °C | 20.7 °C | 82.3 mm |
| November | 2.7 °C | 12.8 °C | 95.25 mm |
| December | -2.1 °C | 6.8 °C | 73.41 mm |
| Annual | 8.5 °C | 19 °C | 1162.56 mm |
| 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 : -17.8°C to -15°C |
| Plant Hardiness Zone - 2012 | Zone 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |