Barnwell, South Carolina has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 11.8 °C (low) to 24.6 °C (high). Annual precipitation averages 1242.8 Millimeters. Barnwell is located in USDA Plant Hardiness Zone 8b. The growing season typically runs from the last frost around Mar. 21 - Mar. 31 through the first frost around Nov. 1 - Nov. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 1.9 °C | 14.8 °C | 99.57 mm |
| February | 3.4 °C | 16.8 °C | 94.23 mm |
| March | 6.6 °C | 20.8 °C | 101.35 mm |
| April | 10.9 °C | 25 °C | 79.5 mm |
| May | 15.7 °C | 28.9 °C | 89.15 mm |
| June | 19.9 °C | 32.1 °C | 144.27 mm |
| July | 21.8 °C | 33.7 °C | 135.89 mm |
| August | 21.3 °C | 32.4 °C | 141.73 mm |
| September | 18.4 °C | 29.9 °C | 107.19 mm |
| October | 12.1 °C | 25.4 °C | 81.79 mm |
| November | 6.2 °C | 20.1 °C | 71.63 mm |
| December | 3.2 °C | 15.8 °C | 96.52 mm |
| Annual | 11.8 °C | 24.6 °C | 1242.82 mm |
| Average First Frost Date | Nov. 1 - Nov. 10 |
| Average Last Frost Date | Mar. 21 - Mar. 31 |
| Average Days With Frost | 40-50 Days |
| Plant Hardiness Zone | Zone 8b : -9.4°C to -6.7°C |
| Plant Hardiness Zone - 2012 | Zone 8a : -12.2°C to -9.4°C |
| Current Drought Conditions | Severe Drought |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |