Castlewood, South Dakota has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 0.3 °C (low) to 12 °C (high). Annual precipitation averages 642.6 Millimeters. Castlewood is located in USDA Plant Hardiness Zone 4a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -16.2 °C | -5.7 °C | 14.73 mm |
| February | -14.1 °C | -3.1 °C | 16.51 mm |
| March | -7.2 °C | 3.7 °C | 27.94 mm |
| April | -0.4 °C | 12.1 °C | 59.69 mm |
| May | 6.9 °C | 19.3 °C | 80.52 mm |
| June | 13 °C | 24.9 °C | 107.95 mm |
| July | 15.4 °C | 27.8 °C | 91.19 mm |
| August | 13.9 °C | 26.7 °C | 75.95 mm |
| September | 9 °C | 22.3 °C | 74.17 mm |
| October | 1.3 °C | 14.1 °C | 55.12 mm |
| November | -5.9 °C | 4.9 °C | 20.83 mm |
| December | -12.5 °C | -2.8 °C | 18.03 mm |
| Annual | 0.3 °C | 12 °C | 642.63 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 4a : -34.4°C to -31.7°C |
| Plant Hardiness Zone - 2012 | Zone 4a : -34.4°C to -31.7°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |