Castlewood, South Dakota has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 32.5 °F (low) to 53.6 °F (high). Annual precipitation averages 25.3 Inches. 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 | 2.9 °F | 21.7 °F | 0.58 in. |
| February | 6.7 °F | 26.5 °F | 0.65 in. |
| March | 19.0 °F | 38.6 °F | 1.1 in. |
| April | 31.2 °F | 53.7 °F | 2.35 in. |
| May | 44.4 °F | 66.7 °F | 3.17 in. |
| June | 55.4 °F | 76.8 °F | 4.25 in. |
| July | 59.8 °F | 82.0 °F | 3.59 in. |
| August | 57.0 °F | 80.0 °F | 2.99 in. |
| September | 48.2 °F | 72.2 °F | 2.92 in. |
| October | 34.3 °F | 57.4 °F | 2.17 in. |
| November | 21.3 °F | 40.9 °F | 0.82 in. |
| December | 9.5 °F | 27.0 °F | 0.71 in. |
| Annual | 32.5 °F | 53.6 °F | 25.3 in. |
| 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 : -30°F to -25°F |
| Plant Hardiness Zone - 2012 | Zone 4a : -30°F to -25°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |