Duncan, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 35.8 °F (low) to 55.5 °F (high). Annual precipitation averages 34 Inches. Duncan is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 7.1 °F | 24.0 °F | 0.84 in. |
| February | 11.5 °F | 28.6 °F | 0.98 in. |
| March | 23.5 °F | 41.3 °F | 2.06 in. |
| April | 35.0 °F | 56.6 °F | 3.85 in. |
| May | 47.6 °F | 68.9 °F | 4.71 in. |
| June | 58.3 °F | 78.8 °F | 5.22 in. |
| July | 61.5 °F | 81.8 °F | 4.08 in. |
| August | 58.7 °F | 79.4 °F | 3.84 in. |
| September | 50.0 °F | 73.3 °F | 3.11 in. |
| October | 37.6 °F | 60.1 °F | 2.3 in. |
| November | 25.4 °F | 43.5 °F | 1.68 in. |
| December | 13.8 °F | 29.7 °F | 1.29 in. |
| Annual | 35.8 °F | 55.5 °F | 33.96 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5a : -20°F to -15°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |