Winthrop, Iowa has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 36.3 °F (low) to 56.7 °F (high). Annual precipitation averages 37.9 Inches. Winthrop is located in USDA Plant Hardiness Zone 5a. 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 | 8.4 °F | 25.8 °F | 1.13 in. |
| February | 12.3 °F | 30.2 °F | 1.21 in. |
| March | 24.1 °F | 43.4 °F | 2.0 in. |
| April | 35.4 °F | 57.9 °F | 3.75 in. |
| May | 47.5 °F | 69.6 °F | 4.67 in. |
| June | 58.4 °F | 79.3 °F | 5.94 in. |
| July | 61.3 °F | 82.1 °F | 4.68 in. |
| August | 58.8 °F | 80.2 °F | 4.26 in. |
| September | 50.3 °F | 74.2 °F | 3.71 in. |
| October | 37.9 °F | 61.0 °F | 2.87 in. |
| November | 25.8 °F | 44.8 °F | 2.09 in. |
| December | 15.1 °F | 31.6 °F | 1.56 in. |
| Annual | 36.3 °F | 56.7 °F | 37.87 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 5a : -20°F to -15°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |