Winthrop, Iowa has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 2.4 °C (low) to 13.7 °C (high). Annual precipitation averages 961.9 Millimeters. 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 | -13.1 °C | -3.4 °C | 28.7 mm |
| February | -10.9 °C | -1 °C | 30.73 mm |
| March | -4.4 °C | 6.3 °C | 50.8 mm |
| April | 1.9 °C | 14.4 °C | 95.25 mm |
| May | 8.6 °C | 20.9 °C | 118.62 mm |
| June | 14.7 °C | 26.3 °C | 150.88 mm |
| July | 16.3 °C | 27.8 °C | 118.87 mm |
| August | 14.9 °C | 26.8 °C | 108.2 mm |
| September | 10.2 °C | 23.4 °C | 94.23 mm |
| October | 3.3 °C | 16.1 °C | 72.9 mm |
| November | -3.4 °C | 7.1 °C | 53.09 mm |
| December | -9.4 °C | -0.2 °C | 39.62 mm |
| Annual | 2.4 °C | 13.7 °C | 961.89 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 5a : -28.9°C to -26.1°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |