Winthrop Harbor, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.8 °C (low) to 13.2 °C (high). Annual precipitation averages 927.6 Millimeters. Winthrop Harbor is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -8.8 °C | -1 °C | 49.28 mm |
| February | -7.3 °C | 0.7 °C | 48.01 mm |
| March | -2.3 °C | 5.8 °C | 58.42 mm |
| April | 3.1 °C | 11.6 °C | 100.58 mm |
| May | 8.6 °C | 17.7 °C | 107.44 mm |
| June | 14.2 °C | 23.2 °C | 104.39 mm |
| July | 17.9 °C | 26.5 °C | 88.65 mm |
| August | 17.6 °C | 25.9 °C | 89.92 mm |
| September | 13.3 °C | 22.3 °C | 86.87 mm |
| October | 6.8 °C | 15.7 °C | 81.03 mm |
| November | 0.3 °C | 8.4 °C | 60.45 mm |
| December | -5.4 °C | 2.1 °C | 52.58 mm |
| Annual | 4.8 °C | 13.2 °C | 927.62 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 6a : -23.3°C to -20.6°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |