Winthrop Harbor, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 40.7 °F (low) to 55.8 °F (high). Annual precipitation averages 36.5 Inches. 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 | 16.2 °F | 30.2 °F | 1.94 in. |
| February | 18.9 °F | 33.2 °F | 1.89 in. |
| March | 27.8 °F | 42.5 °F | 2.3 in. |
| April | 37.6 °F | 52.8 °F | 3.96 in. |
| May | 47.4 °F | 63.8 °F | 4.23 in. |
| June | 57.6 °F | 73.8 °F | 4.11 in. |
| July | 64.3 °F | 79.7 °F | 3.49 in. |
| August | 63.7 °F | 78.6 °F | 3.54 in. |
| September | 56.0 °F | 72.1 °F | 3.42 in. |
| October | 44.2 °F | 60.2 °F | 3.19 in. |
| November | 32.6 °F | 47.2 °F | 2.38 in. |
| December | 22.3 °F | 35.7 °F | 2.07 in. |
| Annual | 40.7 °F | 55.8 °F | 36.52 in. |
| 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 : -10°F to -5°F |
| Plant Hardiness Zone - 2012 | Zone 5b : -15°F to -10°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |