Waterloo, Illinois has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 8.3 °C (low) to 18.9 °C (high). Annual precipitation averages 1110.2 Millimeters. Waterloo is located in USDA Plant Hardiness Zone 7a. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -4.9 °C | 4.5 °C | 70.61 mm |
| February | -2.7 °C | 7.3 °C | 64.26 mm |
| March | 2.1 °C | 12.9 °C | 93.22 mm |
| April | 7.8 °C | 19.8 °C | 128.27 mm |
| May | 13.7 °C | 24.4 °C | 126.75 mm |
| June | 18.3 °C | 29.6 °C | 116.59 mm |
| July | 20.5 °C | 31.1 °C | 100.08 mm |
| August | 19.4 °C | 30.3 °C | 78.23 mm |
| September | 15.3 °C | 26.9 °C | 83.31 mm |
| October | 9.2 °C | 20.4 °C | 79.76 mm |
| November | 2.8 °C | 12.7 °C | 92.71 mm |
| December | -1.9 °C | 6.8 °C | 76.45 mm |
| Annual | 8.3 °C | 18.9 °C | 1110.24 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 75-100 Days |
| Plant Hardiness Zone | Zone 7a : -17.8°C to -15°C |
| Plant Hardiness Zone - 2012 | Zone 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |