Waterloo, Illinois has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 46.9 °F (low) to 66 °F (high). Annual precipitation averages 43.7 Inches. 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 | 23.2 °F | 40.1 °F | 2.78 in. |
| February | 27.2 °F | 45.2 °F | 2.53 in. |
| March | 35.7 °F | 55.2 °F | 3.67 in. |
| April | 46.0 °F | 67.6 °F | 5.05 in. |
| May | 56.7 °F | 76.0 °F | 4.99 in. |
| June | 64.9 °F | 85.2 °F | 4.59 in. |
| July | 68.9 °F | 87.9 °F | 3.94 in. |
| August | 66.9 °F | 86.6 °F | 3.08 in. |
| September | 59.5 °F | 80.4 °F | 3.28 in. |
| October | 48.6 °F | 68.7 °F | 3.14 in. |
| November | 37.1 °F | 54.9 °F | 3.65 in. |
| December | 28.5 °F | 44.3 °F | 3.01 in. |
| Annual | 46.9 °F | 66 °F | 43.71 in. |
| 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 : 0°F to 5°F |
| Plant Hardiness Zone - 2012 | Zone 6b : -5°F to 0°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |