Janesville, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 43.8 °F (low) to 63.7 °F (high). Annual precipitation averages 42.5 Inches. Janesville is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around Apr. 11 - Apr. 20 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 20.2 °F | 36.2 °F | 2.73 in. |
| February | 23.1 °F | 40.9 °F | 2.38 in. |
| March | 32.1 °F | 52.0 °F | 3.03 in. |
| April | 42.9 °F | 64.8 °F | 4.72 in. |
| May | 54.1 °F | 74.7 °F | 4.5 in. |
| June | 63.0 °F | 83.2 °F | 4.89 in. |
| July | 65.8 °F | 86.4 °F | 4.3 in. |
| August | 63.7 °F | 85.2 °F | 3.02 in. |
| September | 56.0 °F | 79.7 °F | 3.11 in. |
| October | 44.6 °F | 67.4 °F | 3.52 in. |
| November | 34.0 °F | 52.8 °F | 3.66 in. |
| December | 25.5 °F | 40.8 °F | 2.68 in. |
| Annual | 43.8 °F | 63.7 °F | 42.54 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 6a : -10°F to -5°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |