Janesville, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 2.9 °C (low) to 14.1 °C (high). Annual precipitation averages 943.6 Millimeters. Janesville is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -12.8 °C | -3.3 °C | 25.91 mm |
| February | -10.4 °C | -0.8 °C | 28.45 mm |
| March | -4.1 °C | 6.6 °C | 50.04 mm |
| April | 2.5 °C | 14.7 °C | 100.33 mm |
| May | 9.4 °C | 21.3 °C | 124.46 mm |
| June | 15.4 °C | 26.8 °C | 146.81 mm |
| July | 17.2 °C | 28.4 °C | 115.06 mm |
| August | 15.6 °C | 27.3 °C | 104.39 mm |
| September | 10.7 °C | 24 °C | 88.9 mm |
| October | 3.8 °C | 16.5 °C | 72.14 mm |
| November | -3.1 °C | 7.4 °C | 48.01 mm |
| December | -8.9 °C | 0 °C | 39.12 mm |
| Annual | 2.9 °C | 14.1 °C | 943.62 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 5a : -28.9°C to -26.1°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |