December in Janesville averages a daily high of 1.1°C with nights around -7.4°C. Expect roughly 9.7 wet days totalling 47.6mm. A hard month to visit for comfort alone.
Across the year, the best time to visit Janesville is May–September.
Based on AFTON JANESVILLE WWTP, WI US (6.3km), 1995–2024. 30 years of December data.