December in Fontana averages a daily high of 19.1°C with nights around 6.4°C. Expect roughly 5.7 wet days totalling 51.4mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Fontana is March–May.
Based on ONTARIO INTERNATIONAL AIRPORT, CA US (13.8km), 1995–2024. 27 years of December data.