December in Fullerton averages a daily high of 20.2°C with nights around 7.7°C. Expect roughly 6.4 wet days totalling 48.2mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Fullerton is April–June.
Based on FULLERTON MUNICIPAL AIRPORT, CA US (5.6km), 1995–2024. 27 years of December data.