December in Rockville averages a daily high of 9.1°C with nights around -1.1°C. Expect roughly 10.3 wet days totalling 97mm. Demanding conditions — pack for it and plan indoor options.
Across the year, the best time to visit Rockville is April–May.
Based on DALECARLIA RESERVOIR, MD US (16.5km), 1995–2024. 28 years of December data.