December arrives on Tuesday. Average high temperatures slide from 51 degrees to 42 degrees at BWI, while average lows fall from 31 to 24 degrees. Average snow for the month is just 1.7 inches, but we've seen as much as 11.5 inches in a day (Dec. 17, 1932). There's a full moon on the 2nd. The earliest sunset is on the 7th. The Geminid meteor shower peaks on the 14th, and the winter solstice arrives on the 21st.