December arrives after midnight tonight, and with it comes the start of the meteorologists' three-month winter season. It's a month when the average high temperatures for Baltimore drop from 51 degrees on the 1st to 42 degrees by the 31st.  The average daily lows, meanwhile, sink from 31 degrees to a seriously cold 24.

Of course, the potential extremes are, well, extreme. The warmest December day on record for Baltimore was 77 degrees, set on a balmy Dec. 29 in 1984. The coldest saw the mercury bottom out at minus-3 degrees, way back on Dec. 30, 1880.

Those old 19th century records are remarkably stubborn, expecially considering that they kept records for only 29 years in that century - beginning in 1871. There are still 5 record-cold 19th century days on the books for December in Baltimore, and 3 record-warm dates.

December also brings the region its first serious chances for significant snowfall. Although the average accumulation for December is a mere 1.7 inches, bigger snowfalls are quite possible. The snowiest December on record for Baltimore was in 1966, when more than 20 inches fell. The snowiest day in December was Dec. 17, 1932. Whoever stuck a foot-long ruler into the snow on that date almost lost it: 11.5 inches had fallen.

The average snow accumulation for an entire winter season in Baltimore is just over 18 inches. We've already had an official half-inch of snow this season. That fell on Nov. 23. Here is a list of the biggest winter storms in the history of Baltimore and Washington. The folks at the National Weather Service forecast office in Sterling., Va. need to do some updating, however. The chart does not include the President's Day Weekend storm of 2003, which dumped 28.2 inches at BWi over four days. For the season, the airport recorded 58.1 inches of snow that year, including the snowiest February ever, with 40.5 inches. The snowiest winter in Baltimore was 1995-96, when 62.5 inches fell.

Global warming theory, by the way, predicts more such extreme precipitation events. The fact that the two snowiest winters on record for Baltimore have occured in the last decade may tell us something.

The official start of winter comes with the arrival of the Winter Solstice, which occurs this year at 1:36 p.m. on Dec. 21. That marks the shortest day (and longest night) of the year. Sunrises continue to come later during December, moving from 7:07 a.m. on the 1st  to 7:26 a.m. by month's end. But there's good news for victims of Seasonal Affective Disorder: in December the sun begins, at last, to set a bit later in the afternoon. After reaching a early limit of 4:43 p.m. between Dec. 3 and 11 in Baltimore, sunsets advance by month's end to 4:53 p.m.  It's not much, but it's a harbinger of longer days to come, and a promise of spring. Eventually.