The Arrival of Astronomical Winter

Although meteorological winter got underway on December 1, astronomical winter begins tomorrow.  So far this month, the Nation’s Capital has experienced a wide range of weather, including two days with high temperatures in the 60s and two days with low temperatures in the teens.  That’s kept average monthly temperatures (combining daily high and low temperatures) within less than half a degree (0.3F) of average at National Airport through December 19.  

It has been neither warmer than nor cooler than average for very long in the Mid-Atlantic Region so far this month.  The extremes have been noteworthy, though.  For example, the high temperature in Washington, D.C. on December 10 was only 39 degrees.  While being well below the average high of 48 degrees, it was also noteworthy since the first such day with a sub-40 degree high temperature didn’t occur last winter until January 4.

On December 16, 2016, Washingtonians experienced their coldest December day since 2010, as high temperatures remained below freezing.  Dulles Airport even set a record low (9F) and saw its first December single-digit temperature reading since 2002.  Midwest cities like Minneapolis and Chicago all saw near record cold with temperatures below zero.  However, the frigid air mass moderated by the time it reached the East Coast.  Due to increased urbanization and the urban heat island effect of large cities like Washington, D.C., Baltimore and New York City, temperatures don’t often get as cold as more rural areas in the Midwest.

Record winter cold has actually become relatively rare in the Mid-Atlantic Region.  The Nation’s Capital hasn’t had a record low in December since 1989.  The last time Washingtonians experienced record lows in January was in 1994.

National Airport only experienced one record low high temperature in recent years on January 22, 2014 (19F).  Meanwhile, the more rural Dulles Airport experienced a total of 16 record low high temperatures and 13 record low temperatures during the 2013-2014 and 2014-2015 winters.  While records in Washington, D.C. are much older than at Dulles Airport (1871 vs. 1962), the data shows convincingly how urban areas are remaining warmer than their more rural counterparts.  Part of this phenomenon is attributed to an increase in urbanization and development in the urban and suburban areas with a commensurate increase in automobile traffic.  

Cold air outbreaks remain a regular event across the United States during the winter months.  However, the frequency and duration of such cold air outbreaks fluctuate from one winter season to the next.  Yesterday was the third sub-40 degree high temperature of the season in the Nation’s Capital, while Washingtonians didn’t experience their third such day last winter until January 13.