How does Snowpack Make it Colder?

Clear skies enhance "radiational cooling"

The coldest part of the day is typically just after sunrise.  That’s because all the heat from the previous day will have radiated upward from ground level.  This process, known as “radiational cooling,” occurs every night.  However, there are certain variables that can either help or hinder this process.

Cloud cover, for example, reduces the amount of heat that is able to radiate upward, and that keeps the ground temperature warmer than it otherwise would be.  Being in or near an urban area also hinders the process, as the heat from buildings and automobile traffic keeps urban area mornings several degrees cooler than in suburban areas.  Since water heats up more slowly than air and cools off more slowly, being near a body of water reduces radiational cooling as well.

Ideal conditions for radiational cooling include clear skies and calm winds.  During the winter months, snowpack enhances radiational cooling.  Since white is a reflective color, areas with snowpack tend to remain a few degrees cooler during the day as solar energy is reflected away.  Given the solar energy lost to reflection, temperatures tend to fall rapidly at night under clear skies with calm winds and a fresh snowpack.  That’s happened several times in the last week since the Mid-Atlantic Region saw it’s first appreciable snowfall of the winter last Tuesday, January 6. 

Last Wednesday night under clear skies, with calm winds and a fresh snowpack across the region, temperatures plummeted to their lowest levels since January 2014 in the Nation’s Capital.  By Thursday morning, January 8, the low temperature had plunged to 12 degree at National Airport.  That was significantly colder than the daily average low of 29 degrees.  It was even colder at Dulles Airport where it was 3 degrees.

Such cold temperatures during the last week have left ground temperatures near the freezing mark.  That meant the snowfall early this morning began sticking as soon as the first flakes started falling, creating treacherous roads and sidewalks.  Although accumulations were generally light, with a dusting around the immediate DC Metro area, St. Mary’s and Calvert Counties in southern Maryland saw 1” to 2”.  Since both the air and ground temperatures were sufficiently cold, the snow began accumulating almost immediately causing school delays and closures.  That demonstrates, too, why listening to the latest weather forecasts is so important.