Posted at 8:26 AM on December 7, 2007
by Craig Edwards
Snow cover this early December is allowing temperatures to drop overnight to very frigid readings when skies clear and the air goes nearly calm. During the day, snow reflects the heat of the sunshine back into space. The albedo of fresh snow can result in a 90 percent reflection of solar energy.
The layer of greenhouse gases works to moderate the daily temperatures, both with incoming solar energy and outgoing radiation from the earth.
Clouds are even more effective in restricting the outgoing nighttime radiation. As noted in Paul Huttner’s blog on Thursday morning, when clouds moved in on Wednesday night the temperature rose at the surface when there was really no change in air masses. Outside my weather lab the thermometer went from -11 at midnight to 17 above by sunrise.
With daylight continuing to shorten, and sunset at its earliest, darkness falls early. Temperatures by early Saturday morning, absent cloud cover, will be in the teens below zero in the northland. Thermometers positioned at observing sites located in a valley will read the coldest, where the cold air prefers to settle.