Arctic Air Masses

[phantom power]

For the record, an air mass is a large chunk of air with horizontal dimensions on the order of several hundred to a couple of thousand miles. Within any air mass, temperatures and dew points near the earth's surface (or at any other arbitrary altitude) vary only gradually with increasing distance away from the center of the air mass (a center of high pressure).

Winter's most frigid air masses, like the air mass shown on the 09Z surface analysis, are tagged continental-Arctic air masses (cA) to herald their extreme cold and very low dew points. There are a couple of tests for Arctic air masses. For starters, I avoid looking at nighttime temperatures because sometimes, nocturnal cooling can mislead you into thinking the air mass is colder than it really is (this happens sometimes in Nevada and other parts of the Intermountain West on clear, calm nights during the cold season).

No, I like to look at daytime temperatures in order to determine the presence of Arctic air. When Arctic air masses invade the United States, daytime high temperatures are typically in the single digits or they're below zero degrees Fahrenheit. Yesterday's high temperatures over central and western Canada (right panel) seal the deal on the Arctic nature of this air mass.

I also look at 850-mb temperatures (standard altitude is 1500 meters) to verify the presence of Arctic air. As a general rule, 850-mb temperatures equal to or lower than minus 20 degrees Celsius are consistent with Arctic air. Check out the 06Z GFS 3-hour forecast for 850-mb temperatures (valid at 09Z this morning) and note the large area over Canada where 850-mb temperatures were predicted to be minus 20 degrees Celsius or lower.

http://www.wunderground.com/blog/24hourprof/comment.html?entrynum=0

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