Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: 24hourprof , 5:14 PM GMT on December 12, 2012
At 65, I'm one of those old timers who still enjoys cold and snowy weather. In the early 1980s while I was in graduate school at McGill University (Montreal, Quebec), I would chase lake-effect snow squalls downwind of Lakes Ontario and Erie, inserting myself into convective bands with snowfall rates of six inches per hour (or higher) with thunder and lightning. Those were the days, my friends.
Here in central Pennsylvania, cold weather has been hard to come by this autumn and this winter, so I looked enviously at the Arctic air mass that has dipped into the Upper Middle West this morning (09Z surface analysis).
Take a closer look at the 09Z temperatures and dew points on the station models over western and central Canada (09Z is 4 A.M. EST). With temperatures and dew points as low as minus 44 degrees and minus 47 degrees Fahrenheit respectively, there's no question that this is a genuine Arctic air mass. What is an air mass? Are polar air masses colder than Arctic air masses?
The source regions for air masses that affect North America. Credit: Courtesy of A World of Weather: Fundamentals of Meteorology.
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.
For your benefit, here's a close-up of the Arctic front on the 09Z surface analysis this morning, which marks the leading edge of Arctic air. In case you're wondering, the cold front shows where Arctic air was advancing, and the stationary front indicates where Arctic air stalled.
Despite its name, continental-Polar air masses (designated cP) are not as cold as continental-Arctic air masses during winter. Yes, I agree, the convention of using "polar" to describe these air masses is somewhat confusing, but revisit the source-region map above to drive home my point that continental-Polar air masses form farther equatorward than continental-Arctic air masses.
A Pet Peeve
Earlier, I made a reference to "nocturnal cooling." For the record, I always try to avoid "radiational cooling" whenever I'm discussing low nighttime temperatures. That's because I often hear people describe the reason for very low nighttime temperatures as "great radiational cooling." Aaaarrrgggghhhh...
If the truth be told, the greatest radiational cooling occurs around the time of the daytime maximum temperature. Indeed, radiational cooling is proportional to the fourth power of absolute temperature (in Kelvins), according to Stefan-Boltzmann's Law. So that's why I prefer "nocturnal cooling," a term that gets to the heart of the real reason for very low temperatures on a clear, calm night...yes, there is radiational cooling, but the amount of radiation from the atmosphere is relatively small. So the ground runs an energy deficit, and temperatures continue to fall until solar energy arrests the decline after sunrise. I'll have more to say about nocturnal cooling in future blogs.
So, yes, I'm sort of a stickler for the accuracy of science. There will be an underlying theme of science in all of my blogs, and I hope readers learn more about meteorology and atmospheric science as I delve into deeper topics.
I am very grateful for this opportunity.
Comments will take a few seconds to appear.