Roanoke Times
                 Copyright (c) 1995, Landmark Communications, Inc.

DATE: THURSDAY, February 24, 1994                   TAG: 9402230034
SECTION: EXTRA                    PAGE: 1   EDITION: METRO 
SOURCE: Joel achenbach
DATELINE:                                 LENGTH: Medium

IT'S WINTERTIME, THAT'S WHY IT'S COLD!

A: A good way to understand, scientifically, the nature of cold weather, is to think of cold air as containing fewer molecules of nitrogen and oxygen and carbon dioxide and so forth than warm air.

This is not technically precise, because it is completely wrong, and in fact is the opposite of the truth (cold air is actually much denser than warm air), but it's still a convenient thought, you must admit. When it gets to minus-3, as it did this very morning outside the Why bunker, we can tell ourselves that there's basically nothing out there but a vacuum. It's like outer space. It's dangerous to go outside because the pressure gradiant may cause your eyeballs to pop out and dangle on your cheeks.

Anyway, there are two ways it can get cold, gradually and suddenly. The gradual way is that it gets dark and the ground radiates much of its heat into space and the little molecules in the air slow down bit by bit. Air molecules are always slowing down unless they get input from some other energy source, like the sun.

(Aha! you say. So how come it's colder than February than in December even though there's more daylight in February? Faithful readers of this column know the answer: The days are still short in February and there's not enough sunshine to replace the heat that radiates away during the long nights. All that heat built up over the previous summer continues to dissipate.)

The fast way that the air gets cold is that a big air mass sweeps in from the north. We are so used to these Arctic fronts that we don't stop to think that they are a bit peculiar.

Ground-level weather systems are controlled by the jet stream, a river of air flowing at an altitude of 40,000 to 50,000 feet. For the most part the jet stream flows west to east (it's a function of the Coriolis Effect and relatively dense high-altitude tropical air moving toward the low-pressure air masses high above the Arctic, but let's not get into that).

What makes the weather complicated is that the jet stream also veers northeast and southeast and sometimes almost due north or due south. These meanders, which can be predicted with moderate accuracy some months in advance but which are essentially chaotic in nature, are known as "ridges" and "troughs," because that's the shape they take when you see a graphic representation of the jet stream on your TV during the evening news. A deep trough means that the jet stream is meandering far to the south before going east again and back north.

Here's the key fact: The jet stream controls the weather way down below it. So when it veers south, for example, it can pull masses of ground-level air from frigid northern climes. The repulsively cold weather over the eastern U.S. in January was caused by a deep trough in the jet stream that yanked a big chunk of Arctic air down from the Yukon.

Now, for no reason at all we will tell you why polar bears don't get cold: They're fat and round.

"Their mass-to-exposed-area ratio is very very high," says Larry Kalkstein, a climatologist at the University of Delaware.

Look at a polar bear sometime. The creature is nearly spherical! But don't let on that you notice; they are very sensitive.