Don't think of it as cold outside, think of it as "not hot," because it's the absence of heat in the Arctic during the winter that brings subzero weather to the region.

And while more normal weather in the teens and 20s is expected to return to the Midwest and Utah the next few days, the "not hot" Arctic air will probably be back by the weekend.Why does the air, even over the North Pole, get so cold and why does it descend like a Siberian nightmare on Minnesota and other parts of the Upper Midwest almost every winter?

Two reasons, said Bruce Watson, a private meteorologist in the Twin Cities: the long winter night in the Arctic and calm air.

Because the Earth's Northern Hemisphere is tilted away from the sun during the winter, it is perpetually dark in the Arctic. No solar energy - which generates heat - is added to the Arctic; and, because of the long night, the heat that is there at the start of winter quickly radiates into space.

The result is an extreme lack of heat.

The truly cold, subzero Arctic air tends to stay in high latitudes during much of the winter, but active low-pressure systems of warm air occasionally push into the northern regions. As the lows push north, rotating counter-clockwise as they go, some Arctic air is forced south.

So air that was over the Arctic Ocean, Alaska or the Yukon Territory only a few days ago found itself over the Upper Midwest Wednesday.

During some winters, the interplay of high- and low-pressure systems shoves little extremely cold air down over the Midwest, but some winters it shoves a lot, Watson said.

Even with computers and years of records and research, understanding all the complexities of the weather can defy some of the best meteorologic minds, and trying to predict what will happen more than a few days in advance is extremely difficult.

While the interaction of highs and lows accounts for cold air pushing south, it takes another ingredient to create the extremely bitter cold of the past week, Watson said: still air.

"Cold air is denser than warm air and settles near the ground," he said. If there is turbulence, the coldest air will mix with the warmer air above it, raising the ground-level temperature slightly. But when the air is still, the cold air falls to the ground and stays there.

"Sometimes when you get this far north," Watson said, "you find that the air is actually warmer at 1,000 or 2,000 feet above the ground," as was the case last week when rain turned to ice as it landed on streets and cars.

The recent movement of high- and low-pressure systems has resulted in a ridge of high pressure that runs northwest from Minnesota through Canada, and that promises a lot of cold weather for some time to come.

But take heart. There is an absolute limit to how cold it can get - a limit known to scientists as "absolute zero."

That temperature, so cold that even atoms can't move, is minus 459 degrees Fahrenheit.

Distributed by Scripps Howard News Service