Nancy's Almanac, Jan. 26-28, 2013: The skinny behind the freeze

 


An unusual weather pattern was behind this past week’s freezing temperatures.

It was so unusual that temperatures in southern Canada dropped well below those at the North Pole.

The pattern had to do with upper level air temperatures and circulation at the North Pole.

First, a definition of terms. The bottom layer of the atmosphere is known as the troposphere and the next layer is called the stratosphere:

The troposphere is the first 6 to 12 miles of air, and it is where we live and weather occurs.

The stratosphere is the next layer, and typically occurs between 6 and 30 miles up. There’s no real “weather” as we know it in the stratosphere. It’s a relatively stable layer of atmosphere.

The first sign that something unusual was about to happen was in late December, when climate models forecast sudden warming in the stratosphere in early January, according to AccuWeather Inc., The World-Herald’s private weather forecaster.

It’s not unusual for the air in the stratosphere to be relatively warm, but a sudden change in temperature is unusual, according to AccuWeather.

When it happens, it’s known as sudden stratospheric warming, and somewhat predictable consequences occur in the troposphere.

Chiefly, meteorologists can be fairly confident that a few weeks later, Arctic air will flood southward to the mid-latitudes (the lower 48 states are located in the mid-latitudes).

And that’s what happened in mid- to late January after the sudden stratospheric warming early in the month.

At one point in the past week, temperatures were about 40 degrees below zero in south-central Canada, but in the single digits to just below zero at the North Pole, according to AccuWeather.

Nebraska was on the western edge of the Arctic outbreak, so it didn’t suffer the worst of the bitter cold.

Further outbreaks of Arctic air are likely in February, according to AccuWeather, as this phenomenon continues to unwind.

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