What is the "Polar Vortex"?

A wall of wind that encircles the North Pole can have a big wintry effect on weather patterns across much of the U.S. when it occasionally falters.

When a significant snowstorm or cold snap arrives, you may hear it blamed on the "Polar Vortex."

It sounds like a candidate for the latest Marvel character, and in some cases, it can be the hero of snowy weather fans thwarting their arch-nemesis of a mild winter pattern. 

But the term describes a band of strong winds that circle the North Pole from around 55 degrees North Latitude (the latitude of Ketchikan, Alaska) at about 10-30 miles above the ground. 

Why do those winds matter? Think of a spinning top: As long as the rotation is constant and stable, the top stays spinning in a nearly fixed pattern. When the Polar Vortex is stable, the spinning winds circling the pole act like a wall that keeps the arctic air bottled up atop the planet. 

In that situation, the mid-latitude areas, including much of the U.S., will typically avoid severe cold conditions.

But about every other year, weather events in the lower atmosphere will send strong atmospheric waves up into the stratosphere, where they can interact with the polar vortex.

Going back to our example, this has the effect of knocking our spinning top off its axis, causing it to lose speed and severely wobble or even topple over.

In the atmosphere, disrupting the Polar Vortex will cause it to slow down and wobble, allowing the collapse of the protective wall around all that frigid air. 

As cold air heads south toward southern Canada and the U.S., milder air can rush into the polar regions now that the protective wall is gone. Suppose you ever hear the term "sudden stratospheric warming" start getting tossed around; it's a signal that the Polar Vortex has been disrupted, and the weather will start getting interesting across the mid-latitudes in the days and weeks ahead.

How does it get cold at my house?

The cold air doesn't flood the entire hemisphere equally; it interacts with the polar jet stream, creating a wavy pattern with large troughs of low pressure filled to the brim with this escaping arctic air.  These troughs can bring colder air farther south than usual, and those inside the trough will experience an extended and intense cold snap, with potential for heavy snows. 

Meanwhile, the weather for those outside the troughs will head in the opposite direction -- becoming milder.  Deep troughs will push ridges of high pressure on the other sides, generating an alternating pattern across the northern hemisphere of strong, cold troughs and corresponding strong, mild ridges that could bring unseasonably warm conditions.

Where the troughs and ridges set up will vary, but in North America, the Midwest and East Coast tend to end up inside the cold troughs while the West usually ends up in the milder sector. According to NOAA, Nor'easters are more common across New England when the Polar Vortex is in its faltering state.

That's how "Polar Vortex" became such a national household name as it tends to bring cold and potentially stormy weather to a large portion of the U.S. population. Though the name has only garnered attention in the past decade, it's always been part of the weather.

How can I track the Polar Vortex?

Meteorologists can use the Arctic Oscillation measurement to keep an eye on the winds circling the poles. The calculations are rather complex, but when the polar vortex is in its robust, stable state, the AO indicator will tend positive, and cold snaps will be less frequent.

When the Polar Vortex begins to falter, and ensuing cold snaps could loom, the AO goes negative. The AO had a robust negative reading in January -- its lowest in eight years -- just weeks before the record arctic blast hit Texas and the Midwest in early February. 

So next time you hear the word Polar Vortex being bandied about, and if you live in the central or eastern U.S., it just might be time to find that heavier coat and keep that snow shovel someplace easily accessible.

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