SciJinks It's all about weather!

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What is a polar vortex?

photograph of Niagara Falls partially frozen.

The American side of Niagara Falls on January 9th 2014. The massive falls partially froze over during a polar vortex event. Credit: Michael Muraz.

Polar Vortex. It sounds like it could be some sort of alien death-ray or an extremely powerful washing machine, but what does it have to do with cold weather?

A polar vortex is an area of low pressure—a wide expanse of swirling cold air—that is parked in polar regions. The one up north can cause some pretty wild weather and sub-zero temperatures in the United States. But it's not a new thing—this low-pressure system is almost always up there.

Sometimes this low-pressure system, full of cold Arctic air, strays a little bit too far from home. Part of it can break off and migrate southward, bringing all of that cold air with it. Just like that, areas as far south as Florida get to experience their own little taste of life in the Arctic.

photograph of Chicago with a frozen Lake Michigan in foreground.

A Frozen Lake Michigan in front of Chicago on January 7th 2014 during a polar vortex event. Credit: Edward Stojakovik.

We actually want a strong polar vortex to stay warm?

The breaking off of part of the vortex is what defines a polar vortex event. But it actually occurs when the vortex is weaker, not stronger. That might sound weird—but it actually makes sense. Normally, when the vortex is strong and healthy, it helps keep a current of air known as the jet stream traveling around the globe in a pretty circular path. This current keeps the cold air up north and the warm air down south.

But without that strong low-pressure system, the jet stream doesn’t have much to keep it in line. It becomes wavy and rambling. Put a couple of areas of high-pressure systems in its way, and all of a sudden you have a river of cold air being pushed down south along with the rest of the polar vortex system.

Diagram of a regular polar vortex system and a polar vortex event showing jetstream and low and high pressure systems.

Regular, strong polar vortex with fairly stable jet stream (left) compared to the early 2014 weak polar vortex with a detached low pressure system over the United States and wavy jet stream (right).

photograph of Chicago with a frozen Lake Michigan in foreground.

The fountain in front of the National Museum of the American Indian in Washington, DC is frozen solid on the January 8th 2014 during the polar vortex event. Credit: Phil Osrtoff.

That’s what happened in early 2014. The polar vortex suddenly weakened, and a huge high-pressure system formed over Greenland. The high-pressure system blocked the escape of all that cold air in the jet stream, and allowed part of the polar vortex to break off and move southward. Places as far south as Tampa, Florida experienced the wrath of this wandering polar vortex. Most of Canada and parts of the Midwestern United States had temperatures colder than Alaska at the height of this cold snap!

It's important to remember that not all cold weather is the result of the polar vortex. While the polar vortex is always hanging out up north, it normally minds its own business. It takes pretty unusual conditions for it to weaken or for it to migrate far south, and other things can cause cold arctic air to travel our way, too.