What is the polar vortex?

The polar vortex is a large area of cold, low-pressure air with fast circling winds that sits high in the atmosphere over each of the Earth's poles. Despite how the term gets used in the news, it is not a specific storm or a rare event, it is a permanent, always-present feature that strengthens every winter, essentially a river of strong winds high up (in the stratosphere and upper atmosphere) that normally spins in a tight circle around the pole and acts like a fence, penning the coldest Arctic air up near the top of the globe. When the vortex is strong and stable, it keeps that frigid air bottled up over the Arctic, and the mid-latitudes (like most of the United States and Europe) have relatively milder winter weather.

The dangerous cold outbreaks happen when the polar vortex weakens, stretches, or wobbles off the pole, sometimes splitting into pieces. When that circular wind fence breaks down or gets distorted, often triggered by disturbances in the atmosphere, it can bulge or send lobes of the trapped Arctic air spilling far southward, plunging places like the central and eastern US, Canada, and Europe into extreme, sometimes record-breaking cold snaps. This is why you hear about the polar vortex specifically during those brutal deep-freeze events, the phrase has become shorthand for the frigid air itself, though technically the vortex is the whole system and the cold outbreak is what happens when it destabilizes and leaks. Scientists are actively studying whether a warming climate, particularly a rapidly warming Arctic, is making the polar vortex more prone to these disruptions, which would counterintuitively mean that global warming could contribute to some severe cold outbreaks in certain regions, a debated but actively researched connection. So the polar vortex is the permanent swirl of cold high-altitude air over the poles that usually keeps the worst cold contained, and the headline-grabbing freezes occur when it weakens and lets that Arctic air escape southward.