What is the polar vortex, and why does it bring extreme cold?
A ring of frigid air normally locked over the Arctic can sometimes spill southward, unleashing bitter cold.
When temperatures plunge to dangerous extremes across North America or Europe, the "polar vortex" often gets the blame. But the term is widely misunderstood. It is not a storm, but a permanent feature of the atmosphere whose occasional disruptions can send bitter cold spilling far to the south.
The polar vortex is a large area of low pressure and frigid air that sits high in the atmosphere over the Arctic, strongest in winter. A band of fast-moving winds, the polar jet stream, normally acts like a fence, keeping that cold air corralled near the pole.
At times, however, the vortex weakens or becomes distorted, often following a sudden warming high in the stratosphere. When this happens, the containing winds buckle, and lobes of frigid Arctic air can break away and plunge southward into the mid-latitudes, bringing extreme cold to places unaccustomed to it.
This is what drives many of the most severe cold outbreaks, including historic freezes that have reached as far south as Texas. Paradoxically, some research suggests that a warming Arctic may be making these disruptions more common, meaning a warming world could still experience occasional blasts of brutal cold.
Understanding the polar vortex helps explain why extreme cold can strike even as the planet warms overall. It is a reminder that climate change is not simply a story of uniform warming, but of a more volatile atmosphere capable of dramatic extremes in both directions.