weather term
Polar Vortex
A large, persistent area of low pressure and cold air that circles each of Earth's poles, strongest in winter.
The polar vortex is always present, but when it weakens or is disrupted, some of that cold air can spill southward into the mid-latitudes, often behind a wavier, more southward-dipping jet stream. This is the mechanism behind many of the sharpest cold snaps in North America, Europe and Asia, and can bring a genuinely severe cold spell even during a winter that is, on average, forecast to be mild.