What is the polar vortex?
A ring of strong west-to-east winds 10 to 30 miles above the North Pole, far above the weather, that forms every winter and holds the coldest air near the pole. When it's knocked off balance, the odds of bitter cold reaching the US can rise for weeks afterward.
When its cold escapes: wind chill and frostbite risk.
Bitter, bone-dry air that can tighten airways.
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Not the same as the jet stream
The polar vortex lives in the stratosphere, well above the jet stream and the clouds. It spins up each fall as the pole goes dark and cold, and fades away in spring. When it's strong and round, the jet stream below tends to stay farther north and frigid air stays bottled up in the Arctic.
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When it breaks down
Sometimes waves rising from the weather below shove into the vortex and stall its winds. The stratosphere over the pole can then warm by tens of degrees in a few days, an event called a sudden stratospheric warming. The vortex stretches, splits or wanders off the pole.
The effect works its way down slowly, over a few weeks, and it's indirect: it loads the dice for a wavier jet stream that can send Arctic air into the eastern US, Europe or Asia. Not every disruption delivers a cold blast, and plenty of cold snaps happen without one.
What forecasters watch
Scientists measure the vortex by the wind speed about 19 miles up at 60°N latitude. When those winds slow sharply or reverse, they start watching the following weeks for cold.
Questions people ask
- Is the polar vortex a storm?
- No. It's a large, permanent winter wind pattern high above the pole, not a storm you can see. News reports often use the name for the cold air that sometimes escapes from it.
- Does the polar vortex exist in summer?
- No. It forms in fall as the polar night sets in and breaks down in spring.
- How long after a disruption does cold arrive?
- Usually one to several weeks, if it arrives at all. The signal works its way down from the stratosphere slowly.