This article was originally featured on The Conversation.
Over the past few days, extremely cold Arctic air and severe winter weather have swept southward into much of the U.S., breaking daily low temperature records from Montana to Texas. Tens of millions of people have been affected by dangerously cold temperatures, and heavy lake-effect snow and snow squalls have had severe effects across the Great Lakes and Northeast regions.
These severe cold events occur when the polar jet stream–the familiar jet stream of winter that runs along the boundary between Arctic and more temperate air–dips deeply southward, bringing the cold Arctic air to regions that don’t often experience it.
An interesting aspect of these events is that they often occur in association with changes to another river of air even higher above the jet stream: the stratospheric polar vortex, a great stream of air moving around the North Pole in the middle of the stratosphere.
When this stratospheric vortex becomes disrupted or stretched, it can distort the jet stream as well, pushing it southward in some areas and causing cold air outbreaks.
The current Arctic cold blast fits into this pattern, with the polar vortex stretched so far over the U.S. in the lower stratosphere that it has nearly split in two. There are multiple causes that may have led to this stretching, but it is likely related to
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