A man on his way to work feels the effects of the icy cold weather in Janesville, Wisconsin
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North American freeze: What is a polar vortex?

6 January 2014 Last updated at 23:50 GMT

As the east cost of America prepares to face dangerously cold temperatures that have already hit the Midwestern states, meteorologist Jacqui Jeras explains what a polar vortex is to the BBC's Katty Kay.