Polar vortex: What is it and how does it happen?
The deep freeze gripping much of the US has been blamed on a polar vortex.
It's brought extremely low temperatures to parts of North America, with people in Chicago experiencing temperatures of -30C (-22F) - colder than parts of Antarctica.
The US National Weather Service is warning that frostbite is possible within just 10 minutes of being outside in such extreme temperatures, and hundreds of schools, colleges and universities in the affected areas have been closed.
But how exactly did all that happen? Weatherman Tomasz Schafernaker put on his winter wear to explain.
