More than half of the U.S. is enduring a dangerously cold start to the week as a whirlpool of frigid, dense air known as a ‘polar vortex’ descended on Monday morning, pummeling parts of the country with a dangerous cold and adding to the brutal weather that has grounded more than 4,400 flights.

Record low temperatures have already been set; at Chicago's O'Hare International Airport, temperatures of minus 16 degrees were recorded at 8am on Monday, beating the previous record of minus 14 set in 1988.

In Minnesota, officials took the rare step of closing all of the state's public schools on Monday - the first time in 17 years. Schools across Chicago, Milwaukee and St Louis were also closed, while officials in Washington D.C. and as far south as Atlanta have announced school closures for Tuesday.

With wind chill warnings stretching from Montana to Alabama, much of the U.S. is experiencing the coldest temperatures in almost 20 years, according to the National Weather Service. They are expected to be 30 to 50 degrees below average in some cities - and the deep freeze is expected to last into Tuesday.

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