Large parts of the US are experiencing temperatures not seen for over 20 years, known as a Polar Vortex, producing temperatures as low as -50C in some of the 21 States affected.

This type of weather system, featuring very dense cold air, very rarely makes it out of the polar regions, trapped there by huge rotating winds but causes life threatening conditions when it does. Frostbite can occur on exposed skin in less than ten minutes.

The deep freeze has large parts of the Midwest in its grip but even Florida temperatures are not going to be far short of freezing on Tuesday, a  major concern for the orange growers that lose crops when the temperature gets down to -2C.

The short term outlook is for the extreme temperatures to ease mid week but for now hundreds of schools are closed and 10% of internal flights in the US have been cancelled.