As parts of the US shiver through some of the lowest temperatures in two decades Professor Ted Shepherd of the University of Reading explains the climatic phenomena known as a 'polar vortex' that has hit the country
The US has been struck by a whirlpool of frigid, dense air known as a polar vortex which has sent temperatures plummeting from Montana to Alabama.
The polar vortex is a fast flowing stream of air that circles the North Pole during the winter months, said Professor Ted Shepherd of the University of Reading.
"This air is basically whipping around the pole and trapping in the cold Arctic air", he said.
This polar vortex can get what are called, Rossby waves, which move along the boundary between the warm upper layer of air and the cold deeper of air. These waves can become unstable and grow.
It is rare for them to swing as far as to affect densely-inhabited areas, but "if you look at the weather maps at the moment, there is a particularly strong meander that is going particularly far south", he said.
It is this "exceptional wave" which flinging the brutally cold air so far southwards.
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