Many parts of the northern United States are experiencing record cold temperatures this week, attributed to the polar vortex, but what exactly is the phenomenon?

Here's the science bit:

The polar vortex is a band of strong winds high in the atmosphere around the north pole that normally keeps bitterly cold air locked around the Arctic region. 

How come it is affecting the US?

Normally, the cold air remains in the far north. However, higher temperatures than usual in the Arctic region have forced it to travel south.

The vortex has broken into two air masses, with one sitting over North America and the other over Europe and northern Asia.

The NASA graphic above shows how the vortex has been broken up by the warmer air in the Arctic region and forced to the south.

Why is it happening?

The short answer is that the vortex has moved south due to warmer air in the Arctic.

The longer answer is, of course, slightly more complicated. Disturbances to the jet stream - the flow of warm air across North America and Europe - have sent warmer air to the north.

Climate scientists believe this has combined with a general warming of the Arctic due to melting ice to cause disturbances in the upper atmosphere, where the vortex normally hangs out.

Just how cold is it?

To state the obvious - very. Yesterday morning in Chicago the temperature was -30C, and that is before you factor in the wind chill, which brings it down to a balmy -50C. That is colder than temperatures recorded in parts of Antarctica at the same time.

For further comparison, the coldest temperature recorded in Ireland in modern times was -17.5C, recorded by Met Éireann in Co Mayo on Christmas Day 2015.

Has this happened before?

Yes. The term polar vortex entered public consciousness in January 2015, when a similar blast of Arctic air brought what were then 20-year record low temperatures to parts of the US.

It was also known as the Polar Pig, but that term did not take off.

How are people coping?

There have been a number of deaths attributed to the cold snap, with at least 12 reported in the states of Michigan, Iowa, Indiana, Illinois, Wisconsin and Minnesota since the weekend.

Schools and many businesses have closed, and people are advised to stay indoors wherever possible.

Flights, buses and train services have been cancelled, the US Postal Service has suspended deliveries in some states and hundreds of 'warming centres' have been opened.

People who do have to go outdoors are advised to cover any exposed skin, and even then, not to spend more than 15 minutes outdoors to avoid hypothermia and frostbite.

They are also advised to avoid talking or taking deep breaths while outside.