A POLAR vortex can lead many countries being hit with extremely low temperatures.
It is low pressure between the two artic regions, which can spread and affect the weather across the globe.
What is the polar vortex?
As its name suggests, the polar vortex is found around the north pole.
A band of strong winds high up in the atmosphere keeps bitterly cold air locked around the Arctic region.
But sometimes it undergoes changes and moves south, creating alarmingly cold conditions.
It has been more than 20 years since a similar Arctic blast last covered a large swath of the Midwest and Northeast, according to the weather service.
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The impact of the vortex has caused widespread closure of schools, businesses, government offices and flights.
It has also prompted the US Postal Service to take the rare step of suspending mail delivery.
How does a polar vortex happen?
The term "vortex" in itself refers to the counterclockwise flow of air helping to keep the colder air near the Pole.
But sometimes the vortex can become distorted and meander far further south than normal.
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Making matters worse for the United States at the moment, the vortex has broken into two swirling blobs of ultra cold air, ushering in killer temperatures as low as -40C.
When was the last polar vortex?
Chicago is one of the places worst affected by the weather system.
The wind chill brought temperatures down as low as -29C in 2019, nearly 10 degrees colder than the 1985 record.
Windchill temperatures have reached -60C – colder than Antarctica – and emergency measures have been put in place.
Authorities advised that such low temperatures could cause frostbite in minutes, and kill in not much longer.
Chicago residents were even advised not to take deeper breaths while outside, or talk too much when outdoors.
Some residents experiencing the painful cold dubbed their city “Chiberia” while one local newspaper ran with the headline “COLD AS ºF”.
At least 21 have died across the country during the polar vortex-induced weather, according to Reuters.
In fact it was so cold railway engineers had to set fire to train tracks to keep them from locking up.
Metra Commuter Rail spokeswoman, Mel Riele, told Chicago Tribune: "Basically, what the fires are, they're gas powered switch heaters and essentially you're looking at a giant gas grill."
Temperatures have begun to crawl back up, but as they tipped above freezing, authorities expected burst pipes.
NBC Storm Team 5 meteorologist Andy Avalos warned those in Chicago: “Wind chill values are -45F to -55F which could lead to frostbite and hypothermia in just a matter of minutes to any exposed skin."
Does the polar vortex happen every year?
It always exists near the poles, but weakens in summer and strengthens in winter.
People in the UK will rarely hear about it unless it is heading towards them though.
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