US shivers in 'once-in-a-generation' polar vortex
Deadly cold weather has brought what meteorologists call a "once-in-a-generation" deep freeze to the US.
The extreme Arctic blasts, caused by a spinning pool of cold air known as the polar vortex, could bring wind chill temperatures as low as -53C (-64F).
Weather officials in the state of Iowa have warned people to "avoid taking deep breaths, and to minimise talking" if they go outside.
At least five people have been reported dead across several states.
More than 55 million people currently face below-freezing temperatures.
A state of emergency has been declared in the Midwestern states of Wisconsin, Michigan and Illinois as well as in the normally more clement southern states of Alabama and Mississippi.
John Gagan, a National Weather Service (NWS) meteorologist, said: "The intensity of this cold air, I would say, is once in a generation."
The NWS is warning that frostbite is possible within just 10 minutes of being outside in such extreme temperatures.
The Illinois city of Chicago has set up 62 "warming centres" for the homeless, where the Chicago Coalition for the Homeless (CCH) estimates there are about 80,000 rough sleepers.
Police stations have also opened their doors to shelter homeless people.
But the New York Times reports that homeless people are still out on the streets in the city, and spoke to one man who was trying to collect $45 for a motel room.
"A lot of us don't go to the shelters because of bedbugs, we don't go because people steal from you, we don't go because you can't even really sleep in the shelter," he said.
Chicago police say people are being robbed at gunpoint of their coats.
Those wearing Canada Goose jackets, which can cost as much as £900 ($1,100), have been targeted, local media report.
Reuters reports that hundreds of schools have been closed in the affected states and more than 2,000 flights cancelled.
Animal rights organisation Peta has warned people to bring animals inside.
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The coldest temperatures are blasting the Midwest from Tuesday to Thursday, with forecasters predicting that Chicago will be colder than Antarctica.
The city experienced a low of -33C (-27F) overnight, with freezing winds making that feel closer to -46C (-50F), officials say.
As much as 2ft (60cm) of snow is forecast in Wisconsin, and 6in is expected in Illinois.
Alabama and another southern state, Georgia, are expecting snow too.
How did President Trump inject controversy?
President Donald Trump, who has questioned whether humans are responsible for climate change, tweeted about the conditions.
"What the hell is going on with Global Waming [sic]?" he said. "Please come back fast, we need you!"
But one of the US government's own meteorological agencies, the National Oceanic and Atmospheric Administration, posted what was widely interpreted as a rebuttal to the president.
Forecasters are attributing this cold snap to a sudden warming above the North Pole, caused by a blast of hot air from Morocco last month.
This weather system split the polar vortex and caused it to drift south, Judah Cohen, a winter storm expert for Atmospheric Environmental Research, told AP news agency.
The world's leading scientists have said that climate change is primarily human-induced and can lead to harsher winters.