At least 21 people have died in one of the worst cold snaps to hit the US Midwest in decades. Ninety million people - a third of the US - have seen temperatures of -17C (0F) or below. Some 250 million Americans overall have experienced the “polar vortex” conditions. Hospitals have been treating patients reporting frostbite as parts of the country ground to a halt.
Temperatures are expected to swing to above average over the weekend. Homeless people have been particularly at risk, with warming shelters set up across cities. But some still braved the freezing conditions and one woman, aged 60, was found dead in an abandoned house in Lorain, Ohio. Some people were found dead a short walk away from their homes.
Officials said a Michigan man who froze to death in his neighbourhood was “inadequately dressed for the weather”. In a wind chill of -46C (-51F) an 18-year-old student was found unresponsive a short walk from his dorm on Wednesday and later died in hospital.
On Tuesday, a man froze to death in a garage in Milwaukee, Wisconsin, having “apparently collapsed after shovelling snow”, according to a medical examiner.
Dangerous roads have also been a factor in the deaths. A man was fatally struck by a snow plough near Chicago on Monday and in northern Indiana a young couple died after a collision on icy roads. The icy cold is expected to loosen its grip on Friday. By the end of the weekend, Chicago could see temperatures as high as 10C (50F).
“It’s going to be at least a 60-degree swing for Chicago,” David Hamrick, a National Weather Service forecaster, told Reuters news agency.
More than 30 record lows were broken across the Midwest.
Cotton, Minnesota, was the coldest place in the US on Thursday with a low of -48C (-56F) based on preliminary data.
Chicago passed the record low for 31 January.
The chill drifted eastward on Thursday, bringing sub-zero temperatures to north-eastern cities such as Boston.
With wind chill factored in, temperatures of -40C (-40F) in the Midwest and Great Lakes have felt closer to -53C (-63F), which is enough to cause frostbite in less than five minutes.
The Arctic weather could cost the US billions of dollars. In 2014, a similar polar freeze cost the country an estimated $5bn (£3.8bn).
In Minnesota and Michigan, residents were asked by gas companies to turn down their home thermostats to help handle heating demands.
Native American tribes in the northern Midwest states helped their members obtain heating supplies as many live in poor-quality housing, the Associated Press reported.
More than 2,300 flights have been cancelled and another 3,500 delayed due to the polar vortex.
Most of Canada was under some sort of weather warning - from extreme cold in the Prairies, Quebec and Ontario to heavy snows in Alberta and Nova Scotia. In Toronto, where winters tend to be milder compared to cities such as Montreal and Ottawa, temperatures plummeted to -18C (0F).
Icy roads and several transit delays made for a hellish commute for the city’s residents.
Environment Canada issued extreme cold warnings for most parts of Ontario, Quebec and Manitoba, urging residents to limit their exposure to cold and keep pets indoors.
Advocates expressed concern for homeless people living in cities hit by the extreme temperatures.
In Winnipeg, the capital of Manitoba, temperatures were -40C (-40F), with wind chill factors making it feel like -50C (-58F).
In parts of rural northern Alberta, 15in of snow were expected to fall.