Frozen water mains and strained natural gas supplies left the US Midwest struggling Thursday as a deadly Arctic air mass had tens of millions of Americans shivering for a second day.
Ten deaths have been attributed to the extreme weather since the weekend.
Schools and businesses remained closed in several midwestern states, people were encouraged to stay home, and travellers were stranded by grounded flights and halted trains.
Natural gas supplies were under threat in Michigan and Minnesota, where authorities asked residents to reduce their heat consumption if at all possible.
Frozen water mains broke in Detroit and parts of Canada.
More than 1,600 flights were canceled in Chicago by mid-morning. Airport crews worked in 15-minute increments on the tarmac to avoid frostbite.
Amtrak planned to begin restoring service to some trains after canceling all service Wednesday out of Chicago.
The deadly, sub-zero temperatures were expected to lift by Friday, but the misery would not end quickly in the roughly dozen states most affected.
"Temperatures will slowly begin to moderate as the air mass slowly warms," the National Weather Service said.
The agency forecast wind chill temperatures Thursday would still range between -20 to -50 degrees Fahrenheit (-29 to -46 Celsius) over parts of the Upper Midwest.
The cold has frozen sections of Niagara Falls and sent blocks of ice floating down the river winding through Chicago's downtown.
The Arctic air mass that descended from its usual northern rotation on Wednesday caused the second coldest day ever recorded in the Windy City, where residents reported hearing "frost quakes."
Local television station WGN said booms heard by residents were likely frozen, water-saturated ground cracking under their feet.
Officials in multiple states warned that the extreme weather should be taken seriously, with the risk of hypothermia and frostbite setting in within minutes of exposure.
"They are life-threatening temperatures, and they should be treated accordingly," Chicago Mayor Rahm Emanuel cautioned residents Wednesday evening.
Hundreds of warming centers were opened for vulnerable residents such as seniors, and shelter capacities increased for the homeless.
Among the 10 reported dead this week was an 18-year-old University of Iowa student.
He was found unresponsive behind a campus building Wednesday morning, when wind chill temperatures in Iowa City were -51F (-46C), according to local TV station KCCI.
Officials warned residents to remain on guard Thursday while the sub-zero weather persisted.
"Remember to check on your neighbors and loved ones to make sure they're safe during this brutally cold weather," Illinois Governor J.B. Pritzker tweeted.
Authorities in Michigan and Minnesota were also asking residents to turn down their thermostats to conserve natural gas.
Supplies were strained due to high demand from home heaters and from a fire at a natural gas compressor station in Michigan, officials said.
"Due to extremely high demand for natural gas with these record low temperatures and a facility incident, Consumers Energy has asked that everyone who is able to turn down their thermostats through Friday at noon so we can all get through this with minimal harm," Michigan Governor Gretchen Whitmer said in a statement.
Detroit's three major auto makers all suspended or curtailed manufacturing and other operations to conserve natural gas, according to The Detroit News.
City crews braved sub-zero weather to repair a number of frozen water main breaks that plagued Motor City neighborhoods.
America's northern neighbor Canada was also contending with extreme cold, with frozen water pipes, snarled travel on a major waterway, and temperatures as low as -40F (-40C) on Wednesday.
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