A brutal cold wave moved eastward on Thursday after bringing temperatures in the US Midwest lower than those in Antarctica, grounding flights, closing schools and businesses and raising fears of hypothermia.
The extreme weather has now been blamed for as many as eight deaths, US news outlets reported.
Mail deliveries were suspended and people were encouraged to stay home in nearly a dozen US states where the mercury plunged into the negative double digits, the worst deep freeze to grip the region in a generation.
There were surreal scenes throughout the region, such as steam rising off the waters of Lake Michigan — the result of extremely cold air passing over warmer water below. Parts of Niagara Falls froze, and blocks of ice covered the river winding through downtown Chicago.
America's third city was in the direct path of the harsh weather and experienced its second-coldest day . Chicago was on track to break new records overnight when temperatures were forecast to reach their lowest in the early hours Thursday.
"This is a historic cold, obviously," Chicago mayor Rahm Emanuel told a Wednesday night news conference.
"They are life-threatening temperatures, and they should be treated accordingly," he cautioned.
The most intense cold was expected overnight on Wednesday into Thursday, said Ben Deubelbeiss, a meteorologist with the National Weather Service (NWS), The New York Times reported.
The Wednesday morning temperature in the Windy City was -30°C but felt like -46°C with the wind chill.
It was colder than Alaska's state capital and even colder than parts of Antarctica.
"It feels like being close to dry ice," Leon Gilbert, 31, told AFP. "I can feel my skin tighten up."
Unlike most Chicago residents, Mr Gilbert was required to report to his job at a Starbucks on a downtown street largely devoid of its usual bustle of people and traffic.
Sandwich shop manager Daniel Gonzalez, 37, also reported to work — at 5am.
"I have two shirts on … I have a hoodie, I have my big winter coat, I have a face mask and a skullcap and am still cold," Mr Gonzalez said.
At Chicago's O'Hare airport, a major regional hub for airlines, ground crews struggled with freezing equipment and were told to avoid spending more than 15 minutes at a time exposed to the cold, causing delays for flights that managed to take off.
More than 1,800 flights were cancelled at Chicago's two major airports, while rail operator Amtrak scrapped train services from its hub in the city.
The US Postal Service suspended deliveries in parts of Indiana, Michigan, Illinois, Ohio, Iowa, the Dakotas and Nebraska.
The cause of the sub-zero chill affecting tens of millions of Americans was a swirl of Arctic air that broke away from the polar vortex that usually encircles the North Pole.
"The dangerously cold wind chills could cause frostbite on exposed skin in as little as five minutes," NWS said.
Hundreds of warming centres were opened for vulnerable residents such as seniors, buses were employed as mobile warming spaces, and shelter capacities were increased for the homeless, including the approximately 16,000 living on the streets of Chicago.
