The coldest, most dangerous blast of polar air in decades gripped the U.S. Midwest and pushed toward the east and south on Monday, closing schools, grounding flights and forcing people to pull their hoods and scarves tight to protect exposed skin from nearly instant frostbite.

Polar vortex?

The polar vortex refers to winds that whip around the polar ice cap, trapping Earth's coldest temperatures there.

Its deterioration with global warming, however, can send arctic weather south into areas as far away as the southern U.S. and Europe, according to National Oceanic and Atmospheric Administration scientists.

"When the polar vortex … breaks down, this allows cold air to spill south, affecting the eastern United States and other regions," says NOAA's Dr. James Overland.

"This can result in a warmer-than-average arctic region and colder temperatures that may include severe winter weather events on the North American and European continents."

Nearly 187 million Americans, more than half of the population of the U.S., were under a wind chill warning or advisory Monday.

For a big chunk of the Midwest, the extremely cold temperatures were moving in behind another winter wallop: more than 30 centimetres of snow and high winds that made travelling treacherous. Officials closed schools in cities including Chicago, St. Louis and Milwaukee and warned residents to stay indoors and avoid the frigid cold altogether.

The forecast is extreme: –35 C in Fargo, N. D., –29 C in Madison, Wis. and –27 C in Minneapolis, Indianapolis and Chicago. Wind chills could drop into the –40s and –50s.

The Indianapolis mayor upgraded the city's travel emergency level to "red," making it illegal for anyone to drive except for emergencies or seeking shelter. The last time the city issued such a travel warning was during a blizzard in 1978.​

Elsewhere in Indiana, state police closed a 142-km stretch of I-65 on Sunday when conditions became too hazardous, Indianapolis media reported.

Several deaths were blamed on the snow, ice and cold since Saturday, including the death of a 1-year-old boy who was in a car that went out of control and collided with a snowplow Monday in Missouri.​

Coldest in 20 years

"It's just a dangerous cold," said National Weather Service meteorologist Butch Dye in Missouri.

It hasn't been this cold for almost two decades in many parts of the country. Frostbite and hypothermia can set in quickly at those temperatures.

Lorna West, a 43-year-old student and consultant from Columbus, Ohio, said she doesn't believe people unaccustomed to such weather are ready for what's coming.

A Chicago native, she said thermal underwear, lots of layers and "Eskimo coats" with zipped hoods to block the wind were the norm growing up.

"And don't go out if you don't have to," she said.

Deep Freeze

A woman in Chicago walks briskly as she arrives at the La Salle Street commuter rail station with temperatures well below zero and wind chills expected to reach 40 to 50 below Monday. (Charles Rex Arbogas/Associated Press)

It was –15 C at kickoff Sunday inside sold-out Lambeau Field for a playoff game between the Green Bay Packers and the San Francisco 49ers, one of the coldest ever played.

In the parking lot, Craig and Renee Heling of Waukesha, Wis., set up a camouflage hunting blind behind his white pickup truck and tailgated next to a propane heater. He wore four layers of clothing up top, two on his legs: "Two wool socks on — right now, I feel comfortable," he said.

"Well, my nose is about frozen. It feels like — I jumped in the lake the other day — it feels about like that," his wife said with a laugh. She was completely dry, unlike New Year's Day when she took part in a "polar plunge" into Lake Michigan.

Thousands of cancelled flights

For several Midwestern states, the bitter cold was adding to problems caused by a weekend snow storm. The National Weather Service said the snowfall at Chicago's O'Hare International Airport totalled more than 28 centimetres as of 6 p.m. Sunday — the most since the Feb. 2, 2011, storm that shut down the city's famed Lake Shore Drive.

Missouri transportation officials said it was too cold for rock salt to be very effective, and several Illinois roadways were closed because of drifting snow.

The airline flight-tracking service FlightAware.com listed total cancellations within, into, or out of the United States today as being 4,010 at 9:30 p.m. ET. 

Airline officials said de-icing fluid was freezing, fuel was pumping sluggishly, and ramp workers were having difficulty loading and unloading luggage. JetBlue Airways stopped all scheduled flights to and from New York and Boston on Monday, and Southwest ground to a halt in Chicago.

Many cities came to a virtual standstill. In St. Louis, where more than 25 centimetres of snow fell, the Gateway Arch, St. Louis Art Museum and St. Louis Zoo were part of the seemingly endless list of things closed. Shopping malls and movie theatres closed, too. Even Hidden Valley Ski Resort, the region's only ski area, shut down.

School was called off Monday for the entire state of Minnesota, as well as cities and districts in Wisconsin, Illinois and Indiana and Iowa, among others. Chicago Public School officials reversed an earlier decision to keep schools open, announcing late in the day Sunday that classes would be cancelled Monday.

Government offices and courts in several states closed Monday. In Indiana, the General Assembly postponed the opening day of its 2014 session, and the state appellate courts, including the Indiana Supreme Court, said they would be closed.

'Scrambling' to save Florida fruit

Temperatures are expected to dip close to freezing in parts of Florida on Tuesday. Though Florida Citrus Mutual spokesman Andrew Meadows said it must be at –2 C or lower four hours straight for fruit to freeze badly. Fruits and vegetables were a concern in other parts of the South.

'We did learn with the ice storm that you can wake up in the 19th century.'- Smithland, KY, farmer David Nickell

With two freezing nights ahead, Louisiana citrus farmers could lose any fruit they cannot pick in time.

In Plaquemines Parish, south of New Orleans, Ben Becnel estimated that Ben & Ben Becnel Inc. had about 5,000 bushels of fruit on the trees, mostly navel oranges and the sweet, thin-skinned mandarin oranges called satsumas.

"We're scrambling right now," he said.

In western Kentucky, Smithland farmer David Nickell moved extra hay to the field and his animals out of the wind. He'd also stocked up on batteries and gas and loaded up the pantry and freezer. The 2009 ice storm that paralyzed the state and knocked out power to hundreds of thousands of people is fresh in his mind.

"We are hoping this isn't going to be more than a few days of cold weather, but we did learn with the ice storm that you can wake up in the 19th century and you need to be able to not only survive, but be comfortable and continue with your basic day-to-day functions," Nickell said.

With files from CBC News