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

A whirlpool of frigid, dense air known as a "polar vortex" was expected to suppress temperatures in more than half of the continental U.S. starting into Monday and Tuesday, with wind chill warnings stretching from Montana to Alabama.

snow spiderman
Patrick O'Brien jogs around Lake Harriet in Minneapolis in his Spider-man costumeReuters
snow girl
A tourist catches snowflakes on her tongue in Times Square, New YorkReuters
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Eastern United States viewed from NOAA-GOES satellite imageReuters
snow bus tyres
New York City bus on tyre chains as it is driven down 5th Avenue in the snowReuters
snow car
Brad Sachs shovels snow from around his car near his home in Indianapolis, IndianaReuters
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A snow-covered bus shelter in downtown ChicagoReuters
snow chicago
Chicago skyline rises above mountains of snow along Lake MichiganReuters
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A man cycles along the Rideau Canal in Ottawa, CanadaReuters
snow steam
A woman walks in cold temperatures through downtown Chicago, IllinoisReuters
snow snowmen
Walking across frozen Lake Harriet in south MinneapolisReuters
snow dogs
Dog-walker in Cedar Hill in New York's Central ParkReuters
snow luggage
Travellers leave the Back Bay train and subway station in Boston, MassachusettsReuters
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A traveller reacts to news of delays at the check-in counter of JetBlue Airways terminal at John F Kennedy International Airport in New YorkReuters
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Pedestrians skirt mounds of snow during bitter cold temperatures in downtown DetroitReuters