After days of an arctic chill so cold that a Chicago zoo brought their polar bear inside, the United States woke up to a break in the weather Wednesday.

The National Weather Service said "a much anticipated warm up" was expected over the eastern two-thirds of the United States which had shivered through a week of record-breaking cold and two massive snowstorms.

But it will still be long-underwear and scarf weather for many: the weather service predicts temperatures will be 15 to 25 degrees Fahrenheit below average in parts of the upper Midwest.

And every US state except sunny Florida and tropical Hawaii were forecast to dip below freezing either during the day or overnight Wednesday.

Air travel, which had been an utter nightmare with more than 18,000 flights cancelled since Thursday, was starting to slowly return to normal.

On Wednesday morning 664 US flights had been cancelled and more than 1,000 were delayed, according to FlightAware.com, a site that monitors air travel.

The slight warming comes after some truly stunning temperatures -- in some cases lower than the surface of Mars.

Chicago was colder than the South Pole when officials at the Lincoln Park zoo decided to keep Anana the polar bear inside on Monday. Unlike her wild cousins she hadn't built up the protective fat stores to insulate herself against the chill.

It was so cold in Kentucky that an escaped inmate begged to be let back into prison so he could warm up after spending the night shivering in an abandoned house, officials told AFP.

And more than 500 passengers spent the night aboard trains stranded by snow drifts in Illinois.

The most dangerous cold -- cold that can cause frostbite in minutes and death in a matter of hours -- hit the Midwest, as a weather phenomenon called the 'polar vortex' brought frigid air from the Arctic.

Schools, businesses and government offices were closed. Water mains and household pipes froze. Airplanes were grounded, trains were halted and roads and sidewalks became ice rinks. The National Guard was called in to help rescue hundreds of stranded motorists.

There were reports of at least 20 deaths caused by the extreme weather. Many were from traffic accidents, but four women were found dead in the snow, including an elderly woman with Alzheimer's disease in New York and a young woman found outside her home in Minnesota.

Hypothermia was blamed for the death of a man in Wisconsin and contributed to a death in Ohio. And at least four men suffered heart attacks while shoveling snow in Chicago.

The town of Embarrass, Minnesota, recorded the lowest temperature in the United States Tuesday at a frigid -37 Celsius (-35 Fahrenheit). It had warmed up to -25 Celsius (-13 Fahrenheit) on Wednesday morning.

Then there was the wind chill: a calculation that represents how much colder it feels when the blinding gusts hit you in the face.

That dipped as low as -52 Celsius (-61 Fahrenheit) in Montana and was in the -40 to -50 Celsius (-40 to -58 Fahrenheit) range in parts of Illinois, Indiana, Iowa, Maryland, Michigan, Minnesota, Nebraska, North Dakota, Ohio, Pennsylvania, South Dakota, Virginia, West Virginia and Wisconsin on Monday and Tuesday.

That's cold enough to toss a cup of boiling water into the air and watch it turn into snow before it hits the ground.

The Mars Rover has been sending back daily temperature readings from its tour of the Red Planet ranging from -25 to -31 degrees Celsius (-13 and -24 degrees Fahrenheit).

"To be fair, though, Mars is still way colder," the Smithsonian Institute wrote in a blog post. "The Curiosity rover is driving around in a crater at, roughly, the equivalent latitude of Venezuela."

But then again, Mars is 78 million miles further away from the Sun.

Even hardy Canadians were complaining of the cold amid widespread power outages.

Temperatures as low as minus 50 degrees Celsius (minus 58 Fahrenheit) with a wind chill plunged the western Prairies region into a deep freeze.

The streets of Toronto, Montreal and the capital Ottawa were coated with black ice after unusually warm temperatures brought rain Monday before the mercury plunged overnight.

Toronto's Pearson Airport temporarily halted ground operations early Tuesday because of "equipment freezing" and out of concern for the safety of airport personnel, it said on its Twitter account. A quarter of its flights were cancelled on Wednesday morning.