After two days of US weather so cold that at least one polar bear at a zoo had to be taken inside, some degree of relief was forecast for Wednesday. The National Weather Service gave no precise temperatures but said "a much anticipated warm up" was expected over the eastern two-thirds of the United States. The arctic air mass that has caused this week's bone-chilling cold will begin to moderate, the service said on its web site. 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 air travel that has been a nightmare, with thousands of flights delayed or cancelled outright, still looked tricky. As of early Wednesday, 551 flights into, out of or within the United States had been cancelled and 206 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 on the surface of Mars. It was so cold in Chicago Tuesday that the polar bear at the Lincoln Park zoo -- which had not built up the winter fat stores of her wild cousins -- was brought inside to warm up, according to zoo spokeswoman Sharon Dewar quoted by US media. And in Kentucky, an escaped inmate begged to be let back into prison so he could warm up after spending the night shivering in an abandoned house. All of Canada and all the US states bar tropical Hawaii recorded temperatures below freezing, even usually sunny and warm Florida and California. But the most dangerous cold -- cold that can cause frostbite in a minute and death in a matter of hours -- hit the Midwest, dragged down as 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 town of Embarrass, Minnesota, recorded the lowest temperature in the United States Tuesday at a frigid -37 Celsius (-35 Fahrenheit). 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 in Montana and was in the -40 to -50 Celsius range in parts of Illinois, Indiana, Iowa, Maryland, Michigan, Minnesota, Nebraska, North Dakota, Ohio, Pennsylvania, South Dakota, Virginia, West Virginia and Wisconsin. 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. For a more Earthly comparison, the Amundsen-Scott South Pole Station recorded a temperature of -23 Celsius (-9 Fahrenheit) and a wind chill factor of -35 Celsius (-31 Fahrenheit) on Tuesday. The brutal chill came as people in the Great Lakes region were still digging out from two massive snow storms which dumped more than two feet of snow. More than 500 people spent Monday night stuck on a train that was stranded by snowdrifts on the line between Detroit and Chicago. Commuter rail -- and even Chicago's famed elevated trains -- were also knocked out of service by the snow and frigid temperatures froze switches and signals. Nearly 2,500 flights were cancelled in the United States on Tuesday and 3,400 were delayed. That brought the total of flights cancelled since the latest snowstorm hit Thursday to more than 18,000. 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. The governor of Illinois declared a state of emergency Monday and called up the National Guard to help rescue stranded motorists as high winds whipped up blinding snow. Even hardy Canadians were complaining of the cold amid widespread power outages. Temperatures as low as minus 50 degrees Celsius 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.