BISMARCK, N.D. >> The deadly deep freeze enveloping the Midwest sent temperatures plunging today to rival some of the most frigid spots in the world, triggering widespread closures of schools and businesses, and the canceling of more than 1,600 flights to and from Chicago’s airports.

The U.S. Postal Service even took the rare step of suspending mail delivery across much of the region. Hundreds of public schools and universities from North Dakota to Pennsylvania canceled classes as residents huddled inside amid one of the coldest air masses in years.

The bitter cold is the result of a split in the polar vortex that allowed temperatures to drop much farther south than normal. That meant temperatures in parts of the Midwest were lower today than in Antarctica, where the Amundsen-Scott South Pole Station hit negative 25 degrees — balmy compared to Fargo, North Dakota’s negative 31 degrees and Minneapolis’ negative 27 degrees, according to the National Weather Service.

Snowplows were idled overnight because of the cold in southeastern Minnesota, where temperatures dropped to negative 29 degrees. In Chicago, temperatures were still dropping after plunging early today to minus 19 degrees, breaking the day’s previous record low set in 1966 — and colder than the weather in Barrow, Alaska, the most northern town in the U.S.

And that doesn’t include wind chill, which in northern Illinois made the air feel as cold as negative 57 degrees. The National Weather Service warned that a wind chill of minus 25 can freeze skin within 15 minutes.

Officials throughout the region were focused on protecting vulnerable people from the cold , including the homeless, seniors and those living in substandard housing. Some buses were turned into mobile warming shelters to help the homeless in Chicago.

“These (conditions) are actually a public health risk and you need to treat it appropriately,” Chicago Mayor Rahm Emanuel said Tuesday. “They are life-threatening conditions and temperatures.”

About 1,300 of today’s canceled flights in Chicago were at O’Hare International Airport, one of the nation’s busiest airports. United Airlines spokesman Charlie Hobart said “everything tends to slow down” during severely cold weather, including manpower, fueling and equipment. Calling the temperatures “dangerous,” Hobart said United was bringing in extra workers and providing heated tents for employees.

A popular saying goes: “Neither snow nor rain nor heat …” will stop the mail from being delivered, but extreme cold did so today. The U.S. Postal Service has suspended mail delivery in parts or all of several Midwest states including North Dakota, South Dakota, Nebraska, Minnesota, Wisconsin, Iowa, Illinois, Indiana, Michigan and Ohio.

Governors in Illinois, Wisconsin and Michigan declared emergencies as the worst of the cold threatened today. In Chicago, major attractions closed because of the bitter cold, including the Lincoln Park Zoo, the Art Institute and the Field Museum.

In Michigan, homeless shelters in Lansing were becoming “overloaded,” Mayor Andy Schor said. They also were filling up in Detroit.

“People don’t want to be out there right now,” said Brennan Ellis, 53, who is staying at the Detroit Rescue Mission Ministries.

At least six deaths have been linked so far to the weather system, including two people in the Detroit area, according to local police. Other deaths included a man struck and killed by a snow plow in the Chicago area, a young couple whose SUV struck another on a snowy road in northern Indiana, and a Milwaukee man found frozen to death in a garage.

Hawaii native Charles Henry, 54, was staying at a shelter in St. Paul, Minnesota, and said he was grateful to have a place to stay out of the cold.

“That wind chill out there is not even a joke,” he said. “I feel sorry for anybody that has to stay outside.”

Shelters, churches and city departments in Detroit worked together to help get vulnerable people out of the cold, offering the message to those who refused help that “you’re going to freeze or lose a limb,” said Terra DeFoe, a senior adviser to Detroit Mayor Mike Duggan.

American Indian tribes in the Upper Midwest were doing what they could to help members in need with heating supplies. The extreme cold was “a scary situation,” because much of the housing is of poor quality, said Chris Fairbanks, energy assistance program manager for the White Earth Band of Ojibwe in Minnesota.

The cold weather was even affecting beer deliveries, with a pair of western Wisconsin distributors saying they would delay or suspend shipments for fear that beer would freeze in their trucks.

But it wasn’t stopping one of America’s most formidable endurance tests: the three-day Arrowhead 135 was going on as scheduled in northeastern Minnesota. Competitors can cover the race route by bicycle, cross-country skis or just running.