A torrent of polar air has brought record low temperatures to much of the US midwest, making conditions so unbearable that garbage trucks stopped making pick-ups and mail carriers suspended deliveries.

School classes were cancelled in many cities, including Chicago, home of the nation’s third-largest school system, and police warned of the risk of accidents on icy highways.

All Michigan state offices were to remain closed on Thursday. In a rare move, the US Postal Service appeared to temporarily set aside its credo that “neither snow nor rain … nor gloom of night” would stop its work: it halted deliveries from parts of the Dakotas through to Ohio.

Streets in Chicago were nearly empty, with few people walking outside in the painfully cold air as temperatures hovered around minus 28C. Temperatures in parts of the Northern Plains and Great Lakes plunged to as low as minus 41C in Park Rapids, Minnesota, according to the National Weather Service.

The frigid winds were headed for the US east coast later today and into tomorrow.

National Weather Service meteorologist Andrew Orrison said some of the coldest wind chills were recorded in International Falls, Minnesota, at minus 48C. Even the South Pole in Antarctica was warmer, with an expected low of minus 31C with wind chill.

Banks and stores in the region closed for business.

Waste Management, a major garbage collection company, said it was cancelling pick-ups in counties across the region for the next few days. The bitter cold was caused by a displacement of the polar vortex, a stream of air that normally spins around the stratosphere over the North Pole, but whose current was disrupted and was now pushing south.

At least five deaths related to extreme cold weather have been reported since Saturday in Iowa, Indiana, Illinois, Wisconsin and Minnesota, local media said.

Hundreds of flights, more than half of those scheduled, were cancelled on Wednesday out of Chicago O’Hare and Chicago Midway international airports, according to the flight tracking site FlightAware.

Amtrak said it would cancel all trains in and out of Chicago on Wednesday. Most federal government offices in Washington DC opened three hours late on Wednesday due to frigid weather already impacting the area.

CLIMATE CHANGE LINK?
The Arctic-like deep freeze has gripped the US with the coldest temperatures in decades, but is it linked to climate change?

Experts say it could be, but whether global warming plays a role in this particular extreme weather phenomenon is still up for debate. Here’s why:

What is a polar vortex?
“It is a mass of very cold air that typically sits right on the North Pole and tends to be restricted to the North Pole by the jet stream,” explained Ben Kirtman, professor of atmospheric sciences at the University of Miami Rosenstiel School of Marine and Atmospheric science.

Normally, the jet stream, which is a strong a current in the atmosphere, would keep this cold air in the Arctic, but when the jet stream wavers, or weakens, the chill can spill.

“Occasionally we get meandering of the polar vortex and that is what is happening right now. And if you get a real big one you can have a big blob of cold air penetrate very far south,” Kirtman told AFP.

According to the Weather Channel, this week’s freeze “may be the coldest in more than 20 years in parts of the Midwest and will smash dozens of daily record lows in some areas.” NASA’s Earth Observatory said that steady northwest winds “were likely to add to the misery, causing dangerous wind chills below -40C in portions of 12 states.”

Why is this cold spell so strong and so vast?
“It is not out of bounds with the historical record. They happen. You get storms that are bigger than other storms. There is a big part of this that is part of the natural variability of the climate,” Kirtman said.

The protective band in the atmosphere that typically keeps polar air in the north can and does waver. That, too, is not unexpected.

“There are always undulations in the jet stream,” Kirtman said.

Why does the jet stream waver?
The strength of the jet stream is linked to the temperature contrast between the hot tropics and the frigid poles.

The starker this contrast, the stronger the jet stream, and in theory, the more likely that the polar air will stay in the Arctic.

However, sometimes a jet stream that is too strong can also become unstable, “and these instabilities cause a certain amount of waviness in the jet so you can get these meanders,” Kirtman said.

Some evidence also suggests that when the poles heat up, there is less contrast between the tropics and the poles, and that, too, can make the jet stream become wavier, allowing cold air to spill down from the north.

The Arctic is well known to be heating at twice the rate of the rest of the planet.

“What people are starting to ask is, if you weaken the jet stream, does that mean we are going to have more, stronger excursions of the polar vortex? If that turns out to be true, we can link more extreme cold spells to climate change,” Kirtman added.

Researchers are examining data in the quest to find out.

“There are some hints that it is linked to climate change, but I would emphasise that the jury is still out,” he said.

Scientists are getting better at deciphering the role of climate change in certain weather extremes. So far, the signals are clearest in events like rainfall, drought, heatwaves and wildfires.

But when it comes to cold snaps, the answers are not as clear. “I would say the science is still incomplete. We don’t have proof at this point.”