Why has it been so cold? Here's what science says

Why has it been so cold? Is this unusual? Is the polar vortex to blame? And how can it be this cold if there's global warming? Here are the answers to your questions.

Want to know more? Replay CBC meteorologist Johanna Wagstaffe's interactive webcast

People brave the frigid weather to skate on an outdoor rink on Parliament Hill in Ottawa on Dec. 29. (Patrick Doyle/Reuters)

Whitehorse was warmer than Toronto on Tuesday. The weather in North America is upside down.

That's because the Arctic's deeply frigid weather escaped its regular atmospheric jail, which traps the worst cold. It then meandered south to the central and eastern part of the continent.

And this has been happening more often in recent times, scientists say.

(CBC meteorologist Johanna Wagstaffe answered questions about the cold snap in an interactive live stream. ​Watch a replay below.)

Super cold air is normally locked up in the Arctic in the polar vortex, which is a gigantic circular weather pattern around the North Pole. A strong polar vortex keeps that cold air hemmed in.

"Then when it weakens, it causes like a dam to burst," and the cold air heads south, said Judah Cohen, a winter storm expert for Atmospheric Environmental Research, a commercial firm outside Boston.

"This is not record-breaking for [northern] Canada or Alaska or northern Siberia, it's just misplaced," said Cohen, who had forecast a colder than normal winter for much of the U.S.

Is this unusual?

Yes, but more for how long — about 10 days — the cold has lasted, than how cold it has been. 

That said, on New Year's Eve, record lows were measured in communities across Canada, including Lethbridge and Claresholm in Alberta and Toronto, Ottawa and Kitchener-Waterloo and Trenton in Ontario, as temperatures dipped to dangerous levels in parts of Quebec. 

In Waskaganish, a Cree community on the southeast shore of James Bay, it got as cold as –45.2 C and in the Far North, residents of La Grande Rivière saw the thermometer hit –48.2 C.

Meanwhile, more than 1,600 U.S. daily records for cold were tied or broken in the last week of December, according to the National Oceanic and Atmospheric Administration.

Visitors take photographs at the brink of the Horseshoe Falls in Niagara Falls, Ont., as cold weather continues through much of the province on Tuesday. (Aaron Lynett/Canadian Press)

In Canada, a number of records are falling across the country, mainly for daily cold temperatures. But forecasters also point to new records for how long those cold stretches have lasted.

"Montreal just broke an all-time record for longest-stretch of cold temperatures below –16 C," said  Wagstaffe. "I think you had six days and counting of temperatures that didn't rise above –16."

Is it just North America?

Pretty much. While the United States and Canada have been in the deep freeze, the rest of the globe has been toastier than normal. The globe as a whole was 0.5 C (0.9 degrees F) warmer than normal Tuesday and the Arctic was more than 3.4 C (6 degrees F) warmer than normal, according to the University of Maine Climate Change Institute's analysis .

A small dog wears boots and a coat during frigid weather on Parliament Hill in Ottawa on Dec. 29. (Patrick Doyle/Reuters)

What's next?

For now, in parts of Canada, it has warmed up a little bit and extreme cold warnings have been lifted in places like Montreal, OttawaToronto and London, Ont..

However, Wagstaffe says a weather bomb or bombogenesis is shaping up for the Maritimes, with cold air moving eastward and merging with a storm system that is moving north from the U.S. East Coast.

Children warm themselves next to the Centennial Flame during frigid weather on Parliament Hill Dec. 29. (Patrick Doyle/Reuters)

Winter storm warnings are in place for Prince Edward Island, eastern New Brunswick, and western and northern Nova Scotia, she said. The region could first get a messy mix of rain, ice pellets and snow on Thursday, with conditions deteriorating rapidly overnight and into Friday.

"This is where we're talking a full-blown blizzard… a weather bomb when we get a storm deepening so rapidly in a short amount of time.

"The central pressure of a storm actually has to drop 24 millibars in 24 hours for it to be considered a weather bomb, and we call it bombing out when a storm does this," Wagstaff said.

The risk is great for widespread outages and white conditions with hurricane-force winds, she added, "on top of snowfall totals that could be over 40 centimetres."

Cold air is being pulled into the centre of the storm and colder-than-seasonal temperatures are expected to last throughout the weekend in both the U.S. Northeast and the Maritimes.

Thursday afternoon temperatures are seen in this map, which also shows the undulating jet stream, which is allowing Arctic air to spread south. (CBC)

What makes the polar vortex move?

The polar vortex is the circulation of air that is always happening around the poles. When there are very strong temperature differences between the north and the south, that cold air stays locked up in the north. But when temperature differences aren't as great, the jet stream meanders.

On Thursday, the jet stream was rising high along Canada's West Coast and diving down to the south farther to the East, "and that allowed part of the polar vortex to break off and spread down across the country," Wagstaffe said.

"More research is saying that we've got more meandering jet streams with climate change because we're seeing less of a temperature difference with a warming Arctic. That allows that jet stream to meander more, and allows the polar vortex to break off more often, and we see these deep freezes." 

This map shows how far-displaced the polar vortex is across much of North America. The Arctic air mass is seen in purple and white. (CBC)

What makes the polar vortex move is an area of hot debate and research among scientists and probably is a mix of human-caused climate change and natural variability, said Jason Furtado, a University of Oklahoma meteorology professor. Climate change hasn't made the polar vortex more extreme, but it probably is making it move more, which makes the weather seem more extreme, he said.

A recent study by Potsdam Institute climate scientist Marlene Kretschmer found the polar vortex has weakened and meandered more often since 1990, but that study focused more on Europe. Ongoing research shows that there seems to be a similar connection for more frequent Arctic cold snaps like what the U.S. is now experiencing, Kretschmer said.

A person walks with their face covered in Quebec City on Dec. 27. (Alice Chiche/AFP/Getty Images)

How can it be so cold with global warming?

Don't confuse weather — which is a few days or weeks in one region — with climate, which is over years and decades and global. Weather is like a person's mood, which changes frequently, while climate is like someone's personality, which is more long-term, Furtado said.

"A few cold days doesn't disprove climate change," Furtado said. "That's just silly. Just like a couple down days on the stock market doesn't mean the economy is going into the trash."

With files from CBC News