An Arctic air mass known as a "polar vortex" has descended over much of Canada and the eastern and mid-west U.S., causing temperatures to plunge, nearing -50 C with the wind chill in some regions.

But what exactly is a polar vortex and how does it work? Here are four polar vortex questions answered:

1) What is a polar vortex?

A polar vortex is a large, frigid air mass located near the Earth's geographical poles. The vortex is continually circulating a pool of cold air in a counter-clockwise direction. As the air is being circulated in place, it grows colder and denser.

2) Where is the polar vortex?

On Earth, the vortex hovers around the Arctic, with two centres: one near Canada's Baffin Island and the other near Siberia.

3) Why is the polar vortex making this winter in particular so cold?

While it's normal for the some of the vortex's frigid air to leach southward during the winter, this year has proved to be exceptional.

Dan Riddle, senior meteorologist with the U.S. National Weather Service, told CTV News Channel that the vortex is normally present over Baffin Island and north-central Canada for the winter, with the cold air building up over time.

But this year the jet stream has dipped far south, dragging a "piece" of the vortex with it through the U.S. prairies and beyond, he said.

Riddle said the effects of the vortex are being felt as far south as New Orleans, La. and Tallahassee, Fla., where temperatures are dropping to levels not seen in 30 years.

4) How common is it to have a polar vortex descend south like this year?

Riddle said historical records indicate that outbreaks of unusually cold weather have occurred in U.S. in the past, with a frequency of about one every 20 years.

Some scientists have begun to study whether there is a link between the effects of climate change -- including the melting of the polar ice caps -- and extreme weather events.

"There is a lot of thought about the changes that have been going on in the Arctic and if that has caused colder weather to be displaced further south," Riddle told CTV News Channel.