A polar vortex will bring snow, sleet, and ice to Canada and the eastern United States, starting Friday, due to the arctic air moving south.

The polar vortex is a mass of cold air that circulates counterclockwise in the stratosphere, helping to keep the cold air at the North Pole. Usually, the vortex remains stable, but sometimes it extends southward, causing episodes of extreme cold. Meteorologists emphasize that these extensions of the vortex can lead to heavy snowfall and severe conditions in North America. Research suggests that the Arctic is warming faster than the global average, which contributes to the frequency of these extreme weather phenomena.