Much of the nation east of the Rocky Mountains is hunkered down for what could be the worst winter storm of the season.

Rising Arctic temperatures and melting sea ice could be causing cold air to flow into the Northern Hemisphere. But not all scientists agree.

The meteorologists said the polar vortex system is set up due north of Michigan over Canada's Hudson Bay. It's at least partly responsible for the frigid temperatures around the region that contributed this week to a 100-vehicle pileup in southwest Michigan and led the city of Detroit to declare a cold weather health emergency.

As the U.S. prepares to take on another bout of cold weather, the term "polar vortex" is getting thrown around. But what is it?

Meteorologists warn that a stretched polar vortex could bring severe winter weather to much of the United States.

Tens of millions of people are in the path of a major winter storm. Federal cuts threaten efforts to understand the causes of such weather.

It exists all the time, but it usually gets stronger in winter. When the jet stream (a river of wind) bends and dips south, the edge of the polar vortex can wobble or stretch. That can let Arctic air slide into the United States and Canada and bring a short spell of extreme cold.