Located about 66.5 degrees north of the Equator lies the Arctic Circle — a line of latitude that outlines the border of Earth's northernmost region known as the Arctic.

It occurs in winter north of the Arctic Circle and south of the Antarctic Circle, which are circumpolar lines at latitude 66.6 degrees North and South respectively.

Sea smoke, the result of extremely cold air blowing over warmer water, rises from Lake Michigan in Chicago, Jan. 30, 2019. The deadly cold front was tied to a misshapen Arctic polar vortex.

The polar vortex is the jet stream above the Arctic Circle. It is very cold. Most of the time it stays up there, but there are circumstances where it can dip down into North America.

The Swedes call it the Polar Circle (latitude 66 degrees 32 minutes North) and, though we had cameras at the ready, it raced past so quickly - just a string of white stones in a forest clearing ...

The division between frigid cold temperatures at the North Pole and warmer temperatures into the mid-latitudes helps create a strong jet stream around the Arctic Circle, hence the name polar vortex.

It appears the Learjet Polar Mission plane and crew accomplished a polar circumnavigation. First, they had to cross over 75 degrees south latitude, which they did on Friday .