While we usually hear the term polar vortex used in the singular, there actually are two polar vortices on Earth -- one in the Southern Hemisphere over Antarctica, and one in the Northern Hemisphere over the Arctic [source: NASA].
These vortices exist because in the wintertime, the air that's high over the poles gets really, really cold -- we're talking on the order of close to 110 degrees below zero on the Fahrenheit scale or minus 79 degrees Celsius for the Northern Hemisphere [source: NASA]. That drastic contrast with the temperatures farther away from the pole, and the pressure contrast that results, cause really fast winds -- with speeds of between 120 and 250 miles (193 and 402 kilometers) per hour -- to swirl around the big pocket of cold air [sources: Rafferty, Rutgers Today]. The vortex in the Southern Hemisphere is even colder than the one in the Northern Hemisphere, has faster winds and also much lower ozone levels [source: NASA].
Either way, what results is a huge spinning cyclone with cold air inside it, which hangs high over the pole, almost like a halo [source: Herwick]. If you were looking at it from the side, it starts roughly around the border of the troposphere, the lowest level of the Earth's atmosphere and the region where our weather takes place -- and the stratosphere, the next atmospheric layer that begins at about 12 miles (20 kilometers) in altitude and contains the ozone that protects Earth from too much solar radiation.
The vortex reaches all the way through the stratosphere up to the mesosphere, the next layer, where the air starts to get thin and super-cold [source: NASA]. From a vantage point high above the North Pole, for example, the polar vortex actually would look like an ellipse, often swirling around two centers -- Canada's Baffin Island and a spot in northeast Siberia, and enclosing the icy edges of North America, Greenland, Europe and Asia [sources: AMS, NSIDC, O'Hanlon].
The polar vortex is weakest during the warmer months of the year, when there's less of a contrast between polar temperatures and those in warmer regions, and it tends to become the strongest in the winter months [source: Rafferty]. Usually, that barrier of winds keeps cold air in the Arctic, which prevents rest of the Northern Hemisphere from getting too chilly. But some years, it doesn't work quite as well at doing that. We'll get into the details in the next section.