The polar vortex. It's gotten a great deal of attention lately, being blamed for the crushing cold enveloping much of the country this week. The term has been thrown around seemingly endlessly since the onset of the arctic snap. The problem is that most people do not know what it is.
The polar vortex is a large pocket of cold air in the upper level of the atmosphere centered near each pole -- north and south. Over the north pole, it rotates counterclockwise, which is why we refer to it as a vortex. It exists all year long, but it is stronger during the winter season.
At times, a piece of this large, rotating cold air mass will break off and get pulled south by the polar jet stream. When this happens cold surface air gets dragged south with it, resulting in an arctic onslaught for the people in its path. The diagram above, courtesy of the National Weather Service, shows the position of the polar vortex and its associated offshoot as of Monday morning.
As the diagram shows, and as we have experienced, this week the United States, from the Rocky Mountains to the east coast, is the target of the onslaught.