A blast of brutally cold air – thanks to the polar vortex – is roaring across the central and eastern USA early this week, sending temperatures to levels not seen in 20 years.

 

The polar vortex is a strong area of low pressure that usually wanders around the Arctic throughout the winter. Counterclockwise winds around the vortex occasionally push waves of intensely cold air into the USA.

 

The vortex is strongest during the winter, and usually weakens or even disappears in the summer. The vortex tends to move very slowly or even stay stationary, and its position determines what part of the USA the Arctic air will invade. [Link]