Polar vortex and jet stream. Illustrations showing the the causes and effects of a breakdown of the polar vortex over the Arctic. The polar jet stream (7-12 km altitude, blue arrows) is one of the fast flowing high-altitude narrow air currents that circle the Earth. They can have a major influence on the weather. At left is a stable polar vortex. At right, the system is unstable, and the polar jet stream has weakened and moved southwards with warm air moving north (red arrows). Cold air flowing south from the Arctic brings ice and snow to the north-eastern USA. The switch between these two phases is known as the Arctic Oscillation.