It’s really, really cold again – as the US shivers through at least the eighth blast of air from the Arctic this winter. Here’s why it keeps happening.

Meteorologists warn that the coldest burst of Arctic air this season is coming to put an icy and prolonged exclamation point on America’s winter of repeated polar vortex invasions starting this weekend.

The polar vortex is not synonymous with cold snaps, but when it slides toward the United States, the polar jet stream can mirror the distortion and introduce Arctic air. While this distortion is often caused by weakening of the polar vortex, that’s not the case this time.