Why the polar vortex matters for our weather

The polar vortex plays a key role in winter weather in the Northern Hemisphere. When the polar vortex is strong, cold air is trapped around the North Pole. But when it weakens, the cold air can spill south into the US mainland — like opening the freezer door. As a result, conditions are colder and snowier than normal.

Source: NOAA

Graphic: Jhasua Razo, CNN