A polar vortex dipping down from Siberia will bring a cold front with frigid temperatures to nearly 300 million Americans. See maps of the arctic blast.

The polar vortex is a low-pressure system of cold air that swirls above Earth’s poles. There’s one over the South Pole in Antarctica – but for this story, we’ll be focusing on the one above the North Pole. Over the North Pole, the wind flows counterclockwise in the stratosphere around 16 to 50 kilometers (10-30 miles) high.

According to the National Weather Service, a polar vortex will impact most of the country this weekend, here is what you need to know.

A new blast of Arctic air sweeping from north to south will bring bone-chilling temperatures to millions of Americans this weekend and next week.

A blast of Arctic air is set to cover much of the United States with temperatures below freezing starting on Friday and into next week, impacting millions of Americans in nearly all of the contiguous states.