Why is it so cold? Split in polar vortex brings historic deep freeze to US

The bitter cold sweeping across the US is the result of a split in the polar vortex

A man walks along an ice-covered break-wall along Lake Michigan while temperatures were hovering around -20F (-29C) on 31 January, 2019 in Chicago, Illinois. ((Photo by Scott Olson/Getty Images))

The US is currently experiencing historically cold weather, with Chicago dropping to a low of around -23F (-30C) on Wednesday – but what is the cause of this extreme drop in temperature?

A split in the polar vortex has been identified as the cause of the bitter cold that continued to sweep parts of the country on Thursday.

The polar vortex is a mass of cold air that usually stays bottled up in the Arctic.

But when the split occurred, that air spilled and traveled much further south than usual, causing a painfully cold weater system that put much of the Midwest into a historic deep freeze.

Temperatures still tumbled to record lows in some places on Thursday, though the weather was expected to start easing into Friday.

Record-breaking cold hit northern Illinois early Thursday, when the temperature in Rockford dropped to -30F (-34C). The previous record in the city, northwest of Chicago, was -27F (-33C) on 10 January, 1982.

The blast of polar air that enveloped much of the Midwest on Wednesday closed schools and businesses and strained infrastructure with some of the lowest temperatures in a generation. The deep freeze snapped rail lines, canceled hundreds of flights and strained utilities.

A man walks along an ice-covered break-wall along Lake Michigan while temperatures were hovering around -20F (-29C) on 31 January, 2019 in Chicago, Illinois. (Photo by Scott Olson/Getty Images) ((Photo by Scott Olson/Getty Images))

Chicago dropped to a low of around -23F (-30C), slightly above the city's lowest-ever reading of -27F (-32C) from January 1985. Milwaukee had similar conditions. Minneapolis recorded -27F (-32C). Sioux Falls, South Dakota, saw -25F (-31C).

Wind chills reportedly made it feel like -50F (-45C) or worse.

At least twelve people have died in weather-related circumstances. Some died in weather-related traffic accidents, others from apparent exposure to the elements.

Additional reporting by agencies.