US weather: 'polar vortex' brings big freeze – in pictures

Ice builds up along Lake Michigan as temperatures dip well below zero in Chicago, Illinois. A pool of cold, dense air known as a “polar vortex” has been sweeping through northern and central parts of the United States and has sent temperatures plummeting to their coldest in 20 years.

A woman skis to work through several inches of snow in Detroit. Residents have been warned to stay indoors and stock up on food.

Temperatures in Chicago drop to a record low of -27 degrees Celsius.

A man removes several inches of snow from a pavement in Detroit as the area deals with record-breaking weather conditions. Sub-zero temperatures are expected most of this week with wind-chill driving temperatures down to -50 degrees Celsius.

Mist rises from Lake Michigan.

A commuter with iced-up glasses braves the sub-zero journey to work in Chicago. Experts have warned that exposed skin could suffer frostbite in as little as five minutes in such conditions.