Polar Vortex freezes US with snow

A blast of Arctic air from the polar vortex has brought dangerous, bone-chilling cold to a wide swath of the United States on Tuesday, stretching from the Dakotas through Maine, with snow expected as far south as Alabama and Georgia.

Arctic air and icy conditions led schools to close, flights to be cancelled and railway officials to set fires on train tracks to prevent them from cracking.

Icicles form on the walkway at North Avenue Beach of Lake Michigan in Chicago. All this is the work of a polar vortex, a low-pressure mass that normally sits near the Earth's poles moving from North Dakota to Ohio. Photograph: Pinar Istek/Reuters

A pedestrian stops to take a photo by Chicago River, as bitter cold phenomenon called the polar vortex has descended on much of the central and eastern United States, in Chicago. The Midwest is the hardest-hit region, as temperatures plunged below zero Fahrenheit (minus 18 degrees Celsius). By nightfall the mercury was hovering at 0F in Chicago, 7F (minus 14C) in Detroit and minus 21f (minus 29C) in Minneapolis. Photograph: Pinar Istek/Reuters

Snowplows have been busy, and dangerous roads have caused chaos. Officials have warned Chicago residents, accustomed to chilling winters, to expect an unusually deep and dangerous freeze. Photograph: Scott Olson/Getty Images

Wisconsin has declared a state of emergency, and so have Michigan and Illinois. The bitter cold has raised fears for the homeless. Photograph: Scott Olson/Getty Images

A pedestrian walks by Chicago River, as bitter cold has descended on much of the central and eastern United States, in Chicago. Photograph: Pinar Istek/Reuters

Officials have asked residents to avoid outdoors, and said that if residents need to, they must cover their mouth with a scarf or mask to warm up the air. Photograph: Pinar Istek/Reuters