© John Dixon/APA wrecked semi truck sits in a ditch in Illinois as sub-zero temperatures hit driving conditions.
- Twenty six states under federal warnings for severe wind chills

- Montana recording a record-making wind chill of -52C (-61F)

- Live blog: life-threatening cold hits North America

Millions of Americans from Montana down to as far south as Alabama are being warned that their lives are at risk if they venture out for any length of time into brutally cold conditions that are driving temperatures to their lowest in 20 years.

Severe weather warnings from the National Oceanic and Atmospheric Administration for Minnesota and Wisconsin said that wind chills caused by gusts of up to 30 mph were causing temperatures to plummet to between -37C (-35F) and -46C (-50F). "Exposed flesh will freeze and cause frost bite in only five minutes," the warning observed, adding that such dangerous conditions were likely to last until at least Tuesday afternoon.

Twenty six states continue to be under federal warnings for severe wind chills as bitterly cold air is swept down from the Arctic, with Montana recording a wind chill of -52C (-61F). The severe weather has already caused havoc in the Northern Plains and is expected to reach the north-east on Tuesday, extending disruption to airports and travellers. More than 3,000 flights were cancelled on Monday and Chicago's O'Hare International airport has been particularly badly affected.

© ReutersA man clears snow from his car in Indianapolis. A red travel warning has since been imposed in the city making it illegal to use the roads except for emergencies.
As a sign of the extraordinary nature of the weather pattern, the federal government has even issued a wind chill warning for Florida. It said that the very cold Arctic air mass would drive temperatures in the north of the state below zero fahrenheit.

Local governments in states across the country were taking exceptional measures to protect their populations. Every school in Minnesota was closed on Monday - the first such blanket precaution in the state in 17 years. Children were also kept away from school in Chicago, dubbed for the duration "Chiberia".

In Indianapolis, where thousands were struggling without power, mayor Greg Ballard raised a travel warning to red making it illegal to use the roads unless for emergencies. He used his Twitter feed to inform residents how to find shelter.

© Michelle Tessier/APChildren in Grand Blanc, Michigan, play in the snow.
The strife caused by the extreme weather is being blamed on a "polar vortex", a spinning wind that in normal years has a beneficial effect in that it keeps sub-zero air trapped above the North Pole. A weakened polar vortex has allowed the trapped air to spill out of the Arctic and hurtle south in an anti-clockwise movement across the face of the US.

Just why the polar vortex has proved to be too weak to contain the cold air this winter will be a matter for intense scientific study and debate for some time to come. The phenomenon might in large part be explained by the natural changes in climate that occur in the Arctic in winter, including the North Atlantic Oscillation that has a dominant impact on wind patterns and storm tracks in the region.

But Noaa, a federal agency, has also floated the possibility that a reduction in summer sea ice cover caused by climate change could be a factor behind the weakening of the polar vortex. That would have the paradoxical effect that while Arctic waters are getting warmer, North America experiences much colder snaps such as the present severe weather as a result of Arctic air spilling out from the North Pole and moving south.
© Rex Arbogast/APThe Chicago skyline sits as a backdrop as fog drifts across Monroe Harbour with temperatures well below zero and wind chills expected to reach 40 to 50 below.