Parts of the US are braced for potentially record-breaking low temperatures as a polar vortex brings more freezing weather.
A winter storm has already blanketed areas of Canada and the north-eastern US with up to two feet of snow.
It has been blamed for 16 deaths in recent days and the cancellation of some 3,700 flights.
Schools have been closed in several US states and residents urged to stay indoors for their own safety.
The plunging temperatures result from the polar vortex, an anti-clockwise pool of cold, dense air.
The vortex has been very strong, with the cold air locked in to Arctic Canada for a long time.
Stuck in one place, the air has gotten colder and colder.
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