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Clashing air masses create North American ice storm

23 December 2013 Last updated at 20:54 GMT

Hundreds of thousands of households in Canada and the northern US are facing a Christmas without electricity after a severe ice storm on Sunday.

The storm was the result of clashing warm and cold air masses that led to a 28C difference between New York and Montreal - just 600km away - over the weekend.

BBC Weather describes how freezing rain forms and creates weather havoc.