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.