<P> A white Christmas refers to the presence of snow at Christmas; either Christmas Eve or Christmas Day depending on local tradition . This phenomenon is most common in the northern countries of the Northern Hemisphere . Because December is at the beginning of the Southern Hemisphere summer, white Christmases there are extremely rare, except in Antarctica, in the Southern Alps of New Zealand's South Island, and in parts of the Andes in South America . The Irving Berlin song, "White Christmas", sung by Bing Crosby from the film Holiday Inn, is the best - selling single of all time and speaks nostalgically of a traditional white Christmas . </P> <P> The definition of "White Christmas" varies . In most countries, it simply means that the ground is covered by snow at Christmas, but some countries have more strict definitions . In the United States, the official definition of a white Christmas is that there has to be a snow depth of at least 1 in or 2.5 cm at 7: 00 a.m. local time on Christmas morning, and in Canada the official definition is that there has to be more than 2 cm (0.79 in) on the ground on Christmas Day . In the United Kingdom, although for many a white Christmas simply means a complete covering of snow on Christmas Day, the official definition by the British Met Office and British bookmakers is for snow to be observed falling, however little, (even if it melts before it reaches the ground) in the 24 hours of 25 December . Consequently, according to the Met Office and British bookmakers, even 91 cm (3 ft) of snow on the ground at Christmas, because of a heavy snow fall a few days before, will not constitute a white Christmas, but a few snow flakes mixed with rain will, even if they never reach the ground . In the United Kingdom the most likely place to see snowfall on a Christmas Day is in North and North Eastern Scotland, in Aberdeen, Aberdeenshire or the Highlands . </P> <P> Although the term' White Christmas' usually refers to snow, if a significant hail accumulation occurs in an area on Christmas Day, as happened in parts of Melbourne on 25 December 2011, this can also be described as a White Christmas, due to the resulting white appearance of the landscape resembling snow cover . </P>

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