Quote:
Originally Posted by Tancredi
Thank you for the answer, but I don't understand the reference to 7 a.m.; so if the snow is on the ground on Christmas Day before or after 7 a.m. but not exactly at 7 a.m., is it not a 'White Christmas'? 
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Yes. That's the time of measurement they picked for their standard.
So they review the weather data on December 25 using these steps:
1) Look at snow depth at 7 a.m. and if it's >= 2 cm, it counts as a "White Christmas"
2) If (1) was met, and snowfall during the day was > 0 cm, it's a "Perfect Christmas"
It's got some limitations. If the snow is there at 7 a.m. but melts by 8 a.m., it still counts as a White Christmas. If it snows 0.1 cm and rains 30 mm, it can be a Perfect Christmas. Also I'd say 2 cm snow depth in an instrument doesn't necessarily mean consistent snow cover outside due to drifts and melt in the sun. A real "perfect Christmas" is more like 10 cm+ of snow depth and either sun or snow, with no rain. If that were the standard I think places like Winnipeg or Quebec City would still often have a "perfect Christmas", but Vancouver/Victoria/Toronto/Halifax would be pretty low. I think Quebec City is pretty much the "Christmas capital" of Canada.