Ray found on Wikipedia.....
White Christmases defined
The view of what constitutes a white Christmas varies from country to country. 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 inch (2.5 cm) 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 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 3 ft (91 cm) 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.
You need official, technical definitions of "white christmas"? How crazy are humans?
In Britain you can legally bet on any legal activity or natural phenomenon. The book maker and the better need to agree on terms. A legal definition for a popular bet, like white Christmas, will save a lot of aggravation down the road, and may have been settled in a court of law, with Met Office input.
For recording history, again a subjective term like White Christmas has no true meaning.
Interesting, thank you. Having never gambled, I didn't see that angle.