The Met Office definition of a white Christmas is for one snowflake to be officially observed falling in the 24 hours of 25 December at one of their sites in the UK.
Lying snow on the ground on Christmas Day doesn't count, although many would probably class that as a white Christmas.
It wasn't a white Christmas in the UK last year - in fact, temperatures didn't even fall below zero anywhere.
But the four years before that were white Christmases.
The last time all four UK nations had a white Christmas was back in 2010, the year snow lay most widely across the UK since 1959.
In 2010, 83% of stations reported lying snow - very rare - but snow also fell at 19% of stations.
2010 was also the last time Northern Ireland had a white Christmas.
The deepest snow on Christmas Day was 47cm at Kindrogan (Perthshire) in 1981.
Check the weather forecast where you are
Falling snow happened most widely on Christmas Day in 2004, when 64% of stations officially observed falling snow, with 34% reporting lying snow.
December, however, is only at the start of the period when it's likely to snow across most of the UK.
Lying snow occurs an average of three days in December, compared to 3.3 days in January, 3.4 days in February and 1.9 days in March, according to the Met Office.
How likely is it this year?
It's still too early to tell whether this year will bring a technical white Christmas or not - but looking at the statistics, the odds are in our favour.
Forecasting snow can be a big challenge in the UK just a few days ahead, let alone with 24 days to go.
Read more:
Wintry blast could be a dry run for a cold December
Snow hits parts of the UK
The first half of December is likely to be influenced by a cyclonic pattern, with generally mild and unsettled conditions.
There's more uncertainty from the middle of the month, but global weather patterns, which have some influence on the UK, suggest something more settled and potentially colder for the end of the month.
(c) Sky News 2025: What counts as a white Christmas - and how likely is one this year?
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