Will it be a white Christmas this year? What counts as a white Christmas and how often do we have one.
For many of us, snow is synonymous with Christmas. Christmas cards, songs and movies all portray a 'white Christmas'.
However, for most parts of the United Kingdom, Christmas is right at the beginning of the period when it's likely to snow. Looking at climate history, wintry weather is more likely between January and March than December. Snow or sleet falls on average five days in December, compared to 7.6 days in January, 6.8 days in February and six days in March.
White Christmases were more frequent in the 18th and 19th centuries, even more so before the change of calendar in 1752 which effectively brought Christmas day back by 12 days. Climate change has also brought higher average temperatures over land and sea and this generally reduces the chances of a white Christmas. However, the natural variability of the weather will not stop cold, snowy winters happening in the future.
For many people, a White Christmas means a complete covering of snow falling between midnight and midday on 25 December.
However, the definition used most widely, notably by those placing and taking bets, is for a single snowflake (perhaps among a mixed shower of rain and snow) to be observed falling in the 24 hours of 25 December at a specified location.
Traditionally the location was the Met Office building in London. However, with the increase in betting on where will see a White Christmas the number of locations have increased and can now include Buckingham Palace, Belfast (Aldergrove Airport), Aberdeen (Pittodrie - Aberdeen FC), Edinburgh (Castle), Coronation Street in Manchester and the Millennium Stadium in Cardiff.
We also analyse the data from our observing stations around the UK to provide a complete picture of where snow has fallen or was lying on Christmas Day.
We can accurately forecast if snow is likely on any given Christmas Day up to five days beforehand. In terms of the statistical likelihood of snow based on climatology, we know that a snowflake has fallen on Christmas Day 38 times in the last 52 years, therefore we can probably expect more than half of all Christmases to be a 'white Christmas' in this sense.
Snow lying on the ground on Christmas Day - as we would expect from typical Christmas scene - is much rarer. There has only been a widespread covering of snow on the ground (where more than 40% of stations in the UK reported snow on the ground at 9am) four times in the last 51 years.
Christmas 2010 was the last white Christmas. It was extremely unusual, as not only was there snow on the ground at 83% of stations - the highest amount ever recorded - but snow or sleet also fell at 19% of stations.
We also had a white Christmas in 2009, 13% of stations recorded snow or sleet falling, and 57% reported snow on the ground.
Will it be a white Christmas this year? Find out by checking the forecast for Christmas Day.
Last updated: 10 December 2014