Will we have a white Christmas?

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With exactly a month to go until Christmas it’s the question every weatherman gets asked; will it snow at Christmas?

Today the Met Office has released its long range 30 day forecast and predictably the headline is that it’s too early to say for sure. However, there are a few trends:

For the two weeks leading up to Christmas day the weather looks like being colder than average. The mean daytime temperature in Central London is usually 9C and drops to 3C at night. There’s no clarity on whether temperatures will drop below freezing but it is possible for it to snow in milder temperatures (or rain when it’s below freezing).

It also looks as if high pressure will be in charge in the lead up to Christmas. The likely outcome of that is to see similar weather conditions to those we have at the moment. More settled, cloudy, frosty and misty at night but predominantly dry. So that would suggest it is slightly less likely to snow on the 25th.

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For most parts of the UK, Christmas is right at the beginning of the period when it's most 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.

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So, what are the odds of a white Christmas in London?

According to the bookies it’s 5/1 but that doesn’t offer great value.

Accurate forecasts can only be given up to 5 days in advance. 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. In London it’s far less common. It actually only snows on Christmas Day in London 6% of the time. We've had snow on the ground at other times (like three years ago) but for the Met Office to record it as snowing at least one flake must fall on the 25th. The last time snow was recorded on Christmas Day in London was back in 2001.

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It used to be that one unfortunate member of staff from the Met Office would have to physically go on the roof at the first hint of snow and keep look out but now technology has replaced humans (in London at least).

Where the snow is monitored has also changed. Since the Met Office moved to Exeter in 2006 their official statistics for London are now recorded at Heathrow Airport. But for the sake of the bookies and us weathermen the unofficial site is now Buckingham Palace.

So history and the odds are stacked against it being a white Christmas in London. But the weather (and the forecast) can change a lot in 30 days.