Where can you find a White Christmas?
Weather forecasters get asked every year whether there might be a White Christmas – snowy weather.
The idea that Christmas should feature snow may be relatively recent – many point at the sentimental song “White Christmas” from the musical “Holiday Inn” as the origin. Composer Irving Berlin won an Oscar for the song in 1943. But there are also longer-term climate patterns that may have played a role.
Some of the associations between Christmas and snow may have been forged by the “little Ice Age”, a period from the 16th to the 19th centuries when temperatures were typically lower than normal. Christmas-themed art and literature created particularly in the 1800s made snow a regular feature of Christmas.
Now, in the twenty-first century, there are varying definitions of a “White Christmas”. The United States National Weather Service requires one inch (2.54cm) of snow to be on the ground for it to declare a White Christmas. The NWS does not require snow to fall on the day. The United Kingdom Met Office uses a different definition – there need not be any snow lying in the UK, but a trained weather observer must see at least one flake of snow hit the ground.
During December, the different weather services in the northern hemisphere have been looking ahead at their chances of snow.
The US National Oceanic and Atmospheric Administration posted a map of the probabilities of snow, based on climate data records from 1981-2010.
You can easily pick out the big mountain ranges where snow is most likely. There is also a clear pattern, as you might expect, of declining probability of snow as you move southwards. But several of the southern states have already had snow this winter, during a severe cold snap in November, so historical climate information can only tell some of the story.
Computer models have improved weather prediction so that now we can have reliable forecasts about five days in advance. Satellite data, along with surface observations, and measurements taken by balloon, help us to understand the conditions in the atmosphere, and the models calculate what is likely to happen next.
Models can run predictions for up to two weeks in advance, but these forecasts are not as reliable as short-term predictions. However, repeated model runs – especially if they are consistent in their results – can give an idea about what kind of patterns will be in place, allowing for some assessment of whether the weather might be warmer or colder than usual.
General predictions can be made depending on what kind of airmass is dominating over Europe.
The specific weather for where you live will depend on lots of smaller factors – how far north you live, whether you live high above sea level, whether you live close to the sea or large lakes. Each of EUMETSAT’s member states has its own meteorological service, or weather service, that uses the available data about the atmosphere and different combinations of computer models to make predictions.
As far back as December 15, the German meteorological service, DWD, said it did not expect a weather pattern conducive to snow at Christmas, with the exception of areas in the mountains.
The Austrian weather service said statistically that its provincial capitals could expect a white Christmas only once every three to five years. They have had some very snowy holidays in the past though – Innsbruck recorded almost one metre of snow on 24 December 1961. ZAMG has calculated percentage chance of snow on the ground on 24 December based on averages since 1991. The only weather stations with 100% chance are those over 1000m above sea level.
The mountainous areas of Norway typically have no shortage of snow, but that does not follow for western and southern Norway. A map published by the national meteorological services news website shows that the mountains have had snow at Christmas for all of the past 30 years, but in the town of Bergen there was snow only in 2009 and 2010.
New model runs suggested Finland might be getting snow further to the south than originally thought (via Google Translate: “The latest ECMWF monthly forecast, the white Christmas opportunity to grow also in the south. http://bit.ly/J5WwrR # winter , # Christmas , # Snow”
— Ilmatieteen laitos (@meteorologit) December 12, 2014
The Estonian weather service held a special competition to predict not only whether there would be snow for Christmas Eve, but also the exact temperature recorded at noon at the Tartu weather station.
The UK Met Office, with its broad definition of a White Christmas, says that 38 of the last 54 years have qualified. As you might expect, snow at Christmas is more common in northern parts of the UK, but according to statistics England, Scotland, Wales and Northern Ireland get most of their snow between January and March, and not in December. The latest blog post from the Met Office highlights how changeable December has been.
Ireland is also expecting relatively mild weather in the run up to Christmas, but there could be colder weather on the way after that.
Of course, some parts of Europe are much too warm to expect snow.
And for those making long journeys to visit friends and family over the winter holidays, mild and dull weather can be a whole lot easier.
Whatever the weather, meteorologists around Europe will be working over the holidays to let us know what might happen next, as they do all year around.