by Jon Shonk
With Christmas lights sparkling in cities all around the UK, department stores festooned with lights and decorations and Christmas songs starting to be played on the radio, it is clear that the festive season is just around the corner. But will it snow…?
The last widespread white Christmas in the UK was in 2010, when over 80% of the country woke up to snow on the ground, and about 20% experienced falling snow. The official definition of a white Christmas according to the Met Office, however, is based on the occurrence of falling snow rather than lying snow – to be precise, a single flake of snow falling on 25 December anywhere in the UK. Hence, 2015 was actually our most recent white Christmas – snow fell over about 10% of the country, but it did not settle on the ground anywhere.
Snow is not an uncommon wintertime feature in the UK. The average number of days of snowfall in the UK ranges from about 10 in the south of England to over 60 in the Scottish Highlands. However, snow days tend to happen later in the winter – on average, there are fewer snow days in December in the UK than in any of January, February or March.
Figure 1: Snow at the Atmospheric Observatory at the University of Reading, although not at Christmas – on 18 January 2013. Photo © Jon Shonk.
For snowflakes to reach the ground, we need the temperature of the air to be cold (less than 2 °C) all the way to the surface. In the UK, air that cold typically comes either from the north in the Arctic, or the east from Siberia. If they are to land and settle, we also need the ground to be cold so that they do not immediately melt.
Forecasting snow, however, is a challenge: small forecast errors in the atmosphere could affect whether precipitation falls as rain or snow (or indeed sleet). The chaotic nature of the atmosphere means that any errors creeping into the forecast will grow with time and, the longer the range of the forecast, the greater the uncertainty in the prediction. This makes reliable snowfall forecasts for individual days impossible beyond about five days ahead.
However, at longer ranges, we can still make useful forecasts based on longer-term trends in the weather. For example, forecasts can show whether the weather is likely to be dominated by settled, high-pressure conditions, or unsettled low-pressure conditions, or indeed whether it is likely to be warmer or colder than average. At a month ahead, we cannot predict weather conditions for a single day, but we can begin to get an idea of what the overall weather pattern might be like.
Figure 2: Rainfall equivalent at the University of Reading Atmospheric Observatory on Christmas Day since 1980, with snow days highlighted with pale blue lines. Despite several recent years being declared white Christmases on a national scale, the previous white Christmas in Reading was in 1999. There was snow on the ground in 2010, but it fell on preceding days. Note also that the snowfall on 1996 must have been very light and short-lived with zero rainfall recorded in the rain gauge.
So what can we expect from the weather this year? Well, this far ahead, unsurprisingly, there is a great deal of uncertainty. Currently, the Met Office is forecasting unsettled conditions in the week of the festive period and slightly warmer-than-average temperatures, which suggest that a white Christmas is less likely. But recent news headlines have also been predicting very cold and snowy conditions over Christmas – on the weight of these forecasts, despite the contradiction with the Met Office, many bookmakers have slashed their odds of a white Christmas.
At this stage it is too early to predict with any reliability whether we will have a white Christmas. The best advice is to keep watching the long-range forecast and see how it evolves as Christmas gets closer. It is only in the last few days running up to Christmas that we will get an idea of the real chances of a white Christmas. In the true spirit of the festive season… here’s hoping.