Snow is synonymous with Christmas. Advent calendars and festive cards often depict snow-filled scenes, and of course, the great Bing Crosby dreamt of it. But what is the science behind a white Christmas? Dr Richard Pope, Reader in Climate Change at the University of Derby explains all.
What is a white Christmas?
According to the Met Office a white Christmas occurs when a snow flake is observed falling at a single location – traditionally the Met Office building in London within 24-hours of 25 December. However, with betting on the likelihood of seeing a white Christmas becoming increasingly popular, a single snow flake falling at Edinburgh Castle, the Millennium Stadium in Cardiff, or even Coronation Street in Manchester will guarantee a white Christmas.
Of course, this doesn’t necessarily mean a white Christmas is fast becoming the norm. Met office records show that for significant parts of the UK, the Christmas period falls just outside the main period of snow (typically January to March). Nevertheless, a single snowflake falling somewhere in the UK on 25 December has occurred 38 times since 1963, while a Dickensian-type white Christmas (characterised by significant snow on the ground reported by weather stations at 09:00 am) has occurred four times since 1966 with significant Christmas snowfall recorded across the UK as recently as 2010.
What weather conditions guarantee a white Christmas?
Recent weather reports suggest that the likelihood of snow around Christmas time is closely linked with strong La Niña conditions and reduced sea surface temperatures in the Eastern Central Pacific Ocean and, closer to home, weather patterns linked to the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO).
The NAO is a measure of the seasonal average air pressure difference between stations located in Lisbon and Reykjavík, while the AO is an index of the dominant pattern of non-seasonal sea-level pressure variations north of latitude 20°N. A combination of La Niña conditions and strongly negative NAO and AO phases can increase the likelihood of snow through their influence upon narrow bands of fast moving air within the tropopause termed the Jet stream.
The high altitude polar jet stream adopts a distinctly meandering flow pattern forming a series of Rossby waves as it moves eastwards across the Northern Atlantic towards the UK and northern Europe. Superimposed on these waves are smaller scale waves that move along through the flow pattern around large scale ridges and troughs within Rossby waves (Fig. 1).
During negative phases of the NAO and AO, the meandering behaviour of the polar jet stream becomes increasingly erratic with accentuated northerly and southerly swings linked to the weakening and subsequent displacement of the polar vortex and changes in surface pressure over the polar region (O’Hare, Sweeney and Wilby, 2005).
The subsequent ‘buckling’ of the jet stream and repositioning of high pressure and low pressure zones along the polar front plays a crucial role in determining whether snowfall occurs across the UK. When a persistent and extensive ridge of high pressure or so-called ‘blocking system’ develops over central Greenland, cold air that was previously ‘locked in’ the polar region feeds into a dominantly north to north-easterly flow that brings very cold and dry Polar maritime air into contact with comparatively warmer, moist air over lower latitude areas, including the United Kingdom (Fig. 2).
What makes a snowflake?
For Polar maritime air to produce snow over the UK atmospheric dust, ice particles, and supercooled water droplets are essential. The latter are absorbed by the ice particles, which grow into larger ice crystals at the expense of any surrounding water droplets.
Once formed , the ice crystals (now some several hundred mm to several mm in size) fall through the atmosphere due to their mass, often colliding or sticking together to form aggregates known as snowflakes (Barry and Chorley, 2010).
Predicting where these snowflakes are likely to fall within the UK is no longer rocket science! Data form the Met Office data show a fairly consistent pattern in respect to north to north-easterly air flows and snowfall pattern with more than 50 cm of the white stuff lying on the ground on 2 December 2010 in significant parts of Scotland and Wales, and upland areas of northern England.
Whilst parts of Surrey and Sussex recorded up to 30 cm of lying snow on the same day, this might be considered a rarity, and for significant parts of southeast and southern England a Dickensian-type white Christmas might perhaps be little more than a fairy tale given the changes occurring in the atmospheric system?