When, 1080-odd years ago, King Wenceslas looked out, the scene that greeted his eyes was hardly unusual for a late-December morning in Bohemia.
The countries of central and eastern Europe are used to much colder and snowier winters than we have in the UK, and Bohemia - now comprising the greater part of the Czech Republic - will have several weeks of snow cover during a typical season. In Prague, for instance, the ground is snow-covered for over fifty days a year on average, including roughly fifteen days in December. According to the eminent climate historian, the late Professor Hubert Lamb, tenth century central Europe was one to two degrees colder than it is now so Wenceslas may well have seen snow more frequently than does, say, President Vaclav Klaus.
The probability of Prague Castle being snowbound on the Feast of Stephen is approximately fifteen times higher than a white Boxing Day at Buckingham Palace, four times higher than one at Sandringham, and only fractionally higher than one at Balmoral. Although we usually associate snow with Christmas in the UK, it is in fact one of the rarer meteorological elements to visit lowland Britain in December. The Christmas card snow scenes are more wishful thinking than a reflection of reality. In many years December is much more a late-autumn month rather than the first month of winter as climatologists regard it, and quite often the month passes by without a flake of snow to be seen, and occasionally we even have a December which is completely frost-free.
When the pattern of high and low pressure systems over Europe and the Atlantic is just right, a southwesterly airstream can deliver to us weather which has originated in sub-tropical latitudes, somewhere between Bermuda and the Azores. The ocean surface there is still very warm even this late in the year, and although such weather when it reaches us is usually grey and damp and windy it can also be remarkably mild. Districts to the lee of high ground may also benefit from a föhn effect on these occasions. This allows the clouds to break, the sun to shine, and the temperature to climb to exceptional levels. On just such a day, December 2, 1948, the mercury peaked at 18.3°C at Achnashellach in Wester Ross. Almost as warm was December 18, 1972, when 18.0°C was recorded at Aber, located between Bangor and Llandudno on the north Caernarvonshire coast.
Much more rarely does a really severe December occur: indeed the last one was in 1890. Although a good case could also be made for including 1981 in this category, the fact is that we have had three colder Januarys and five colder Februarys than December 1981 during the present century. The lowest December temperature of all in Britain was -27.2°C, equalling the UK record for any month, and this happened on December 29, 1995, at Altnaharra in Sutherland.