This selection of surface weather charts (for midnight on December 16th, 18th and 20th) shows how deep, cold air from the Arctic swept south across the British Isles then stagnated over the country, only clearing the south after Christmas.
The term ‘deep’ describes the character of the airmass which, as well as being cold, was also very unstable. This led to the snowfall being showery in nature and, in South Devon, it was accompanied by thunder. Some of the snow showers that affected the northeast in November had also been thundery.
As can be seen, atmospheric pressure remained low and, apart from early blizzards over the north, winds were generally light.
This pattern of development differed significantly from the two severe winters that are often quoted in comparison – 1946/47 and 1962/63. On both these occasions, the cold air came from Continental Europe, squeezed between high pressure to the north and low pressure in the south. The airmass was stable, hence the snow was not showery. It came from relatively warm, moist air that had originated further south and was falling along a broad, active frontal zone. The cold continental air was near the surface and this ensured that snow fell rather than rain.
The difference in pressure between the northern high and the southern low led to strong winds and, most notably in 1947, blizzard conditions over much of England and Wales.
by Nick Ricketts (former forecaster at the Met Office)