It is known as the North Atlantic Oscillation, and when it occurs it disrupts the normal winter pattern of low pressure over Iceland and high pressure over the Azores, which brings a steady stream of warm westerly winds from the Gulf Stream over the British Isles.
The NAO replaces this with a high between Iceland and Scotland and a low near the Azores, and this leads to Britain being blasted by freezing winds from Siberia and Scandinavia and the whole snowbound central European land mass.
Over the last century the NAO has had fluctuating timescales lasting from several years to a few decades. The westerly winds form was prominent from 1988 to 1995, which was why winters until last year had been so mild.
But now the change has set in, and it was signalled two months ago by a small arctic bird, the snow bunting, which began arriving in Britain in large numbers. Scientists have found a link between the numbers of snow buntings heading south and the NAO. Between 1988 and 1994 their arrival on Fair Isle, Shetland, averaged 40 a day, but in November they were descending on Fair Isle at a rate of 300 a day.
No one knows exactly why the North Atlantic Oscillation happens. It does involve the ocean, and the atmosphere and climate expert William Burroughs sees the Atlantic playing a major part in prolonging runs of good or bad winters: "The ocean acts like a huge fly wheel, so once the pressure systems have changed they hang on in there for some time."
Does this cold snap put the nail in the coffin of global warming? Even with the climate warming up we can still expect runs of cold winters in the future - it's the average world temperatures over decades and centuries that counts. But the North Atlantic Oscillation does show that we still have not taken into account all the natural variability of the climate, and until we do it is still very difficult to detect how much of the global climate change is man made.
In fact, between 1940 and 1970 all the talk was of an approaching Ice Age as world temperatures dipped dramatically. One of the worst winters struck almost exactly 50 years ago, in one of the most punishing cold spells in recent history. In early 1947 Britain's post-war economy was already devastated by shortages of food and fuel. Then the cold bit deep on 23 January with heavy snowfalls which carried on into March and the list of miseries piled up. Coal stocks ran so low that power rationing was started with domestic supplies cut off for up to five hours a day. The economy nose-dived, production fell by 25 per cent, exports fell by pounds 200 million, unemployment doubled and the Labour government was forced to make savage cuts in spending, leading to devaluation of sterling.
But if it's any consolation, things were even worse in the past. From around 1550 to the early 1800s Europe was decimated by a bitterly cold era called the Little Ice Age, when frost fairs were held on the frozen Thames, and glaciers grew larger in the Alps and Scandinavia. Some winters were so cold that people walked out to ships trapped in ice in the Firth of Forth.
In 1632 Sweden invaded Germany by marching across the frozen Baltic Sea during the Thirty Years War. In France, a succession of ruined harvests from the bad weather led to food riots which culminated in the French revolution of 1789.
From weather records of the time, the cause of the Little Ice Age can be pieced together and a familiar culprit emerges - persistent blocking high pressure systems over Scandinavia which swept Britain with cold air.Reuse content