So how unusual is this cold spell? The TV news people tell us that it's the longest spell of freezing weather for almost thirty years, since the winter of 1981-82, presumably quoting a Met Office spokesperson. A spokesman for the Local Authorities Association yesterday described it as unprecedented. But that does not quite stand up to close analysis.
In the southern half of the UK, up to and including much of Lancashire and Yorkshire, the mid-December cold snap relaxed its grip during Christmas week, most of the snow at low levels disappeared, and the temperature climbed to 8°C in north Wales and Lancashire, and as high as 11°C in the West Country. Thus, here, we have had a week of wintry weather before Christmas, and ten days of frost and snow this month. With the proviso that the cold weather is not finished with us yet, that is less impressive even than last winter when southern Britain had 16 freezing days in December/January and a further 14 in early-February. For the 85 per cent of the population that lives in the southern half of the country, then, it has - so far - been the longest cold spell for precisely one year, not for thirty.
In the northern half of the UK the situation is very different. Here, there was no relaxation of the cold weather over the Christmas holiday. Rather, it tightened its grip between Christmas and the new year, and today is the 24th consecutive day with snow on the ground and sub-zero temperatures on the thermometer. This is now longer than the three-week freeze-ups endured in the winters of 1996-7, 1995-6, and 1990-1, and we have to go back to the mid-1980s (though not as far as 1981-82) to find a longer period of wintry weather. During the winter of 1985-86, there were several snowy episodes in Scotland in December and early-January, but the really cold weather set in during the last week of January and lasted until the first week of March - six weeks with a continuous snow-cover, penetrating overnight frosts, and daytime temperatures struggling to reach zero. Quite why the Met Office spokesperson forgot about February 1986 is a mystery, for, averaged nationally, it was considerably colder than either January 1982 or December 1981. Thus for the 15 per cent of the population that lives in the northern half of the UK, it is already the longest cold spell for 24 years.
One thing we can all agree on is how much snow has fallen in all parts of the kingdom during the last four weeks. True, there has been appreciably less than last February in and around London, in parts of the Midlands, and in inland parts of the West Country, but most other regions have had rather more than last winter and in uplands parts of east and northeast Scotland, Northumberland and Durham, level snow is 40-60cm deep, with drifts to three metres - the deepest snow in these areas since early-2001, and in a few well-scattered locations for even longer. A stunning satellite image from Thursday lunchtime, taking advantage of extensive breaks in the cloud-cover, revealed very nearly the entire United Kingdom blanketed with snow. The only areas showing up as largely snow-free were low-lying parts of the Inner Hebrides, the coastal fringes of Kintyre and Galloway, the west coast of Cumbria, the narrow lowland strip of north Wales between the mountains and the sea, scattered districts around Swansea, Plymouth and Bournemouth, and the Isles of Scilly. It is rare to find virtually the whole of the UK snow-covered as different mechanisms are required to produce heavy snow in different parts of the country. The last occasion, from memory, was in February 1991.
The questions I am asked most frequently are: "Why has the cold come back with such a vengeance last winter and this after such a long run of mild winters?" And "Surely this means that global warming has ended?"
In middle latitudes of the northern hemisphere our winter weather is controlled by the jet stream. This is a powerful conveyor-belt of winds in the upper atmosphere which encircles the hemisphere, and which drives the weather systems which deliver our day-to-day weather. In some years the jet stream is powerful and blows hard and true from west to east round the entire hemisphere. Here in western Europe such a winter will be characterised by an endless succession of big Atlantic depressions, delivering frequent rain and strong winds, while temperatures remain resolutely above average, and snow, if it occurs at all, is slight and fleeting. In other years the jet stream is relatively weak, and like a sluggish river it will meander widely on its travels. It still flows essentially from west to east, but it will meander first deep into the sub-tropics, then into polar latitudes, and then back to the sub-tropics. This winter, here in the British Isles, we find ourselves on the cold side of one of those meanders, and that means that the Atlantic weather systems are tracking across Portugal and Spain and into the Mediterranean, rather than heading directly for us. This in turn leaves the back door open for northerly or easterly winds to blow straight out of the Arctic or from northern Russia, bringing severe frosts and heavy snow with them.
This also explains why there are news stories about extreme cold and disruptive snowstorms in eastern China and Korea, and along the Atlantic seaboard of the USA and Canada, because those areas also find themselves on the cold side of the jet-stream as it wanders far to the south. Between these cold zones, though, are abnormally warm sectors, including Greece, the Balkans, Turkey and southern Russia, and also the Pacific seaboard of North America. In Crete on New Year's Day the temperature reached 30°C (86F), a new all-time record for Europe in January, while Vancouver, Seattle and Portland have been experiencing one of those winters of relentless rain and wind with temperatures stuck around the 10°C mark.
It is also clear, looking back through the archives, that the strong-jetstream winters and the weak-jetstream winters tend to cluster in groups. The result is that we will have a series of mild and stormy winters, followed by a grouping of three or four cold and snowy ones, and then we are back to the mild and rainy regime. We can easily identify the cold clusters in the last half century - 1962-65, 1968-70, 1978-82, 1985-87, and 1994-97. What we don't know is whether the cluster we are in now is simply 2009-10, or maybe 2009-12.
The question about climate change is an important one, because it emphasises how much confusion there is about the difference between weather and climate. Weather is caused by disturbances within the atmosphere, mostly in the lowest 10-15km of it, but climate is controlled by external influences, such as latitude, the distribution of oceans and continents, the position of mountain ranges, the distribution of ice and snow-cover, variations in the amount of energy emitted by the sun, changes in the chemical composition of the atmosphere ,and changes in land-use. Climate is the underlying average, or if the climate is changing it is the underlying trend, whereas weather is the noise in the system. We have always had huge day-to-day and year-to-year variations in our weather, and we always will do, and a single cold winter is no more evidence that climate change has stopped than a single summer heat-wave proves that global warming is happening.
Everyone, of course, wants to know how long this freeze-up is going to last. There is certainly no sign of a major change within the next week, although the cold may once again relax its grip slightly in western and southern districts at times, and by next weekend it may well have turned milder across the whole of the UK.
However, it is interesting to note that when deep cold air has become established across western Europe well before Christmas, it does not normally last beyond the third or fourth week of January. The historic winters of 1963 and 1947 began on December 22 and January 21 respectively, whereas in 1981-82 the very cold and snowy weather set in on December 7, but cleared away on January 17, never to return. The last time a truly severe winter set in before the middle of December and lasted throughout the following January and February was in 1683-84 - the Great Winter of "Lorna Doone". If you are an optimist that should suggest that this particular big freeze will be all over before the end of the month. If you are a pessimist you will undoubtedly be thinking that we are long overdue another winter like 1683-84.
