Tropical warmth in the Uk in December...

As it been so cold recently I thought it might be worth taking a look at Decembers' when we have marvelled not at the cold and snow but at the warmth. They do occur regularly and although the obvious solution to many of those wanting a warm Christmas is to jet off somewhere well south (very well south in fact!..) sometimes in the right set ups it can almost feel warm in the Uk in December. Three main synoptic situations can lead to this.

The first is when very mild tropical air reaches our shores from the Azores in a strong SW'ly regime. It will usually be very moist though so cloudy with often copious rainfall too. Sometimes though if cloud breaks up enough to the east of high ground it can feel pleasantly warm in sunshine. The mean daily temperatures occasionally may reach as high as 12C across England in such a set up.

The second is when a light northerly flow descends in the south along the South Downs, if sunny and well shelteted from the wind,south coast resorts especially in areas against reflective white chalk cliffs can feel pleasantly warm.. usually but not always with a coat on.

Lastly and generally when we see the highest December temperatures is when a foehn effect occurs; that is when tropical maritime air descends down the side of a large mountain ridge or hill range and both dries out and warms up notably as it does so. There are certain places that capture this every so often, the north coast of Wales being a good example in a S'ly or SW'ly regime.

December 1931 was pleasant for the winter period as it was actually a very dry month and it set the equal record highest maximum for Christmas Eve with 15.6C at locations in Scotland. December 1936 was also pleasant as it was a very sunny month, with some mild periods when the cloud of tropical SW'lies broke up to give some relatively warm sunshine especially pre Christmas; between the 17th and 21st temperatures reached 15C in places.

However, the record December temperature of 18.3C was recorded as far north as Achnashellach (Wester Ross, Scotland) on the 2nd December 1948, as a result of an unusually warm southerly airmass and was due to the foehn effect. That same day high temperatures were also recorded at other places in the lee of hills in the southerly flow; such as 17.7C at Aber and Llandudno in North Wales. (However on the same day, nearly 100mm of rain fell in South Wales!) In a similar set up Colwyn Bay recorded 15.6C on the 30th December 2004.

The period from 1985-1988 saw a run of mild Decembers though. In 1985 there was some late exceptional warmth in the first three days of the month. The foehn effect enabled a reading of 17.7C at Chivenor (North Devon) and 17.2C at Bude (Cornwall). Most mild Decembers are wet as well though and 1986 was no exception. In fact it was the wettest December of the century in Scotland. 1987 was again mild but saw 16C at Minehead on the 17th, and -more remarkably- 18C at Cape Wrath in the far north of Scotland on the 18th again due to the foehn effect. If I had to pick just one reading I would say this was perhaps the most remarkably warm temperature ever recorded in December. 1988 was again exceptionally mild overall , at 7.5C the second warmest in the 20th century. It was very mild on both Christmas Eve (with 14.9C at Torquay) and on Christmas Day (14C). I well remember this as my family in Cornwall went out to the Lizard and spent an enjoyable afternoon walking off the Christmas dinner at Kynance Cove, it was remarkably sunny and very pleasant in just a warm t-shirt.

Whether December 2010 gives us any such temperatures remains to be seen as I write this, though the omens are not especially promising. However if you don't fancy your chances here (and you are probably wise not too) then you may be joining the one million Britons who jet off to warmer climes for the Festive period, in fact the record year over Xmas 2002 saw as many as 1.5 million people leave these shores. Personally? Well, I've always been a bit of sucker for a 'traditional' white Christmas I must admit, and I suspect I'm not entirely alone!