With December being the first meteorological month of winter, expectations are that it may be cold and perhaps wintry at some point in the month, even some snow in places, but equally there will be some milder spells too. The odds are stacked against a white Christmas in the UK though, more so in the south. The odds of a white Christmas are about 1/10 in the south of England to 1/4 in the north. More often than not, December is often mild, wet and windy. This is thanks to the polar vortex beginning to strengthen, as the upper polar air cools with long darkness there, bringing strong westerly jet winds aloft that drive low pressure systems across the Atlantic which bring mild air to the UK. December 2015 and 2013 were exceptionally mild months, 2015 the warmest December since records began, and we have to go back to 2010 for the last below average December in the UK, which was the coldest since 1890. Since the early 80s, milder than average Decembers have outweighed colder than average ones against the rolling 30 year averages of those years.
Courtesy of the Met Office
Following a below average November this year temperature-wise and the cold and frosty first 5 days of December, the weather has reverted to that familiar pattern of a mild and changeable southwesterly flow driven by Altantic low pressure systems moving northeast to the north of Britain.
The mercury climbed to 17.0C at Harwarden in North Wales on Wednesday. Last December saw 17.2C recorded at Achnagart in the Highlands on the 16th. December 2013 saw a maximum temperature of 17.0C at Llanfairfechan on the coast of North Wales. The highest all-time maximum for December stands at 18.3C recorded at Achnashellac, Wester Ross in the western Highlands.
One common feature of these weather stations that have recorded the high temperatures is that they are not in the south and, more importantly, their locations are in the lee of high ground. The sun is too low in the sky in December to directly create temperatures of 17C+ under normal conditions, so the warmth needs to be imported from way south. But that is only part of the story, because even when there is a strong southwesterly wind blowing bringing air of subtropical origin from latitudes around the Azores but sometimes even as far south as Bermuda, temperatures would only typically reach 13-15C across the south and west. However, the coast of north Wales has over the years seen weather stations record abnormally high temperatures of 17C or higher when there is such a strong southwesterly flow sourced from the sub-tropical regions of the North Atlantic. This warmth in north Wales is down to a mechanism known as the Föhn Effect. The south to southwesterly winds are usually initially very moist when they arrive across Wales. But as the moist air is forced upwards over the Welsh hills, depositing large amounts of rain there, the descending air over the northern flank of the Cambrian mountains has lost much of the moisture Descending air is warmed by compression, and this warming is enhanced if the air is dry. The Fohn effect is not unique to north Wales in terms of the UK as whole though, it also occurs to the lee of the Highlands and Grampians in Scotland, the lee of the Pennines across NE England and the lee of the Moors of SW England.
The Foehn Effect explained - image courtesy of the Met Office
Over the decades December is probably the coldest month of the year only about 20% of the time and in more recent years, temperature-wise it’s been more like a continuation of autumn than a winter month. However, as recent as 2010, December that year was well below average temperature wise and was the coldest since 1890 and the coldest of any winter month since 1986. The lowest minimum of that month was -21.1C at Altnaharra on the 1st. The lowest maximum of the month, -15.8C, was also recorded at Altnaharra, on the 22nd.
December 2010 was well below average for temperatures across the UK, the coldet since 1890.
5 years later, December 2015 was the warmest on record!
The lowest minimum UK temperature since 1900 was recorded in the month of December and we only have to go back 21 years for that, when -27.2C was recorded at Altnaharra in the Highlands on 30 Dec 1995. *Click here* for article from Weather on the extreme cold in Scotland in Dec 1995. The lowest minimum temperature for England in December stands at -25.1C recorded at Shawbury (Shropshire) on 13th Dec 1981. These places are infamous frost hollows, where they lie in valley bottoms where cold air sinks down from higher ground because cold air is dense and heavier than warmer air. Frost hollows are widespread in the Welsh Borders and Scottish Glens – hence why we have seen the lowest temperatures recorded here.
Frost hollows occur in deep valleys where cold dense air sinks to the valley floor and creates the lowest temperatures recorded in the UK.
Top ten coldest temperatures recorded in the UK, mid December and through January generally has brought the coldest temperatures. Image courtesy of Met Office.
December, on average, is not as cold as January or February – because even though it has the shortest day/longest night of the year, there is lag time from the effect of these short days on airmasses and also the seas surrounding the UK don’t reach their coldest point until late winter into early March. Certainly the last 30 odd years have seen mild Decembers outnumber cold ones. Though the UK’s coldest minimum temperature occurred in late December, thanks to the long nights. Though only a year ago we lived through the warmest December on record, the month had a UK temperature anomaly of +4.1C of which was easily the warmest December for both the UK and the CET series since 1659. Temperatures were closer to those normally experienced in April or May
Atlantic in charge for now, bringing mild and changeable conditions, but signs from model ensembles that blocking may re-establish close to the east later next week, bringing colder and drier conditions:
With regards to the rest of this month, there are no signs yet of any particularly cold and wintry weather on the horizon. Despite the signal back in late November, for high latitude blocking high pressure, this hasn’t occurred as of yet. Instead we’ve seen high pressure has slowly retreated into Europe over the past week, while the jet stream has cranked up several gears across the Atlantic, as an intense pool of deep cold air has been dropping south across Canada into northern U.S. states, steepening the north-south temperature gradient and this strengthening the upper winds. So we have seen a milder southwesterly flow taking over since mid-week and temperatures are likely to stay above average with a lack of frost well into next week as westerly or southwesterly winds continue to blow, bringing spells of rain east at times too. Though there are now hints, especially from model ensembles and increasingly high-res runs, that high pressure will build and expand to the east late next week to allow a colder and drier continental southeasterly flow which may allow frosty nights to return to southern and eastern areas at least. There is some uncertainty how long this high will influence the weather, as some ensembles expand it across the UK keeping Atlantic systems at bay – which would suggest a blocking high for the UK and a chillier but dry outlook with greater chance of frost as we head toward Christmas. But equally there is still some support for a more progressive pattern returning of Atlantic low pressure systems affecting the UK weather bringing westerly winds and spells of rain and perhaps hill snow in the colder polar maritime interludes.