November - December 2010
 
After the cold winter of 2009 - 2010 in the UK, which brought very cold weather and disruptive snowfalls to the whole of the country, further extremely cold weather made itself felt very early in the season, late in 2010. This cold spell was significant in that it borught freezing conditions and snow in late autumn; the snowfalls were judged to be the most significant and widespread in late November and early December since late November 1965. The conditions resulted in an exceptionally cold December for much of the country: it was the coldest December since 1890 in the Central England Temperature dataset, with a CET of -0.7°C, and the coldest month in the UK since December 1986. 
 
The potential for below-average temperatures were initially forecast by the Met Office at the beginning of November, several weeks before the event occurred. From the 20th November onwards, high pressure over the Arctic Ocean began to ridge into the north Atlantic, ultimately leading to strong north easterly and easterly winds across the UK for the rest of November. Similar conditions prevailed for the beginning of December, with high pressure generally dominant close to the UK for much of the month. 

 
 
 
 
Surface pressure analysis for 0000 on 24th November 2010 and for 0000 on 1st December 2010. Note the large blocking high pressure over the north Atlantic and north-west Europe during the 1st December. 
Credit: Met Office. 

The above two surface pressure analyses indicate the typical synoptic setup for the UK and northern Europe for late November and early December 2010. The easterly winds brought very cold air from the European continent (in some cases originating from western Siberia) to the UK, leading to temperatures well below average, severe overnight frosts and significant snowfall. The blocking high pressure dominated the north Atlantic for the entirety of December, allowing cold air to pool in regions that would normally be much milder. This forced the jet stream to remain well south of the UK, meaning that Atlantic cyclones tracked much further south than would normally be expected. The extremely cold air helped the areas of high pressure to become self-sustaining, feeding in ever colder air from the north and east - the high pressure was very stubborn to move from this position during December. 

When the high pressure did allow an Atlantic intrustion of fronts from the west, they generally produced snowfall over the UK, since the air sitting here was so cold. Such intrusions only lasted for a day or so, before the cold air returned - allowing December to, ironically, be one of the sunniest and driest on record in the UK! Due to the sunlight being very weak at this time of the year, any calm nights allowed the temperatures to plummet - especially in areas with lying snow. 


 
 
 
 
Overnight minimum temperatures for the night from 2nd - 3rd December 2010. 
Credit: Met Office. 
Recorded snow depths at 0900 UTC on 2nd December 2010.
Credit: Met Office. 

From the 24th November - 9th December, perstent north-easterly or easterly winds brought heavy snow to Scotland and north-east England. The snow accumulated without melting during this cold spell, and reached depths of over 2 feet over higher ground by the end of the spell. The persistent easterly wind produced snow lines of snow showers as it passed over the much warmer north sea (analogous to                                in the USA), that led to snow accumulation of 1-2 feet over the North and South Downs in south-east England. Temperatures struggled to rise above freezing, with night-time minima dipping into minus double digits even into southern parts of the UK (above right). A new Welsh minimum temperature record for November was set on the 28th November, at -18.0 °C. 

Slightly milder conditions were present during the middle of December (enough to partially melt any lying snow), before a bittlerly cold arctic blast of northerly winds gripped the country. This brought snow showers to most of the country, along with a freezing wind chill. A couple of occluded fronts associated with a large upper low produced significant snowfall over southern counties on the 19th and 20th December. 


 
 
 
 
lake-effect snow
Daytime maximum temperatures recorded on 20th December 2010.
Credit: Met Office.
Visible satellite image of lying snow over the UK, on 24th December 2010. 
Credit: Dundee Satellite Receiving Station. 
The heavy snowfalls led to a Christmas with lying snow on the ground for much of the country, after which there was a gradual thaw of the snow, before temperatures slowly climbed in January. The lowest minimum temperatures for England and Wales were recorded on the 3rd December, when winds lightened after bitterly cold Arctic winds swept through; -20.0°C was recorded at Topcliffe in North Yorkshire, while Altnaharra in the Scottish highlands recorded -21.3°C. A new December minimum temperature record for Northern Ireland was set at -18.7°C on Christmas Eve. The lowest maxima recorded was also at Altnaharra, at -13.8°C on the 21st December. 

Heavy snowfalls during the end of November caused road clousures in Northern England and Scotland, as well as school closures and strain on the local infrastructure. The extreme cold also caused issues with water mains, many of which froze solid - casuing them to burst, meaning that thousands of homes were left without water. Heavy snowfall in the run-up to Christmas also led to the cancellation hundreds of flights at Heathrow and Gatwick airports, along with general travel chaos in southern England - especially on the 20th December, when the AA reported its busiest day ever for car breakdowns. 
December Central England temperatures from 1659 - 2010. 
From: http://ryviewpoint.blogspot.co.uk/2011/02/long-running-temperature-data-set-and.html

Comparison with other winters

It is important to note that our perception of a British winter is governed by those in our recent memory. In the run up to December 2010, there had been a sucession of mild Decembers (and mild winters) from the 1980s. Therefore, few people could recall such persistent cold weather - the mean winter temperature for the UK is around 4°C. Although December 2010 was very cold, it is not an unknown extreme when compared with other Decembers on the Central England Temperature record. The coldest December occurred in 1890, at -0.8°C, and several other bitterly cold Decembers have also been recorded - although such extreme cold is arguably now becoming less common.