The UK winter of 2009/2010 compared with severe winters of the last 100 years

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1 The UK winter of 2009/200 compared with severe winters of the last 00 years Weather January 20, Vol. 66, No. John Prior and Mike Kendon Met Office, Exeter When severe British winters of the last 00 years are considered, those of 963 and 947 are usually the first that come to mind. More recent candidates include 979 and 982. Should winter 200 now also be added to this list? To assist with a ranking in terms of temperature (maximum, minimum and mean), we can analyse monthly series from 90 and daily series from 960, based upon 5km grids. These series were assembled using the methods of Perry and Hollis (2005) and have been used to create (i) areal values for the UK, England, Wales, Scotland, Northern Ireland and regions and (ii) colour-shaded maps. The monthly Central England Temperature series from 659 provides a longer-term perspective (Manley, 974). For snowfall, there are monthly 5km grids of days with snow lying from 96 (based upon 0900 UTC station observations), as well as station records of snow depth at 0900 UTC, mostly digitised from 959. Winter 200 overview put under pressure. The adverse impacts included: Travel delays There was frequent disruption to road transport, with higher routes closed, jackknifed lorries and abandoned vehicles. Motorists were occasionally stranded overnight. Rail services were also affected, an example being services through the Highlands in late February. Flights suffered delays, cancellations and diversions, with every major airport affected by snow and ice at least once during the winter. Accidents on icy roads and pavements Many people sustained injuries through slipping on icy surfaces and in traffic accidents, with several fatalities. For example, just before Christmas, rain falling on frozen surfaces caused problems for drivers in counties bordering the English Channel with 40 accidents on the icy A35 in Dorset. Disruption to electricity and water supplies Snow and ice brought down trees and power lines, cutting electricity supplies. For example, on 25 February some homes were affected across central and northern Scotland. Water supplies to thousands of homes and businesses were also interrupted due to frozen/burst pipes. School closures Disruption to transport and power supplies forced the closure of many schools, including several thousand across the UK in early January. Avalanches A number of large-scale avalanches occurred across the Scottish mountains (see photograph on page 4) with several fatalities. Substantial avalanches were also reported from Snowdonia in mid-january and the Lake District in late February. Winter 200 temperature The UK winter mean temperature was.6 C, 2.0 degc below the average. Winter mean temperatures for England (anomaly.9 degc), Wales ( 2.0 degc), Scotland ( 2.3 degc) and Northern Ireland ( 2.3 degc) were all well below average. Overall, the winter was most severe in northern parts of the UK, with the mean minimum temperature in the central Scottish Highlands at least 3 degc below the seasonal average (Figure (a)). Long nights, clear skies, deep lying snow, local Winter 200 was characterised by prolonged cold spells, hard frosts and frequent snowfalls, particularly across the northern half of the UK. After the cold weather arrived in mid-december, milder interludes were short-lived and mainly in the south. Weather patterns were dominated by winds with easterly or northerly components bringing cold air from northern Europe. Pressure was often relatively low over the UK and wintry precipitation resulted. The combination of sustained low temperatures and deep snowfalls resulted in numerous widespread impacts. Everyday life and businesses were affected in most parts of the UK at various times from mid- December until late February, and the emergency services and local authorities were (a) (b) 4 Winter is defined in this article as the period December to February and referred to by the year in which January and February fall. Figure. Winter 200: (a) mean daily minimum and (b) mean daily maximum temperature ( anomaly values).

2 topography and distance from the sea resulted in temperatures regularly falling well below 0 C in the Scottish glens. Minimum temperatures were also at least 2 degc below the seasonal average across most of Wales and Northern Ireland, and they were also well below average in England, albeit to a lesser extent in the east. Maximum temperatures were lowest across a broad swathe of northern and eastern England and Scotland, with the south and west tending to be less cold (Figure (b)). The coldest periods were in the second half of December and the first half of January (Figure 2). Temperatures in the Highlands dropped to 4 C or below each night from 22 to 30 December, with 8.4 C recorded at Braemar (Aberdeenshire) (Figure 3) early on the 29th; the temperature there had risen no higher than 9.7 C the previous afternoon. The coldest spell was in early January, when many places across the high ground of Wales and northern Britain saw temperatures failing to rise above freezing for a week or more. The night of 6/7 January was very cold across England and Wales, with 7.7 C recorded at Benson (Oxfordshire) and 6.0 C at Yeovilton (Somerset) in each case the lowest temperature since January 982 (Table ). Lying snow would have contributed significantly to these very low temperatures, by reflecting solar radiation by day and reducing the upward heat flux from the ground at night. Local topography was also a factor these minima were quite localised with temperatures at neighbouring stations more typically 7 to 0 C. The following night, 7/8 January, was again extremely cold, particularly over Scotland where Altnaharra (Highland) recorded 22.3 C, the lowest UK reading since December 995. In the last twenty years, temperatures in the UK have rarely fallen below 20 C, the only occasions being 2 and 3 March 200 and December 995 at several Scottish stations. However, temperatures below 20 C were recorded during five winters in the 980s. The reading of 3.0 C at Lough Fea (County Londonderry) on 9 January was the lowest temperature in Northern Ireland since March 200, when 4.8 C was recorded at Katesbridge (County Down). The maritime influence tends to prevent very low temperatures in Northern Ireland, with 20 C never having been recorded. The second half of February was again very cold across Scotland, with temperatures in the Highlands dropping below 5 C each night from 8 to 24 February and Braemar recording 9.2 C early on the 23rd. Notably low daytime maxima included 7.0 C at Strabane Carricklee (County Tyrone) on 24 December, 9.7 C at Braemar on 28 December, and 5.6 C and 5.5 C on (a) Mean Temperature ( C) (b) Mean Temperature ( C) /2/2009 0/2/ /2/ /2/2009 5/2/2009 5/2/ /2/ /2/ /2/ /0/200 UK Figure 2. Winter 200 daily mean temperature for (a) the UK; (b) England and Wales, Scotland, and Northern Ireland. 2/0/ average 9/0/200 26/0/200 02/02/200 England and Wales Scotland Northern Ireland Figure 3. Map showing the location of places referred to in this article. 29/2/ /0/200 2/0/200 9/0/200 26/0/200 02/02/200 09/02/200 09/02/200 6/02/200 6/02/200 23/02/200 23/02/200 The UK winter of 2009/200 Weather January 20, Vol. 66, No. 5

3 Table Lowest air temperatures ( C) of winter 200 for each UK region (the boundaries of these regions are shown in Figure 3). Region Name Date Min. Temp. Coldest since Min. Temp. Comments Weather January 20, Vol. 66, No. The UK winter of 2009/200 North Scotland A ltnaharra 8 January 22.3 December C on 30 December 995 is the UK record East Scotland Braemar 9 January 9.8 December December 995 West Scotland Eskdalemuir 7 January 5. February February 986 Northeast England Kielder Castle 8 January 4.2 March and 3 March 200 Northwest England Woodford 7 January 7.6 Coldest on record January 945 North Wales Llanarmon 8 January 4.5 January January 987 Dyffryn Ceiriog South Wales Llysdinam 8 January 6.2 New record in digital November archive: Midlands Benson 7 January 7.7 January January 982 East Anglia Santon 8 January. January January 2002 Downham Southeast and Lacock 8 January.9 January January 987 central southern England Southwest England Yeovilton 7 January 6.0 January January 982 Northern Ireland Lough Fea 9 January 3.0 Coldest in digital archive December 995 The UK record is shared with Braemar, Aberdeenshire on 0 January 982 and February Woodford opened in The record at nearby Ringway is 3.9 C on 25 January Llysdinam opened in 882. Records have been digitised back to The English record of 26. C was set during this spell, at Newport, Shropshire on 0 January Lough Fea opened in 965. Records have been digitised back to and 8 January respectively at Carlisle (Cumbria). Elsewhere across the UK, daytime temperatures also frequently failed to rise above freezing, significantly prolonging the period of lying snow. Winter 200 snowfall Widespread significant snowfalls occurred several times throughout the winter. They first affected eastern England on 7 December and during the next few days spread to most of the UK, particularly northern, central and eastern England, Scotland and Northern Ireland. Further heavy snowfalls affected central and northern Scotland in late December with depths of more than 30cm. From 4 to 6 January, most parts of the UK again experienced heavy snow with falls of 0 to 20cm widely and more than 20cm across central southern England (28cm at Odiham (Hampshire) on the 6th). By 7 January the UK was almost completely covered by lying snow, deep in many northern areas (55cm at Oyne (Aberdeenshire), 57cm at Westgate (County Durham) and 60cm at Blythe (Berwickshire) on the 7th) (see photograph on page 4). Further heavy falls occurred across southwest England and south Wales on 2 and 3 January (28cm at Tredegar (Gwent) on the 2th and 25cm at Holne (Devon) on the 3th). The second half of January and early February saw a general thaw of lying snow across southern areas, but fresh falls of 0 20cm occurred in east Kent and East Sussex on February. Temperatures re mained lower in the north with less thawing of lying snow, and late February saw further accumulations across northern England and Scotland. More than 30cm of fresh snow fell across the high ground of northern Scotland from 24 to 26 February (70cm lying at Balmoral (Aberdeenshire) on the 26th). The Northern Isles also experienced heavy snow, with 28cm lying at Lerwick (Shetland) on the 26th. (a) Snow depth (cm) (b) 70 Snow depth (cm) /2/2009 7/2/2009 Aviemore Balmoral Aberfeldy Strathdearn Kiltarlity 22/2/ /2/ /2/ /2/2009 0/0/200 0/0/200 06/0/200 06/0/200 /0/200 /0/200 6/0/200 6/0/200 2/0/200 2/0/200 26/0/200 26/0/200 Time series of snow depths at stations in the Scottish Highlands, Scottish Borders, and Pennines show deep lying snow persisting from mid-december to mid-january (Figure 4). Snow depths then declined before further very heavy snowfalls affected Scotland in late February. Much of this snow had thawed by mid-march, except on high ground. Overall, the Grampian Mountains, Southern Uplands and north Pennines Figure 4. Snow depths recorded at 0900 UTC at selected stations across (a) the Scottish Highlands and (b) the Scottish Borders and Pennines. 3/0/200 3/0/200 05/02/200 05/02/200 0/02/200 0/02/200 5/02/200 5/02/200 20/02/200 20/02/200 25/02/200 25/02/200 02/03/200 02/03/200 07/03/200 Blythe Westgate Copley 07/03/200 2/03/200 Malham Tarn Buxton 2/03/200

4 Figure 5. Winter 200: (a) days of snow lying at 0900 UTC and (b) precipitation amount (% of average). 2 experienced most snow, lying for more than 60 days. In contrast, south Dorset and parts of Cornwall experienced few days with lying snow (Figure 5(a)). The precipitation anomaly map for winter 200 (Figure 5(b)) shows the marked east west contrast across the UK, consistent with the predominantly easterly weather type and an absence of rain-bearing Atlantic frontal systems. Winter 200 sunshine anomalies saw a similar east west contrast, with the sunniest areas in the west and the dullest down the North Sea coast (National Climate Information Centre, 200). Comparison with previous winters Comparisons for mean temperature have been made using grid-based monthly and seasonal areal series back to 90 (Table 2). December 2009 was particularly cold across Scotland and Northern Ireland due to the very low mean daily minimum temperatures in the second half of the month. January 200 was cold throughout the UK, whilst February was slightly milder in the south but remained cold across Scotland and Northern Ireland. 200 was the coldest winter across England, Wales and the UK since 979. Across Scotland and Northern Ireland, winter 200 was comparable with those of 979 and 947, with only 963 colder. While no individual month was record-breaking, it 2 Maps are from gridded data produced by interpolating station values, taking into account factors such as altitude. All data have been quality controlled. Estimating precipitation totals from snow can be difficult, particularly when gauges are frozen or if snow is blown in strong winds. The Days of Snow Lying map is based on a relatively small number of stations. was the combination of three cold months that led to this being the equal-second coldest winter in the 00-year series for Scotland and Northern Ireland. For northern Scotland, it was the coldest winter on record and had the highest number of air frosts. Table 2 Rankings for monthly and seasonal areal values for winter 200. All temperatures C. Mean Temperature Anomaly (a) (b) In order to place winter 200 into the context of an even longer period, the Central England Temperature (CET) series has been used. CET is representative of an area from the south Midlands to Lancashire and has been produced using observations from a small number of stations: three in recent years (Manley, 974). Mean monthly and seasonal temperatures have been calculated from 659. The mean CET for winter 200 was 2.4 C, making it equal 54th coldest in the 35-year series and the coldest since 979 (Table 3). Only winters 740 ( 0.4 C) and 684 (.2 C) were colder than winter 963 in central England. Winter 947 was also particularly severe, as was, more recently, 979. Winter 200 has also been compared with previous winters in terms of both the persistence and depth of snow (Tables 4 and 5). Records of snow depth at 0900 UTC each day have been digitised from 959. Across England and Wales, winter 963 had the most days with snow lying, followed by 979, with 200 similar to 982 and 985 (Table 4). The winters of 963, 979 and 200 were all comparable across Scotland. However, winter 200 saw the most days of snow lying across Northern Ireland. Rank (nth coldest) in series from 90 Coldest since Coldest in series December 2009 UK th equal 995 (.7) 98 (0.) England st equal 996 (2.7) 98 (0.2) Wales th equal 996 (2.6) 98 (0.8) Scotland th equal 995 (0.5) 98 ( 0.7) Northern Ireland th equal 98 (.6) 950 (.4) January 200 UK th equal 987 (0.7) 963 (.8) England th equal 987 (0.5) 963 ( 2.3) Wales th 987 (0.9) 963 ( 2.5) Scotland th equal 985 (0.2) 94, 945, 963 (.) Northern Ireland th 985 (0.8) 963 ( 0.3) February 200 UK.9.6 9th equal 99 (.4) 947 ( 2.0) England nd equal 996 (2.3) 947 ( 2.0) Wales th equal 996 (2.) 947 ( 2.3) Scotland th equal 986 (.2) 947 ( 2.4) Northern Ireland th equal 986 (0.5) 947 ( 0.5) Winter 200 UK th 979 (.2) 963 ( 0.2) England th 979 (.4) 963 ( 0.6) Wales th 979 (.6) 963 ( 0.3) Scotland nd equal 979 (0.4) 963 (0.2) Northern Ireland nd equal 979 (2.) 963 (.5) Rankings should be viewed as indicative. Uncertainties in estimating areal values may be comparable with the differences between individual years (Legg, 20). The UK winter of 2009/200 Weather January 20, Vol. 66, No. 7

5 8 Weather January 20, Vol. 66, No. The UK winter of 2009/200 Examination of station records for December 946 to February 947 published in the Monthly Weather Reports (Meteorological Office, 947) suggests that the approximate numbers of mornings with snow cover during those three months were 35 over England and Wales, 30 over Scotland and 20 over Northern Ireland (but snow lay for a further 0 to 20 days in March 947 in areas well away from the south coast). Winter 963 saw the most days of air frost in all countries, although in Scotland winters 200 and 979 were comparable to 963. Across the UK, winter 963 saw significant snow lying for at least twice as long as in any other spell in the last 50 years, with the exception of the northwestern parts sheltered from easterly winds by high ground (Table 5). The prevalence of snowy winters between 979 and 99 is apparent, followed by a 9-year break to winter 200. Across northern and eastern Scotland and northeast England, the spell in winter 200 was broadly comparable to those in February 986, January/February 984 and December 98 to January 982. Across the Midlands and southern England, the winter 200 spell was comparable to February 99, January 987 and January/February 985. Western parts of the UK generally saw less snow. The winters of 947 and 963 feature prominently in the temperature and snow analyses and were the most severe UK winters of the last 00 years. Winter 979 was the most recent winter that was colder than 200. These three winters are described in more detail below. The winter of 947 was notable for a succession of snowstorms from late January until mid-march (see cover photograph of this issue), mainly associated with easterly airstreams (Douglas, 947; Roberts, 2003; Booth, 2007). It has been judged to be the snowiest winter since at least the mid-nineteenth century, when reliable records began (Shellard, 968). There were six weeks of continuous snow cover over most of the UK, and level snow depths exceeded 30cm, even in lowland areas for example, 5cm at Cranfield (Bedfordshire) on 6 March. In upland areas depths were even greater with 2cm at Forest-in-Teesdale (County Durham) on 6 February and 9cm at Lake Vyrnwy (Powys) on 6 March. The snowfalls were accompanied by some very low temperatures, at times falling to 5 to 20 C. Strong winds caused severe drifting, up to 5m in some areas, with widespread dislocation of road and rail transport. The snowfalls prevented the movement of coal, upon which the UK was almost entirely dependent to heat homes and generate electricity. Power supplies to homes and industry were severely affected and in turn unemployment rose. Winter 947 was also very dull: for example Kew Observatory (Surrey) recorded no sunshine from 2 to 22 February. When the snow finally melted in the second half of March, serious flooding occurred for example along the Severn valley (Roberts, 2003). This was a time of considerable hardship and austerity following the Second World War. The winter of 963 was one of the most severe on record in the UK (Booth, 968; Shellard, 968). It was probably the coldest since 740 over England and Wales, since 879 over Scotland and since 895 over Northern Ireland. The outstanding feature was its duration from late December until early March. Mean winter temperatures were around 4 degc below average across much of England and Wales, and at many stations it was the coldest winter since records began. The winter was characterised by frequent easterly winds, with gales at times, and was cold enough for the sea to start to freeze in some sheltered locations, Table 3 Mean CET ( C) for winters in the last 00 years that are colder than 200. Winter Mean CET Rank (nth coldest) rd 947. = 2th 97.5 = 23rd = 23rd = 28th = 30th = 44th = 54th Values are rounded to the nearest 0. C. Table 4 Number of days with snow lying at 0900 UTC for winters from 962 for the UK and constituent countries, in rank order. Figures in brackets are the numbers of days of air frost. 200 figures are shown in bold. Winter UK Winter England and Wales Table 5 The 0 snowiest spells since 960 the number of consecutive days of snow lying with depths of 0cm or more at five or more stations The longest spell in each region is shown in bold. Winter North Scotland East Scotland West Scotland Northeast England Northwest England North Wales such as estuaries (Figure 6). There was continuous snow cover over most of the country during January and February, with several blizzards, accompanied by deep drifting, bringing road and rail transport chaos. A snowstorm on 6/7 February Winter Scotland Winter Northern Ireland (68) (73) (62) (49) (58) (58) (6) (50) (52) (43) (6) (56) (44) (47) (55) (29) (46) (47) (5) (36) Based on 5km gridded data from 962. Snow lying means snow covering at least half the ground. The table includes the first three ranked winters for air frost for each country. South Wales Midlands East Anglia Southeast and central southern England cm was chosen as being likely to cause difficulty to pedestrians and drivers. The table excludes stations with an altitude 250m. The values should be used with caution. Snow depths can be difficult to measure and some observations may be unreliable or missing. These statistics may also reflect regional variations in the network density of stations recording snow depth. 2 Winter 963 includes a few days in early March. Southwest England Northern Ireland

6 resulted in a reported level depth of.6m at Tredegar in south Wales. Even so, while the duration of snow lying in 963 was greater than in 947, the overall quantity of snow has been judged to be less. In contrast to winter 947, winter 963 was also sunnier than average, particularly in western areas, which to some extent mitigated the cold easterly winds. During the winter of 979, very cold conditions set in towards the end of December 978 (Burt, 980). The snow that fell after Christmas persisted and was added to throughout January and February, particularly in upland areas of the north. Winter 979 also saw some very low temperatures, with 24.6 C at Carnwath (Lanarkshire) on 3 January, the lowest temperature recorded anywhere in the UK since February 955. Over England and Wales, January 979 is ranked third coldest in the last 00 years, with only 940 and 963 colder. However, winter 979 was noteworthy neither for the degree of cold (surpassed in 963) nor the snow depths (surpassed in 947) but for the number of days with snow cover. Upland areas experienced over 50 days with lying snow as did some areas in the Midlands and eastern England. For example, in the Pennines, Great Dun Fell (847m) reported snow lying continuously from the last week of November until the first week of May. Summary Winter 200 will be remembered for its prolonged cold spells, low temperature extremes and disruptive snowfalls. The spell of freezing and snowy conditions from mid- December to mid-january affected almost all areas of the UK. It was a particularly long and hard winter across Scotland, Northern Ireland and upland areas of northern England, which saw further low temperatures and snowfalls in February. Two photographs of its effects can be found on page 4. It was the coldest winter for 3 years since 979 for the UK overall and for England and Wales. When compared with winters in the last 00 years, the season is ranked seventh coldest over the UK and equal-eighth coldest over England and Wales. However, winter 200 was particularly notable across Scotland and Northern Ireland where it was the equal-second coldest, comparable with 947 and 979, with only 963 colder. Over northern Scotland, winter 200 was even comparable with winter 963, but elsewhere that remarkable winter comfortably retains its position as the coldest in at least the last 00 years. The spells of snow in winter 200 were the most widespread and significant across the UK since the mid-980s, and 200 sits high in lists of the snowiest Figure 6. Ships laid up in the River Blackwater, Essex coast, in the icy winter of 963, looking towards the Tollesbury shore. The boats from Mersea were unable to get out to the ships and the watchmen had to be supplied by helicopter. (Courtesy Mersea Museum Ashley Upsher Collection winters of the last 00 years. However, over England and Wales, it was nowhere near as severe as 947 (in terms of the quantity of snow) or 963 and 979 (in terms of the persistence of lying snow). The most severe conditions of the winter were in northern areas, particularly Highland Scotland which saw prolonged, deep lying snow. Across Scotland and Northern Ireland winter 200 was comparable to, or in some areas even more severe than, these earlier winters. An analysis of the rarity of winter 200, based on historical records, may need to take into account the changing climate. With global warming, the probability of cold winters such as 200 may be expected to decrease. Recent research taking into account this warming trend suggests a return period for the winter 200 temperature of around 00 years in Scotland (van Oldenborgh, 200). For snow cover, it is not yet possible to adjust return periods to take global warming into account: the unadjusted value for Scotland is estimated to be 20 to 50 years. Other parts of northern Europe also experienced a severe winter 200. For example, countries bordering the Baltic saw large negative temperature anomalies during January and February. In Ireland, it was the coldest winter since 963, with heavy snowfalls in late December and mid-january. In contrast, with the polar jet stream displaced southwards, parts of southeastern Europe and the Mediterranean were exceptionally warm and wet. Gibraltar had its wettest winter on record with 379mm of rainfall, easily beating the previous record of 200mm in winter 856 (Ball, 20). It is interesting to note that winter 963 was also very wet in Gibraltar (932mm). Several winters in the last 20 years have been unusually mild. Over England and Wales, five of the ten warmest winters in the last 00 have occurred since 990 with 2007 the warmest of all. There has been a marked absence of widespread and persistent snowfalls in recent winters. Also, in recent decades the public has become more susceptible to travel disruption due to snow and ice with many more vehicles on the road, longer commuting distances and more flights taken. All these factors may have affected the public perception of the severity of winter 200, especially over England and Wales. However, over Scotland and Northern Ireland it was amongst the coldest and snowiest of the last 00 years. References Ball S. 20. Exceptional rainfall in Gibraltar during winter 2009/200, Weather 66: Booth G Winter 947 in the British Isles, Weather 62: Booth RE The severe winter of 963 compared with other cold winters, particularly that of 947, Weather 23: Burt SD Snowfall in Britain during winter 978/79, Weather 35: Douglas CKM The severe winter of 946 to 947. Met Mag, 76: The UK winter of 2009/200 Weather January 20, Vol. 66, No. 9

7 Weather January 20, Vol. 66, No. The UK winter of 2009/200 Legg TP. 20. Determining the accuracy of gridded climate data and how this varies with observing network density, Proceedings of European Conference on Applied Climatology, Zurich, September 200. Advances in Science and Research (in press). Manley G Central England Temperatures: monthly means 659 to 973, Q. J. R. Meteorol. Soc. 00: Meteorological Office Monthly Weather Report, HMSO, London. National Climate Information Centre UK seasonal weather summary winter 2009/200. Weather 65:99. Perry MC, Hollis DM The generation of monthly gridded datasets for a range of climatic variables over the UK. Int. J. Climatol. 25: Roberts CG The winter of 947 in Halesowen, West Midlands, Weather, 58: 3 9. Shellard HC The winter of in the United Kingdom a climatological survey. Met. Mag. 97: van Oldenborgh, GJ How unusual was the winter of 200 world wide? / [accessed November 200]. Further information on the climate of the UK, including monthly, seasonal and annual summaries and statistics, is compiled by the Met Office National Climate Information Centre and available at Snow and gales in eastern England from a North Sea polar low: 6/7 January 200 Correspondence to: Mike Kendon, Met Office, FitzRoy Road, Exeter, EX 3PB, UK michael.kendon@metoffice.gov.uk Crown Copyright, 20, published with the permission of the Controller of HMSO and the Queen's Printer for Scotland DOI: 0.002/wea.735 Editor s note: lower temperatures were recorded in early December 200 than in winter 2009/200 in several parts of the United Kingdom. 0 John Lawson University of Reading Introduction While many people across the British Isles were digging out cars and gritting driveways on the morning of 7 January 200, a mesoscale low was bringing gale force winds and snow showers to the Norfolk and Lincolnshire coasts. The precipitation at this time was illustrated by the 0630 UTC radar image from the Met Office network (Figure ). The mesoscale low was one of many wintry weather events in the winter of 2009/200, and, at the time it formed, 22 people had already died in the UK due to car accidents, avalanches, and ice-related accidents (Times Online, 200). Impacts of the severe winter weather included shortages of road grit, widespread workplace and school closures, and severe disruption to travel (BBC, 200). Because attention was directed elsewhere, the media barely mentioned this mesoscale low. Although the snowfall and winds that affected the Norfolk coast added to the disruption, they were not explicitly addressed in meteorological reports (for example, Brugge, 200). Mesoscale lows can have a large impact in small regions and over short periods of time. Due to their small diameter, which numerical models often fail to resolve, they are often more difficult to forecast than synoptic-scale low-pressure systems. Climatology of such mesoscale vortices (Forbes and Lottes, 985) indicates that they are most frequent in polar regions during the winter, with the term polar low used to designate stronger systems. Rasmussen (98) discovered that certain warm-core variants of these polar lows are different in genesis and structure from synoptic-scale low-pressure systems. The latter are typically associated with strong cyclonic vorticity advection aloft, producing a comma-shaped cloud pattern in satellite imagery. In contrast, many polar lows are driven by convection (so-called Arctic hurricanes ) and produce a spiral-shaped cloud pattern. Synoptic situation At 0600 UTC on 4 January, northwest Europe lay under a diffluent northerly flow at 500mbar, being located downstream of a zonal shortwave trough axis across the Norwegian Sea. As the trough started to move south and amplify, it advected cold air southwards. By 0600 UTC on 6 January (Figures 2 and 3), a 500mbar closed low was analysed over the English Channel, collocated with a filling surface mesoscale low (Low A). A 999mbar pressure minimum lay to the southwest of Denmark (Low B, the subject of this article), and a trough extended to the east of it (analysed as an occluded front). With a 09mbar surface high over Norway, a strong eastnortheasterly surface flow extended from the Skagerrak across the North Sea, as seen in the low-level cloud streets on the infrared satellite image (not reproduced). Close to the Danish and British coasts, deeper convection was breaking out in the vicinity of the occluded front. Figure. Precipitation radar at 0630 UTC on 7 January 200. Note high radar reflectivity along the coast of eastern England, associated with Low B. Blue/green pixels indicate light to moderate precipitation, yellow and red heavier precipitation. ( Crown copyright, Met Office.)