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A White Christmas in Reading?

What is a “White Christmas”?

For many people a White Christmas is a Christmas Day (25 December) with a ‘traditional’ look to the day – that is one with snow covering the ground. For those whose fancy a flutter at the bookmakers (although it is probably too late to get good odds now) a White Christmas is one when snow or sleet is observed during the 0000-2359 h day. This latter definition includes a wet and damp with a few flakes during a shower of sleet on a day when rain is the dominant form of precipitation. In fact William Hill are slightly more restrictive with their White Christmas betting rules which state they pay out only when ‘… one flake of snow will fall on Met office monitoring stations over the 24 hr period of the 25th of December’.

Weather on past Christmas days

Here at Reading weather observations are made at 0900 GMT (9 a.m. in the morning in winter) every day of the year and in this note we will examine the incidence of

  1. Snow/sleet falling at some time during the 24 hours beginning 0000 GMT [SF annotation below],
  2. Lying snow at 0900 GMT [SL annotation below], and
  3. A measurable depth of snow at 0900 GMT.

We currently have records for every Christmas back to 1917 (in 1916 there was almost certainly a fall of sleet/snow on Christmas Day) and these indicate the following years when one or more of the above events was noted on 25 December:

1925 [SF] – The 9 a.m. air temperature in a NE’ly wind was 0.8C (following a slight air frost) and this subsequently rose to 8.2C in the next 24 hours. Some light snow fell during the day but it then turned dull in Reading with light rain. Further north, snow lay for in both Manchester and Glasgow. The state of the ground is not known for this day in Reading although it is not thought that a 50% snow cover prevailed.

1927 [SF] – According to Robin Stirling (in “The Weather of Britain”) “A depression from the Atlantic moved from Ireland to the English Channel and then across France to the Mediterranean. It caused a great snowstorm in southern England. About 18 h on Christmas Day, rain in the south turned so heavy that roads were hopelessly blocked by midnight, and a train was snowbound between Alton and Winchester.” As the storm continued some 15 cm of snow fell in central London. In Reading the 25th began with an air temperature of 6.3C at 9 a.m. – then rising only to 6.8C before dropping to -0.4C the following night. There was moderate rain falling in an E’ly wind at 9 a.m.; the rain became heavy at times and turned to snow fell later in the day (9.9 mm of water equivalent precipitation fell in 24 hours) – although, again, snow depths were not reported at Reading. However, newspaper accounts report that locally, a considerable stretch of the Reading to Henley road was under five feet of snow by Boxing Day (26th). “The Berkshire weather book” by Currie et al. carries an interesting account of the Boxing Day blizzard in Berkshire and adjacent areas.

For intensity, extent,damage and disruption many would argue that this snowstorm was the most damaging since March 1891 and probably the most severe snowstorm of the twentieth century in the UK. South-east of a line from Norwich to Dorchester over 30 cm of snow fell (Eden, 2008, “Great British weather disasters”) with 60-75 cm over parts of the North Downs in Surrey and the Weald in Kent. A-roads were blocked for up to a week – minor roads for much longer. The snow was of a high water content and hundreds of kilometres of lines came down as a result.

1938 [SL] – The 25th dawned with a minimum temperature of -0.3C and lying snow (depth unknown, although it had been lying from the 21st) after several falls of snow that fell over several days back to the 18th and in which time the temperature had only reached 2.3C. The temperature rose to just 0.6C on the 25th; no snow fell on Christmas Day (which was dull and cold) – but did fall on the 26th. Across the local area it was more the cold that caused problems in the run up to Christmas – with many frozen pipes and a very large demand for paraffin for heating.

1956 [SF] – After a dull, cold start to the day, a SE’ly flow brought some snowfall during the 25th with the temperature rising from -1.4C to 1.8C. Some 5.3 mm of water-equivalent precipitation fell leading to about 5 cm of lying snow on the 26th at 9 a.m. (so almost all of this precipitation would have been snow). The snow was initially light and turned heavier later. Locally snow was ‘several inches [~10 cm] deep’ (Currie et al.) later on the 25th in parts of Berkshire.

1968 [SF, SL] – This was one of the few Christmas Days when the Reading weather observer had to contend with falling sleet and snow at the observation time. At 9 a.m. conditions were air temperature 0.6C (after a minimum of 0.1C), sky obscured by falling light rain and snow (there had been a spell of continuous snowfall early in the morning), with a NE wind of 2 kn, visibility of about 400 m and 8 cm of lying (but thawing) snow. The temperature rose to 2.8C later in the day as the clouds thinned out and just 0.3 mm of precipitation fell in the next 24 hours. As a depression moved ESE’wards across Cornwall some places were cut off by blizzards in N England.

1970 [SF, SL] – Christmas Day began with a minimum temperature of -3.9C and 1 cm of lying snow. The temperature rose later to 0.9C and snow fell during the day to give a depth of 5 cm the next morning. However, on the 25th itself the snow was in the form of a shower before noon – more persistent snow was to fall next morning. Despite the morning snow, some 3.5 h of bright sunshine was recorded – with only 2 oktas of cloud at 9 a.m.

1981 [SL] – There was a cold start to this Christmas morning – the temperature had risen from a minimum of -4.5C to -3.4C by 9 a.m., although the grass minimum temperature had been as low as -10.2C. The ground was frozen to a depth of just over 5 cm under 5 cm of lying snow. No snow fell during the day, which was dry and sunny with 4.5 h of bright sunshine. This was one of the snowiest December’s of the twentieth century – on the 12th the snow depth in Reading had been 18 cm.

1996 [SF] – By now many people had begun betting on the likelihood of a ‘White Christmas’ and this year brought a pay-out. In Reading, after a slight air frost, some snow fell – but not enough to give a 50% ground cover at 9 a.m. With 3.5 h of sunshine during the day any lying snow had melted by Boxing Day morning.

1999 [SF] – Another day when the bookmakers had to pay out – certainly snow fell in Reading but not enough to lie on a ground surface that was ‘moist’ after some 43 mm of rain in the preceding 72 hours. In Reading rain turned to snow around 11 a.m., only to turn back to rain after about ten minutes. It was a cloudy day with another 7.2 mm of precipitation being credited to the 25th. It was a windy day with a gust of 43 kn measured at the university.

2009 [SL] – After a minimum temperature of 0.9C it warmed up to reach 8.4C later, the warmth helping to reduce the 5 cm of complete ground cover of snow (this had persisted since the 18th, having caused some severe pre-Christmas disruption to Reading) to a broken cover of 2 cm depth by the morning of the 26th. No snow fell during Christmas Day, which brought some rain and some sunshine.

2010 [SL] – Again, there was no snowfall but lying snow existed at 9 a.m. to a depth of 2 cm. This lying snow remained rather broken in cover and helped the temperature to drop sharply into Boxing Day down to -6.5C in the screen and to -11.3C on the grass.

A White Christmas in 2012?

In summary then, in the past 95 years there have been seven Christmas Days with snow falling and six with snow lying in the morning (1968 and 1970 fall into both camps). So the likelihood of a White Christmas as the bookies judge it is about one in 13.6 – the last one was in 1999 so we are (maybe) due for one soon…

Those who would like to read about the incidence of a White Christmas elsewhere in the UK should start also read Martin Rowley’s account at http://booty.org.uk/booty.weather/metinfo/snxm_cat.htm .

2012 Hurricane season wrap-up

By Ray Bell

The 2012 Atlantic hurricane season officially ended on Saturday as a ‘very unusual‘ year with an abnormally large 19 tropical storms forming and curiously only 1 making it to Category 3 strength. A typical hurricane season has 12 tropical storms, 6 of which are hurricanes and 3 of which are major hurricanes. This year also saw a large proportion of tropical storms forming at high latitudes.  However, 2012 will be long remembered for producing hurricane Sandy: a ‘Frankenstorm’ (a cross between a nor-easter and a hurricane) that was the largest, most powerful, and second most destructive Atlantic hurricane on record, say Jeff Masters.

North Atlantic tropical storms tracks 2012 (National Hurricane Centre)

Based on the combined number, intensity, and duration of all tropical storms and hurricanes (ACE) the season was above-average, but the limited major Hurricane activity helped keep ACE down to 30% above average.

Since 1995 the US has on average been hit by 4 storms per year, with 1.5 of these being hurricanes and 0.6 being major hurricanes. This year, Beryl, Debby and Isaac hit the US. No major hurricanes hit resulting in 2012 being the 7th consecutive year with a major hurricane strike, the longest since 1861-1868 (see here).

Back in June I wrote a blog on the hurricane seasonal forecast that the Met Office issued, as well as the forecast from other centres. The Met Office was at the low end of the seasonal forecasts going for 10 tropical storms (7-13 most likely). Colorado State University were at the high end only predicting 13 tropical storms. So why did all of the seasonal forecasts significantly under predicted tropical storm activity?

Atmospheric-Oceanic conditions

Phil Klotzbach from Colorado State University has great documentation on the Atmospheric-Oceanic conditions over the North Atlantic this season (hurricane.atmos.colostate.edu/Forecasts/2012/nov2012/nov2012.pdf). One reason was than the predicted El Niño did not develop, which usually calms tropical storm activity in the North Atlantic. A heavy influx of dry Saharan air over the tropical North Atlantic is also likely to have damped the intensity of storms forming in the tropics (see here). The figure below shows how stable the air has been in the tropical North Atlantic this season compared to the past 8 years.

Vertical instability over the tropical Atlantic in 2004 - 2012 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from a climatological temperature profile (Jeff Masters)

Dr. Landsea claims the tropical storm count this year is ‘slightly misleading’ due to satellites and an array of technological advances that allows systems to be identified that likely would have gone undetected decades ago (see here). 7 out of the 9 tropical storms this year reached tropical storms strength for only 2 days or less. The method in which the Met Office use currently excludes any systems with a duration of less than two days to prevent picking up transient disturbances and depressions. Planned increases to the horizontal resolution of the Met Office seasonal forecasting system in December 2012 may allow for a better distinction between weak tropical storms and tropical depressions (Jo Camp, Pers. Comm.).

Atlantic tropical cyclones between 1878 - 2012 that spent two days or less at tropical storm strength (top) and more than two days at tropical storm strength or hurricane strength (bottom.) Figure updated from Villarini, G., G. A. Vecchi, T. R. Knutson, and J. A. Smith (2011), "Is the recorded increase in short-duration North Atlantic tropical storms spurious?", J. Geophys. Res., 116, D10114, doi:10.1029/2010JD015493. (Jeff Masters)

Hurricane seasonal forecasts for 2013 will be released in May.

I would like to thank ex-Reading University student and colleague Joanne Camp for proof reading.

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Fifty years ago – The winter of 1962-1963 in Reading

The winter of 1962/63 was one of the coldest on the UK instrumental record, with the mean January temperature ranking fifth coldest (out of 354 months) and the mean February temperature ranking the seventh coldest, according to the Central England Temperature record [1]. Across the country as a whole gas and electricity supplies failed frequently, there were some noteworthy snowfalls and freezing rain (a relatively rare event in the UK) occurred on occasions – notably across southern England on 3 January. Here is the winter temperature in the context of recent ones in East Berkshire [2].

Winter mean temperature in East Berkshire, 1863-4 to 2011-12.

Winter mean temperature in East Berkshire, 1863-4 to 2011-12.

The cold weather started just before the Christmas of 1962, as temperatures from the University’s climatological station (then in London Road) show.

Daily maximum and minimum air temperatures, Reading, 1 December 1962 to 7 March 1963.

Daily maximum and minimum air temperatures, Reading, 1 December 1962 to 7 March 1963.

During early December it had been had been changeable and stormy, although the daily weather diary written at the time by the Reading observer Arthur Moon noted that 6 December (-2.3 C maximum; the third consecutive ice day) had the lowest daily December maximum temperature since 20 December 1938 (-2.8 C). There was also dense fog around Reading on the 4th to 8th – at the 0900 GMT observation on these days a visibility of 50 metres was reported. Indeed, the Reading weather diary notes that later on the 3rd visibility was a low as 15 yards (15 m) with ice needles being observed on the 4th.

(In London “the final ‘major’ old style London smog occurred in this month (4th to 6th): i.e. a combination of domestic coal smoke plus sulphur dioxide products producing an acidic fog droplet, which in turn caused major respiratory problems. About a thousand people died [in London] as a result. During the fog, the smoke & sulphur dioxide content in the atmosphere increased to a maximum of 10 to 14 times the normal concentration. This was noted at the time as the ‘worst’ since December, 1952”.[10])

Then on 22 December a high pressure system moved to the north-east of the British Isles, dragging bitterly cold winds across the country – as the NCEP reanalysis chart shows [3]. As the Reading weather observer noted “Later in the day colder conditions began to set in…”.

NCEP reanalysis surface analysis, 0000 GMT 22 Dec 1962 (white contours).

NCEP reanalysis surface analysis, 0000 GMT 22 Dec 1962 (white contours).

This NE’ly to E’ly flow was to persist for much of the winter, as the hodograph for the 0900 GMT observations at Reading from 22 December to 7 March shows.

Wind hodograph for Reading, 22 December 1962 to 7 March 1963.

Wind hodograph for Reading, 22 December 1962 to 7 March 1963. Hodograph drawn using the eight principal compass points.

The largely E/NE’ly surface flow is also confirmed by the mean MSL pressure maps for January-February 1963 – with MSL pressure being especially high across the British Isles in January. At Lerwick the mean MSL pressure during January was the highest for any January month during 1961-2012 – although at Heathrow the years of 2000, 1989 and 1964 have seen highest January values.

Winter MSL pressure, 1962-1963.

Mean MSL pressure (hPa), (left) January-February 1963 and (right) January 1963.

There was not to be a White Christmas in Reading, After some snow on the 12th, wintry weather began with a vengeance in Reading around noon on 26 December (Boxing Day) – which was the first of ten consecutive days with snowfall. This snow soon began to lie, the ground being frozen.

There was a blizzard on 29-30 December across Wales and south-west England; by the morning of the 30th the snow depth had doubled to 21 cm by 0900 h while the Reading observer noted on the 30th “drifts of 2.5 to 3 feet [75 to 90 cm] were present, the strong wind causing considerable drifting and blowing of snow.” The snow depth reached 31 cm at the 0900 GMT observation on 3 January. Lying snow (50% or more of ground cover) lasted continuously at Reading from 27 December to 14 February. Altogether, snow fell on 38 days during the three winter months (the average for the period 1981-2010 was just 7.6 days [6]). In spite of these snowy conditions it is reported that there were no delays to Reading’s trolley buses by the 28th [8].

The weather diary for 3 January  contains the following: “Dull conditions continued and overnight snow turned to freezing rain mixed with ice pellets (clear ice) at times. This coated walls and trees with a smooth ice glaze. … The ice covering twigs measured around 3 mm thick and where dripping occurred blunt icicles about one inch [2.5 cm] long formed. Trees so covered emitted a cracking sound when blown by the wind.”

However, the Reading and Berkshire Chronicle reported on 4 January that “staggering sums of money and huge quantities of material have had to be used in the fight to clear the snow in Reading and Berkshire: villages on the Downs have been isolated; public transport services and motorists have had difficulty in keeping going; milk, paraffin and vegetables have been in short supply. It is just a small part of the picture presented by this week of Arctic conditions” [8].

The 12th gave the lowest January maximum temperature (-2.2 C) since 20 January 1946 – but colder was to come. The weather diary for 23 January notes the lowest night minimum temperature (-12.5 C) since 15 February 1929 with the 24th noting the lowest day maximum temperature (-5.6 C) yet recorded. Fog on the 23rd led to riming of trees.

Images from the winter can be seen at [12] – an example is shown here of people walking on the Thames at Windsor. The website at [12] also relates many stories and news items about the winter.

Skating on the ice on the Thames in Windsor, 1963.

Skating on the ice on the Thames in Windsor, 1963.

Level lying snow depth.

Level lying snow depth, Reading, 1 December 1962 to 7 March 1963.A similar pattern of lying snow occurred at nearby Hurley (just outside Maidenhead) ref 9.

On 18 January -22.2 C was recorded at Braemar in Scotland. The long bitterly cold spell caused lakes and rivers to freeze and sea water froze in some of England’s harbours. This is also reflected in the soil temperatures measured here in Reading. At a depth of 4 feet (122 cm) the temperature fell throughout the winter, reaching the lowest level in early March. Close to the surface temperatures were sometimes close to -5 C, and frequently below -2 C. Even at a depth of 30 cm the soil froze for a while in mid-January, the coldest part of the winter.

0900 GMT soil temperatures, Reading, 1 December 1962 to 7 March 1963.

0900 GMT soil temperatures, Reading, 1 December 1962 to 7 March 1963.

Minimum temperatures recorded at Reading during the months of December to February 1962-3 were as follows:

Value (C) Date
Lowest maximum temperature -5.6 24 Jan
Minimum air temperature -12.5 23 Jan
Grass minimum temperature -15.1 23 Jan
Lowest bare soil surface temperature -13.9 23 Jan
Lowest 5 cm soil temperature -5.0 23 Jan
Lowest 122 cm (4 feet) soil temperature -2.6 9 Mar

Lower temperatures have been recorded more recently at Whiteknights – for example the air minimum temperature of -14.5 C on 14 January 1982; the grass minimum temperature was -20.1 C that morning. On 12 January -6.8 C was the highest temperature of the day in Reading. But in 1962-1963 it was the persistence of the cold and lying snow that was remarkable.

During January the Thames was almost frozen over at Caversham Bridge while in Wargrave snow ploughs struggled to clear tons of snow in the High Street. On 16 January a USAF officer was killed in a weather-caused accident on the Twyford-Henley Road and, on the same day, two Tilehurst men died in an accident involving a coach on the Bath Road at Sonning.  British Railways Western Region complained of the wrong type of weather for testing their new anti-freeze for points. A N’ly wind was blamed for blowing back coke fumes into Holy Trinity Church in Theale on 13 January; several people collapsed and were taken to hospital. By 18 January the Thames had frozen over completely at Reading and Windsor.

The cold continued into February – on the 2nd the maximum temperature of -1.7 C was the lowest in February since 2 February 1956. Ice days (when the temperature remained below 0 C all day) are very rare in Reading in February – there were two in February 1963 along with another three days that failed to rise above 0.5 C. As days lengthened and temperatures started to rise (slowly) in February the snow slowly thawed; the Reading weather diary notes that by the evening of 14 February the snow cover on the ground was below 50 % for the first time since early on Boxing Day (a period of 50 days with 50 % cover at 0900 h.)

High pressure for much of the wintry part of the winter led to some sunny days, even if they remained cold. 208 h of bright sunshine were recorded during the three winter months (despite problems keeping the snow and frost off the glass sphere of the sunshine recorder) – this amounts to about 20 % more than average.

Sunshine duration, Reading, 1 December 1962 to 7 March 1963.

Sunshine duration, Reading, 1 December 1962 to 7 March 1963.Note the sunshine around the end of February which helped to melt much of the lying snow.

Once the snow at the end of December/start of January had finished it was a mainly dry winter with just 33 mm of precipitation falling in the first two months of 1963 – or about 33 % of average – and most of that fell by 4 January.

Daily precipitation totals (water equivalents), Reading, 1 December 1962 to 7 March 1963.These are for the 24 hours beginning on the date shown.

Daily precipitation totals (water equivalents), Reading, 1 December 1962 to 7 March 1963.These are for the 24 hours beginning on the date shown.

NCEP reanalysis surface analysis, 0000 GMT 4 March 1963 (white contours).

NCEP reanalysis surface analysis, 0000 GMT 4 March 1963 (white contours).

On 4 March a mild SW’ly flow finally reached the British Isles and temperatures gradually rose, allowing snow to melt and winter to end. There had been quite sunny conditions in Reading towards the end of February and in early March so much of the snow had already disappeared – preventing the severe flooding along the Thames that sometimes occur after a cold, snowy spell.

So how cold was it here? This image, extracted from the annual report for 1964 from the Reading & District Natural History Society [11] gives the raw numbers for January.

Notes on the 1962-3 winter by A Moon, the observer at the time.

Notes on the 1962-3 winter by A Moon, the observer at the time.

For comparison, 1931-60 soil and air temperature anomalies, and 1921-50 rainfall anomalies ([4], [5], [7]) for the three winter months of 1962-63 are shown in the next table.

Month Mean max temp (C) Mean min temp (C) Mean temp (C) Mean temp anomaly (degC) Total Precip (mm) Precip anomaly (mm) Mean 30 cm soil temp (C) Mean 30 cm temp anomaly (degC) Mean 122 cm soil temp (C) Mean 122 cm temp anomaly (degC)
Dec 4.7 -0.9 1.9 -3.1 64.1 +5.7 3.7 -1.9 7.0 -0.6
Jan 0.4 -4.4 -2.0 -5.9 22.7 -38.5 0.5 -3.8 4.1 -1.6
Feb 2.6 -2.4 0.1 -4.3 10.6 -34.6 0.3 -3.8 2.9 -2.3

Finally, it is worth noting that it was not only the British Isles that was affected by cold weather this winter. January 1963 was memorable for the extreme severity of the cold weather which simultaneously gripped North America, Europe, and the Far East. [14] In Europe  it  was  one  of  the  coldest  months  ever  recorded,  resulting  in  shortages  of  coal  and  food  as  snowdrifts blocked roads and ports  and waterways were blocked by ice  or  were completely frozen  over. Shortages of  water and  gas  also  occurred  as  a result  of  damage by frost  to exposed  pipelines  in  normally  milder  climates.  Many died or were hospitalized from exposure to the cold. Average temperatures for the month were in excess of 5 degC below normal from southern England across Europe to the Urals. Warsaw reported an average temperature of -12.4C for January, or 10.3 degC below normal, while Paris averaged -2.9C, or -5.5 degC below normal.  Even Mediterranean regions averaged about 3 degC below normal.

700 hPa mean height in January.

700 hPa mean height in January, from ref. 14. Note the ridges to the west of North America and Europe.

In the  Far  East,  abnormal  cold was  accompanied  by blizzards,  notably in western Japan, where snowdrifts of 4 m depth  in  some  districts  paralysed  transportation  and collapsed roofs.  Soldiers and students were pressed into service to dig out trains and remove snow from roofs to save schools and other structures.

What about the winter of 2012-13? Who knows? While you’re awaiting developments why not relive the winter of 1962-3 in realtime via the observation archive at [13]?

Sources

The author is indebted to the intrepid observers at the time, who made daily observations without fail. The chief observer at the time was Arthur Moon who also assiduously compiled a daily weather diary. These observations, and both earlier and later observations, are currently being compiled by the author into a database showing the daily weather in Reading since 1908. The graphs were plotted using this database.

Note that the observation site in 1962-3 was on the London Road; as such mean maximum and mean minimum temperatures were about 0.4 degC and 0.6 degC, respectively, warmer than at Whiteknights.

References

[1] http://www.metoffice.gov.uk/hadobs/hadcet/mly_cet_mean_sort.txt

[2] http://www.met.reading.ac.uk/~brugge/east_berks_temp/eastberks.html

[3] http://www.wetterzentrale.de/topkarten/fsreaeur.html

[4] Averages of earth temperature at depths of 30 cm and 122 cm for the United Kingdom 1931-60. 1968. HMSO, London.

[5] Averages of temperature for Great Britain and Northern Ireland 1931-60. 1963. HMSO, London.

[6] Burt, S.D. and Brugge, R., 2011. Climatological Averages for 1981-2010 and 2001-2010 for stations appearing in the monthly bulletin of the Climatological Observers Link. ISBN 9780956948502.

[7] Averages of rainfall for Great Britain and Northern Ireland 1916-50. 1958, reprinted 1961. HMSO, London.

[8] Currie, I., Davison, M. and Ogley, R., 1994. The Berkshire weather book. Froglets publications. ISBN 1872337481.

[9] http://www.met.reading.ac.uk/~brugge/winter6263.html

[10] http://booty.org.uk/booty.weather/climate/1950_1974.htm

[11] http://rdnhs.org.uk/blog/wp-content/uploads/Naturalist16w.pdf

[12] http://www.thamesweb.co.uk/windsor/windsorhistory/freeze63.html

[13] http://www.met.reading.ac.uk/~brugge/reading_past_weather.html

[14] http://docs.lib.noaa.gov/rescue/mwr/091/mwr-091-04-0209.pdf : J F Connor (1963). The weather and circulation of January 1963. Monthly Weather Review, vol 91, p 209.

The 2012 Antarctic Ozone Hole

A remarkable event occurs each springtime in the Antarctic (Sept-Oct-Nov) lower stratosphere – a dramatic depletion of ozone in a period of just a few weeks.

It has not always been this way. Ozone amounts can be measured in many ways. One useful measure is the total number of ozone molecules above a square centimetre of the surface (“column ozone”) – this is reported in Dobson Units (DU), named after a pioneering stratospheric ozone researcher. The global average value is about 300 DU. The British Antarctic Survey (BAS) have routinely measured column ozone since the late 1950s at their base at Halley. Until the early 1980s, Antarctic  ozone had a weak annual cycle, rising from about 300 DU in late spring to around 370 DU in early summer, before returning back to about 300 DU, with some year-to-year variation in these values.

In the late 1970s and early 1980s,  springtime column ozone seemed to be falling year on year. By 1983, monthly mean values had fallen to less than 200 DU, one-third of  “normal”. Was this just natural variability or  was it to do with human activity? In 1985, three BAS scientists (Farman, Gardiner and Shanklin) published a landmark paper reporting this decline and proposing chemical reactions which implicated human activity as a cause. The paper ignited a huge scientific effort to understand the nature and causes of the ozone hole (as it was, by then, known). Analysis of NASA’s satellite data showed that the ozone hole was an Antarctic-wide phenomena; balloon-borne ozonesondes showed the ozone depletion to be concentrated in the lower stratosphere.

Theories for its cause abounded – from changes in the stratospheric winds to changes in chemical processes (due to both natural effects and human activity). Human activity was expected to cause ozone depletion.  Stratospheric ozone is in a “dynamic equilibrium” being constantly created and destroyed by a set of chemical reactions (as well as being blown from the equator to the poles by stratospheric winds). From the 1970s onwards, it was realised that a number of additional chemicals could destroy ozone – importantly, human activity was increasing their concentrations. These chemicals included the chlorofluorcarbons (CFCs) which were widely used for both domestic and industrial purposes (in aerosol sprays, refrigerators, air conditioning …). Three pioneers of this early work (Crutzen, Molina and Rowland) received the 1995 Nobel Prize for Chemistry.

But there was a problem. This earlier ozone depletion theory did not predict the rapidity of ozone depletion (with about half of the column ozone being lost in a few weeks in the Antarctic spring), why it happened in the lower, rather than the middle and upper, stratosphere and why it occurred preferentially in the Antarctic. Airborne and ground-based measurements in the Antarctic quickly found a “smoking gun” which pointed to the role of chlorine released from the CFCs; lab measurements and theoretical work showed that hitherto unsuspected chemical reactions could rapidly deplete ozone. The key ingredients were as follows: during the winter darkness, the Antarctic lower stratosphere cools to very low temperatures (typically 190 K). Although the stratosphere is very dry, tenuous polar stratospheric clouds (PSCs) could form at such temperatures. Chemical reactions on the cloud particles released chlorine, which was normally locked up in a more inactive form, into a form that could “attack” ozone. These reactions needed sunlight, so it was only as the sun returned in the Antarctic spring that the final ingredient was in place.

The unique conditions in the Antarctic lower stratosphere were ideal for ozone depletion. Subsequently, a more modest global-ozone depletion was detected. The Arctic lower stratosphere is usually much warmer than the Antarctic in winter, so PSCs are rarer – nonetheless during occasional winter/springs, Arctic conditions are conducive to a less severe form of the ozone hole, as happened in 2011.

The political response to the overwhelming evidence of the human impact on the stratospheric ozone was swift, and has been hailed as one of the great successes of the United Nations. Under the Montreal Protocol on Substances that Deplete the Ozone Layer, and its subsequent adjustments and amendments, the emissions of CFCs have been reduced to almost zero.  Because the CFCs are relatively stable molecules (having atmospheric lifetimes of typically 50-100 years), their atmospheric concentrations respond rather slowly to changes in emissions. Nevertheless, the concentrations of the major CFCs are now  falling, in line with expectations.

Back in the Antarctic, the years following the ozone hole’s discovery saw ozone levels fall further as shown in the figure. This shows the October and December monthly-mean column ozone at Halley. The data is taken from Jonathan Shanklin’s BAS webpage using the monthly-means for 1957-2011 and the average of the daily-data for October 2012. Note that this 2012 daily data is still provisional, and subject to up to 10% adjustments following further analysis.

The figure shows that the ozone hole hit rock bottom in the early 1990s. This was partly because the amount of chlorine in the stratosphere was close to its peak. But it is also believed to be due the eruption of Mount Pinatubo, in the Philipines, which resulted in an increased number of stratospheric aerosol particles (tiny droplets of sulphuric acid) for a few years after the eruption. These enhanced the chemical reactions that released chlorine into its active form and deepened the ozone hole. The figure also shows that December values have remained depressed since the early 1980s, showing that a less severe ozone depletion persists beyond spring time

Since 2000, ozone amounts have been more variable – on average higher than the 1990s, but still only around half the values seen before the 1980s. 2012, on the provisional data shown in the figure, was one of the less deep ozone holes of recent times.

There are many ways that the ozone hole behaviour can be visualized. The Halley daily minimum each year (shown on Jonathan Shanklin’s webpage) seems to hint at a gradual recovery. Satellite data gives a more Antarctic-wide view. Animations of the 2012 hole show its dynamic nature (note the dark blue areas that develop during August), as it is influenced by the winds –  on occasions, low ozone values sweep over  populated areas of South America. Mixtures of satellite data and sophisticated atmospheric models also reveal large-scale features such as the Antarctic-wide ozone minima  and  the hole’s area (defined as the area with column ozone less than 220 Du). These plots consistently point to the 2012 ozone hole being less severe than in many recent years. During October 2012, temperatures were higher, and hence the amount of PSCs were lower, leading to less severe ozone depletion. Nevertheless, the strong ozone depletion in the lower stratosphere from late August onwards is clear in animations of ozonesonde observations made at the South Pole.

All things being equal, the expectation is that the ozone layer will continue to recover in years to come, as CFC concentrations decrease,  but their long atmospheric lifetimes means that the ozone will not likely return to pre-1980s levels before about 2050. Continued monitoring of the ozone layer is necessary, as all things are not likely to be equal. First, ozone depletion could be affected if there were further large volcanic eruptions. Second, concentrations of some ozone depleting substances are  rising. Third, climate change will lead to changes in stratospheric temperatures and winds, which could impact on ozone amounts.

Some Web Resources

Jonathan Shanklin’s BAS pages –  a wonderful collection of up-to-date and archive material on Antarctic ozone depletion, at BAS stations and beyond.

NASA’s Ozone Hole Watch pages

NOAA’s Climate Prediction Center stratosphere pages

WMO’s Antarctic Ozone Bulletins

NOAA’s Earth System Research Laboratory ozone pages

The United Nations Environment Programme (UNEP) organises scientific assessments of the state of the ozone layer, the most recent in 2010. Its “20 questions and answers about the ozone layer” has much background information on ozone depletion. The UNEP website includes much useful information on the political and technological aspects of the response to ozone depletion.

Nature – a collection of some of the key papers in the “ozone story” brought together for the 25th anniversary of the Farman, Gardiner and Shanklin paper

summer weather and food production

It can’t have escaped anyone’s notice that summer 2012 has seen some pretty unusual conditions. In the UK its officially been the second wettest summer (June, July and August) since records began with only 1912 being wetter. The only exception was the NW of Scotland with somewhat drier than average conditions. This of course followed a winter and early spring in which we were being warned of possible water shortages over the coming summer thanks to a couple of drier than normal winters.

The wet conditions this summer have had a major impact on cereal, fruit and vegetable crops. In England the National Farmer’s Union estimates that wheat production was down 15% from the 5 year average (http://www.bbc.co.uk/news/uk-19890250). Wet weather around harvest time made the headlines with floods, particularly in the north-east, meaning that some crops couldn’t be harvested at all. The grain that was brought in often had to be dried artificially, substantially adding to production costs. The Scottish strawberry crop was also reported to be decimated, and cold wet weather around apple blossom time in April and May meant that there were less pollinating insects flying and so apples have experienced reduced yields this year.

However, personal experience suggests it hasn’t all been bad news. The raspberry crop in my garden was the biggest I’ve ever seen as long as I could pick them fast enough before they started to mildew. Raspberies are a British native plant growing on woodland margins and have evolved to do well in cool damp conditions. Hence they were quite happy with this summer’s weather. My sweetcorn yield was also up on previous years and potatoes seemed to do pretty well. Tomatoes growing ouside came to very little though. I’ll be starting to dig up the parsnip crop this weekend so we’ll see how well they’ve done fairly soon.

My own apple tree produced only two edible apples, but talking to the farmer at Cross Lanes Orchard (www.users.waitrose.com/~crosslanes/index.htm) just outside Reading suggests that it hasn’t all been bad with apples either. Whilst yields were significantly down for some varieties others have done very well. Cross Lanes is an orchard that grows over 40 different varieties of apples, as well as plums and pears, and so this highlights the importance of diversity in the crop with earlier and later blossoming varieties meaning that there’s a good chance that some varieties will do well.

Over in the USA its been a drought summer, with some big wheat producing states such as Kansas and Nebraska experiencing “exceptional” drought conditions – the most extreme drought category according to the US Drought Monitor at the University of Nebraska-Lincoln (droughtmonitor.unl.edu). As some of our undergraduate students reported in Weather and Climate Discussion last week, this has had an impact on local food production but also potentially on global food prices. Whilst US wheat yields are actually up, corn yields have been down and the quality of pasture for cattle is also very poor (US Department of Agriculture). Reduced grain yields lead to increased prices and because a  lot of grain is used in winter feed for livestock production the impact goes beyond the grain price itself.

These stories all highlight our dependence on the weather for maintaining food production, an issue that has concerned mankind since the earliest civilizations. In ancient Egypt for instance the fertility of the fields along the Nile was dependent on the annual floods, which were themselves dependent on rainfall many hundreds of miles south around the river’s headwaters. These days we have more sophisticated agricultural methods and are able to develop varieties of crops which are more tolerant of particular weather conditions. However, we still have some way to go in our ability to make weather predictions on the seasonal timescale that will allow farmers to make decisions on what crop varieties to plant ahead of time.

Blood Rain for Halloween?

Last week you might have been forgiven for thinking there would be something spooky in the weather for Halloween, with several news websites were pushing headlines along the lines of:

So what is ‘blood rain’ and was there any truth in these headlines?

Every year, millions of tonnes of Saharan dust are lifted from the desert surface of the Sahara, and transported by the atmosphere. Mostly, this dust makes its way westwards, across the Atlantic. However, some of it can be transported northwards towards Europe. This is quite a common affair for our southern European neighbours such as Spain, Italy and Greece, but dust outbreaks rarely make it as far north as the UK – on average only around 2-3 times a year, when you might notice a light coating of reddish dust on your car. On its journey through the atmosphere, dust eventually either settles naturally due to gravity (dry deposition), or it encounters cloud and rain, which cause it to be ‘rained out’ (wet deposition). If dust mixes with rain, it can frequently change the colour of raindrops giving them a reddish hue, due to the high amounts of iron in Saharan soils, thus creating the phrase, ‘blood rain.’ Sometimes it can be noticed on the ground or cars after rain has fallen, leaving a reddish residue after the water has evaporated.

Last week, the articles getting excited about this phenomenon associated it with warm air over the UK originating from the Sahara. Indeed, going back to Sunday 21st October, there was a low pressure centred over eastern Spain, with associated southerly winds over the Mediterranean bringing warm air and dust from the Sahara to Europe, as shown by the surface pressure chart below.

An overpass of the CALIPSO satellite, with its onboard lidar, provides us with a vertical cross-section of the atmosphere over the French-German border on that day. The image below shows a layer of dust from the surface up to around 4km at latitudes of around 45-50 degrees north, shown by the yellow colouring here, confirming that there was dust in the air over Europe.

Accordingly, the ECMWF were forecasting a plume of dust to be transported from the Sahara over Europe for Monday 22nd October, part of which reached as far north as Kent and Suffolk in the UK:

For the part of this dust plume which made it as far as the UK, forecasts were for it to be deposited by rainfall during Monday and Tuesday (22nd and 23rd October) last week. There was a warm front over the UK on Tuesday, and Reading did receive a few millimetres of rainfall, but I didn’t see any sign of ‘blood rain’, and nothing seemed to be mentioned further by the press. It’s quite possible that the dust didn’t make it quite as far as the UK, or perhaps more likely that I wasn’t inclined to stand outside in the wet looking for redness in the rain. Did anybody else see any blood rain? The dust event over the UK certainly seemed to be a weak one, if it did indeed happen – for example, the Met Office describe a much larger event that affected the UK in June this year here.

Either way, I think the statement ‘blood rain for Halloween’ is a bit optimistic more than a week in advance of Halloween. As it is, quite a different weather phenomenon has stolen the Halloween show on the other side of the Atlantic…

The Indian summer monsoon of 2012

In many summers, a normal monsoon is forecast by the India Meteorological Department using their statistical model. 2012 was no different, and in the absence of large external drivers such as ENSO or anomalous snow cover over the Himalaya, the IMD early-stage forecast issued in April (model described here) suggesting that the chance of getting rainfall within 96-104% of the long period average (LPA) was 47%.

The onset of this year’s monsoon took place on 5 June, a short while beyond the forecast of 1 June but well within the normal range. Incredibly the onset as defined over the south-western Indian state of Kerala has a standard deviation of only 7-8 days, giving a clue to how strongly tied to the timing of the monsoon are the lives and work of the Indian population. The onset progresses in a north-westward direction to cover almost the whole country in monsoon rainfall by mid-July near the border with Pakistan.

In mid to late June the monsoon underwent a stall or hiatus, not advancing north of mid-Maharashtra between 6 and 16 June (see Figure 1), causing problems for agriculture.

Figure 1: IMD monsoon onset 2012

Figure 1: 2012 monsoon onset progression (green curves) and normal dates (red curves). Source: India Meteorological Department.

A further delay in advancing over the north of the country in late June led to widespread fears of drought after a very normal monsoon in 2011 should the poor rains continue.

The delay in the monsoon onset in mid- and late-June is reflected in Figure 2, which shows the daily rainfall totals averaged across the whole country. These breaks are part of the intraseasonal variability of the monsoon and are subject to considerable present research owing to their effects on agriculture (for example, the extended break in July 2002 in which the country received only 50% of its normal rainfall, even causing a drop in India’s GDP. From here on in, the monsoon looked like it was never going to recover.
Figure 2: daily rainfall totals for all-India 2012
Figure 2: Monsoon On-Line’s daily rainfall totals for India. Source: Indian Institute of Tropical Meteorology.

Fortunately, the IMD update their monsoon forecast from late June, when further information on the tendencies of spring-to-summer SSTs in the Pacific Ocean are available. Dynamical forecasts of ENSO conditions being made at that time were for neutral to weak El Nino conditions (see Figure 3), however the emergence of even weak El Nino conditions in the central rather than east Pacific can cause quite large deficits in the monsoon (e.g., in 2002, 2004 and 2009).

Figure 3: IRI's summary of Nino-3.4 ENSO forecasts through 2011-2012

Figure 3: Summary of IRI’s dynamical model ENSO forecasts for the last 22 months. Individual models are shown by the colours and observations are in black. Source: The International Research Institute for Science and Society.

However, conditions are currently neutral in the equatorial Pacific, with temperatures having cooled recently in the central region.

Quite a severe drought looked likely until September, and it wasn’t until 8 September that the accumulated rainfall total came back within normal limits (above the -1 standard deviation line, representing 90% of LPA rainfall) as in Figure 4.
Figure 4: cumulative rainfall totals for all-India 2012.
Figure 4: Monsoon On-Line’s cumulative daily rainfall totals for India. Source: Indian Institute of Tropical Meteorology.

The late surge in rainfall was aided by the eastward passage of the Madden-Julian Oscillation (MJO) along the equator in late August and early September seen by the blue contours near 80E in Figure 5, as its active phase brought enhanced convection to Indian longitudes.

Figure 5: MJO hovmoller

Figure 5: Time-longitude diagram of out-going longwave radiation (OLR) averaged over the northern tropics (2.5-17.5N). Blue regions show areas of reduced OLR, which is increased convection and shows the eastward progression of the MJO near the end of August 2012. Source: Matt Wheeler, the Centre for Australian Weather and Climate Research.

So the 2012 Indian summer monsoon season turned out to be 92% LPA, or 798mm rainfall against the usual 865mm when averaged over the country as a whole. This is much too small a deficit to be classed as a drought, but as one can see from the dramatic variability through the season in Figures 1, 2 and 4, thinking of the monsoon in terms of its seasonal mean area-average tells only part of the story.

Montserrat: There and Back Again

Whilst Reading plunged into a gloomy autumnal daze, a group of researchers namely Geoff Wadge, Antonio Costa and Thomas Webb, from the Department of Meteorology decided to replace their warm coats with T-shirts, their wellies with flip-flops and don excessive amounts of suncream, heading for the tiny island of Montserrat in the West Indies (figure 1) to observe Soufrière Hills volcano. Pyroclastic flows and explosive eruptions from this stratovolcano have caused widespread damage for the island including the destruction of the capital city of Plymouth in 1996. The lava dome continues to grow, occasionally venting ash ahead of another eruption; though it was quiet during this expedition.

Figure 1: Map showing where Montserrat is in the West Indies.
Figure 1: Map showing where Montserrat is in the West Indies.

Tasked with the job of building the largest ground based differential interferometric radar dataset on any andesitic volcano, GPRI2 was hired from Gamma Remote Sensing in Switzerland. Large amounts of surface heating, a ridge of hills and the Trade Winds provided the perfect conditions for sporadic orographic precipitation and deluges that sometimes led to intense but localised thunderstorms. Ingenious improvisation prevented damage to the expensive equipment occurring in the field – GPRI2 was prepared for lightning strikes but was not completely waterproof and so cables were ‘baked’ in a kitchen oven to remove some of the rain water and a large plastic sheet had to be held in place over the rotating antennae during high precipitation events . Despite teething problems with GPRI2, the phase change between two radar signals was measured from four different locations around the volcano each pointing towards the lava dome and for several hours at a time, each measurement taking one minute. An example of differential phase measurement received is shown in figure 2. This phase change may, in future, correspond to deformation of the volcano due to eruptive processes (the main topic of the study). The volcano is not in a deforming state and so this is unlikely to be the signal here. Alternatively this phase change is caused by the atmosphere – the variation of water vapour content around the volcano which changes the refractivity of each radar signal passing through it.

GPRI2 points towards volcano from observatory.
Differential interferogram of same field of view.

Figure 2: (Top) GPRI2 points towards volcano from observatory. (Bottom) Differential interferogram of same field of view where the GPRI2 is to the left of this image. Phase changes on the slopes of the hills surrounding the volcano and particularly around the volcano dome are associated with water vapour as we don’t think the volcano is deforming currently. ‘X’ marks the same spot in both images.

Unless glanced by a tropical storm (Lahar in Belham Valley, 14 October 2012 – MVO), strong Trade Winds mean for the majority of the time clouds and water vapour pile up on the Eastern side of the island. The background water vapour was measured during the field study by a hand-held sun photometer showing the column water vapour at a point and by automatic GPS measurements (figure 3). In addition to the meteorological water there is the magmatic water (sulphur dioxide, carbon dioxide, hydrochloric acid) emitted by the volcano’s plume which complicates the picture. Over 70% of the volcanic plume rising from the top of the lava dome is made up of water vapour, mixing with the Trade Winds and causing a disparity in leeward concentration (western). You know if you’re underneath the plume anywhere on the island because of the strong smell of sulphur.

Figure 3: Outline of Montserrat.
Figure 3: Outline of Montserrat – 18km long and 12km across. Squares represent GPRI2 measurements of volcano, triangles GPS measurements and circles are places where sun photometer measurements were taken. ‘V’ is the location of Soufrière Hills volcano and ‘MVO’ is the location of the Montserrat Volcano Observatory.

During the expedition a numerical model was left to run at the University of Reading – an effort to capture the turbulent mixing of water vapour using the WRF model at a resolution of 200m. The winds generated by this model used to provide high resolution estimates of water vapour concentrations to correct ground and future space based differential interferometric images. Extensive GPS and sun photometer precipitable water vapour datasets, built up during this trip, are being compared with this numerical model using a ray tracing algorithm adapted from computer gaming to account for non-zenith lines of sight. More about this project can be found here.

Arctic Sea ice decline and its impacts

The record minimum Arctic sea ice extent this year caused enormous public and media attention (link). The reducing ice cover puts at risk the way of life of northern communities, threatens polar bears and other aspects of Arctic ecosystems and also creates controversial opportunities for the energy and shipping industries. Combined with this the decline of Arctic sea ice is one of the most visually iconic images of climate change.

Summer sea ice extent 2012 (courtesy of the NSIDC)

The 2012 September sea ice minimum was 3.41 million square km, 16% less than the 2007 minimum. So it makes sense to ask: were the causes of these two minima the same? Looking at the synoptic conditions for summer 2007 one can see a dipole structure with anomalously high sea level pressure (SLP) conditions over Greenland and the Beaufort sea, and low SLP over Eurasia, which resulted in warm southerly winds warming much of the Arctic and encouraging melt (characteristic of the Arctic dipole anomaly). 2012 was quite different, much closer to the climatology; there was however a big storm over the Arctic which was thought to be responsible for some of the loss (link). Perhaps there is a role for ocean heat storage or other potential sources of persistence in the Arctic climate system.

Synoptic conditions in summer 2007 and 2012 (courtesy of NSIDC)

Climate change

While the weather plays a large role in rapid ice loss events as those in 2007 and 2012, such events are superimposed on a strong trend. September sea ice extent has been declining at a rate of 13%/decade since the 1970s. The observations famously lie outside the General Circulation Model (GCM) projections submitted to the Coupled Model Inter-comparison Projects (CMIPs) 3 & 5. According to Vladimir Kattsov, reasons for the mismatch include model insensitivity and vigorous natural variability (link). I think that both play a role. In particular I believe that as the Atlantic Multi-decadal Oscillation (AMO), which has moved from a positive to a negative phase since the 1970s, has accelerated the decline in sea ice extent (link). This is something that one would not expect a randomly initialised ensemble such as the CMIP ensembles to capture. Others see this apparent mismatch as evidence of a sea ice tipping point, which GCMs are too primitive to capture; this is a hotly debated topic! Either way, the models are telling us that the Arctic will be seasonally ice free in a matter of decades rather than centuries.

September monthly mean sea ice extent from the CMIP 3 and 5 integrations (Stroeve et al. 2012). The thick lines show the multi-model mean for CMIP5 (Red) and CMIP3 (Blue). The fans cover ±1 standard deviation about the mean.

Weather impacts

A major aspect of the sea ice story is the impacts on mid-latitude weather. In particular this is related to the Arctic amplification of global warming, whereby the Arctic warms preferentially compared to lower latitudes due to the ice albedo effect. The sea ice is complicit in this since it is rather susceptible to melting, in contrast to terrestrial ice, because it is relatively thin.

Arctic Amplification causes a reduction in the equator-pole temperature gradient. Francis and Vavrus believe that this is responsible for a reduction in zonal wind speed and increased Rossby wave amplitude (link). They suggest that this has slowed down the propagation of Rossby waves in the northern hemisphere and may have caused increased persistence of mid-latitude weather patterns causing drought, flooding and heat waves.

Judah Cohen and others suggest that Arctic warming has, somewhat paradoxically, caused widespread winter cooling of land temperatures over Eurasia (link). Observations show that summer and autumn warming trends are associated with concurrent increases in Eurasian October snow cover. They believe that this increase in snow cover dynamically induces this wintertime cooling.

Studies such as these provide growing evidence that Arctic sea ice plays a large role in both climate and weather. However processes such as rapid ice loss events and the impact of declining sea ice on our weather are not yet well understood. Given the importance of the Arctic in both geo-politics and climate it is not surprising that Arctic Sea ice has been described as “one of the great challenges of climate science”.

The ‘Oxfordshire Tornado’ of 7 May 2012

During the afternoon of Monday 7th May 2012, a tornado was reported in several areas of Oxfordshire, including Bicester, Eynsham, Kidlington and South Leigh. Trees, buildings, and vehicles in these areas were damaged by the storm which also produced large hail and lightning (BBC News). A number of eye-witnesses caught the tornado on film including one – Richard Glazer from Whitney, Oxfordshire – who drove through it as it was forming (or reforming) along the A34 near Kidlington (YouTube video). The photo below, one of the few to show a condensation funnel extending all the way to the surface, was taken from Shipton-under-Wychwood (source: Oxford Mail).

The map below shows the approximate track of the storm which spawned the Oxfordshire tornado – black markers show locations where a tornado was observed as stated in the BBC (and other) news coverage.

This storm was a special kind of thunderstorm, known as a supercell. These storms are characterised by the presence of a mesocyclone: a deep, rotating updraught. In addition to the basic ingredients for ordinary thunderstorms (instability, moisture, and an initial lifting mechanism), supercells require the presence of strong vertical wind shear in the lower troposphere in order to form. This shear results in the generation of horizontal vorticity which can then be tilted into the vertical plane by the storm updraught causing the latter to rotate. The 1200 UTC (1300 BST) sounding from Nottingham (340 km north of Bicester) shows a highly favourable wind profile for supercells, with significant turning of the wind in the lowest 3-km of the atmosphere. However, there is no mid-level instability at this location due to a strong temperature inversion at 850 hPa. This feature is associated with warm, moist air overrunning a warm front to the south. Soundings from the UKV model taken behind this front (in the area where the storm formed; not shown) reveal the presence strong instability below 450 hPa (~6 km) with several hundred J/kg of Convective Available Potential Energy (CAPE).

Observations from the UK radar network can be used to verify that the Oxfordshire storm was indeed a supercell. The left panel of the figure below shows a snapshot at 1530 UTC of the radar reflectivity (at 1° elevation) from the Chenies radar (courtesy of Chris Westbrook). This is compared on the right with a schematic picture of a “classic” supercell from the Oklahoma Climatological Survey. The similarity is striking. Perhaps most significant is the presence of the so-called “hook echo” (albeit less well defined than in the schematic) – this feature appears when rain and hail wrap around the mesocyclone at low levels.

More direct evidence of a mesocyclone is provided by images of Doppler velocity from the radars. These are created by measuring the phase shift between successive radar pulses which can be related to the movement of the targets (in this case rail and hail) in the direction parallel to the radar beam. The measured distances are then combined with the pulse repetition frequency (PRF) to determine the radial velocity of the targets. Images of this quantity at 1530 UTC from the Chenies (left) and Deanhill (right) radars at 1° elevation are shown below (again courtesy of Chris Westbrook). Positive values indicate flow towards the radar and negative values indicate flow away from the radar. Strong “velocity couplets” – adjacent areas of inbound and outbound flow – can be seen in both images indicating the presence of rotation. Similar scans from other elevation angles (not shown) confirm the existence of a deep, rotating updraught.

From a meteorological perspective, an obvious question regarding this case is “was it forecast?” Unfortunately, we were unable to obtain information regarding the Met Office forecasts for this day, although it does seem that the UKV model successfully produced some supercell-like storms in about the right location (Nigel Roberts, personal communication). Aside from the Met Office, there are two other main sources of severe weather forecasts for the UK: the European Storm Forecast Experiment (ESTOFEX) and the Tornado and storm Research Organisation (TORRO). ESTOFEX produces daily forecasts of severe convective weather (large hail, excessive rainfall, damaging straight line winds, and tornadoes) for the whole of Europe. On 7th May 2012, the forecasters issued a Level 1 (5–15 % probability of a severe convective storm within 40 km of a point) for much of Wales and the east-central Midlands (left panel below), i.e. slightly west of where the tornadic storm developed. In the associated discussion they noted the potential for “multicells and low-topped supercells with a chance of small hail, marginally severe wind gusts and maybe an isolated tornado”.  TORRO, which issues “watches” for severe weather in the UK and Ireland, also highlighted the potential for tornadoes – their risk area (shown in the right panel below) agrees very well with the observed track of the Oxfordshire supercell. However, it is not currently known at what time this forecast was issued or the details of the accompanying discussion. We will update with this information if/when it becomes available to us.

Tornadoes are not considered particularly rare in the UK – indeed, it is frequently said that the UK gets more tornadoes per square kilometre than the USA. However, the majority of these are associated with the passage of strong cold fronts during the cool-season (Bolton et al. 2003). In contrast, the 7 May 2012 tornado formed from a long-lived “classic” supercell, a rarity in this country due to the infrequent superposition of strong instability and vertical wind shear*. This event has therefore drawn significant interest in the severe weather community and will no doubt be the subject of future research articles. In our department, Chris Westbrook (in collaboration with Matthew Clark at the Met Office) is writing a paper for Weather focussing on the radar observations of the storm, while Humphrey Lean is carrying out very high resolution simulations of the event using the Met Office Unified Model. Expect to hear a lot more about this event in the future! * On the day this Blog contribution was being prepared (Thursday, 28 June 2012) a series of severe storms developed in the English midlands, producing torrential rain, very large hail, and at least one tornado. Based on examination of radar images we believe that several of these storms were supercells. This will be another exciting case to look into in detail.