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Dublin: 7 °C Sunday 21 September, 2014

Everything you ever wanted to know about snow in Ireland

Met Éireann’s Aidan Murphy explains the patterns of snow in Ireland, why it’s hard to forecast – and the statistical chances of a white Christmas.

Dublin Civil Defence members in snow drifts in Wicklow on 13 January 2010.
Dublin Civil Defence members in snow drifts in Wicklow on 13 January 2010.

AIDAN MURPHY IS a meteorological expert at Met Éireann in Glasnevin Hill, Dublin 9. He wrote this comprehensive analysis of how snow affects Ireland as a response to the “frequent” calls the national meteorological service receives, especially this time of year, on the subject. You can see it in full here on the Met Éireann site but Murphy has kindly allowed TheJournal.ie to bring you highlights of his essay.

When and where is snow most likely to fall in Ireland?

January and February are the months in which snow is most frequent but it’s not uncommon to have  snow in any of the months November to April. Snow has been reported in May and September. On  some of these occasions the falls have been considerable but the snow melted quickly. Generally snowfall in Ireland lasts on the ground for only a day or two. Some of the more notable snowfalls in  recent times had snow lying on the ground lasting from 10 to 12 days.

The number of days with snow cover tends to increase northwards through the Midlands corresponding to the decrease in winter air  temperatures. During the winter, sea temperatures are warmer than land which can often lead to rain around the coasts but snow a few miles inland. Rain showers may fall as snow on higher ground as  temperature generally decreases with altitude. The number of days with snow cover is quite variable from year to year.

Systematic records of snow depths have been made at Synoptic stations in Ireland since 1960. Based  on these records an analysis is made of the snowfall patterns over the country. The mean annual number of days with snow varies from 5 in the extreme southwest to 24 in the North Midlands (Click here for full-screen table).

Extreme western and south western areas have only about 10 per cent of the number of hours with significant snow depths as inland locations. Coastal areas have, on average, one to two days in  the year with depths of 1cm or more, with no significant snowfalls over many years. Inland stations  have, on average, up to 10 days with depths of 1cm or more in the year, while the higher reaches of  the Wicklow mountains have an annual average of up to 30 days. A fall of at least 2cm is likely in most places about every two years while falls of at least 10cm occur every 7  to 18 years at midland  locations, and in the north midlands about once every 6 to 7 years.

The greatest depth of snow recorded at our Synoptic stations was 45cm at Casement Aerodrome, during the winter of 1962/1963. A map displaying the maximum snow depths at 100m above mean  sea level with a 50-year return period is shown here:

This shows the dates on which most snow was recorded at various weather stations (Click here to see full-screen table):

A marked feature of snowfall in Ireland is its variation in depth from place to place. Some heavy  snowfalls can be quite localised. A deep fall of snow can be short lived if it is followed by a sudden  influx of warm air from the Atlantic. Drifting complicates measurements of snow depths. Drifts of six metres or more have been reported in hilly areas. Even in flat countryside, noticeable drifting can  occur especially near buildings or fences. Therefore depths are measured at points judged to be relatively free of drifting. Fresh snow to a depth of 30cm is approximately equivalent to 25mm of rain.

Why is it difficult to accurately forecast snowfall in Ireland?

Often snow that falls from a cloud melts as it descends and reaches the ground as rain. However the melting process extracts latent heat from the surrounding air, causing the air temperature to cool and making it increasingly likely that the subsequent snow will reach the ground. The ideal conditions for snow are temperatures close to and just below zero, rather than colder temperatures. This is because the warmer the snow, the more moisture it contains and hence the bigger the flakes will be. A temperature close to zero facilitates the melting of snow, refreezing and the combination into larger flakes. Consequently very slight changes in temperature can mean the difference between rain and snow. This makes
accurate forecasting of snow particularly difficult in Ireland.

Snow can settle on the ground in different forms, depending on wind, temperature and humidity. Air temperatures well below freezing produce small powdery flakes. Snowflakes that form closer to 0°C are larger and wetter and tend to stick to surfaces.

In winter, snow often occurs when two different air masses collide, the cold continental air from the north or east meets the relatively mild moist maritime air from the south or west. Different parts of Ireland are more affected with snow falls that are associated with particular air masses. Snowfalls in the Northwest and West are most commonly associated with Polar Maritime and Arctic airstreams.

These airstreams bring in frequent snow showers, due to convective activity and instability over the relatively warm seas.  Occasionally significant snow falls under anti-cyclonic conditions in an easterly Polar Continental airflow, for example in February 1947. Under these conditions Eastern and Midland areas are more affected. Eastern and North-Eastern areas are prone to unstable north-easterly winds blowing onshore from relatively warm waters of the Irish Sea which can often produce significant snowfalls, as occurred in January 1987, 2009 and 2010.

How often have we actually had a White Christmas?

In modern times, snow is synonymous with Christmas. Christmas cards, songs and scenes from the Victorian period, notably in Charles Dickens’s novels, all portray snow falling at Christmas. The origins of a White Christmas may originally have come from the ‘Little Ice Age’ that occurred during the period 1550-1850.

In Ireland snow occurs most frequently in the months from December to March. Countrywide snow fell on 17 Christmas Days, at a least one of our Synoptic stations, since 1961 (1961, 1962, 1964, 1966, 1970, 1980, 1984, 1990, 1993, 1995, 1998, 1999, 2000, 2001, 2004, 2009 and 2010). There were nine Christmas Days (1964, 1970, 1980, 1993, 1995, 1999, 2004, 2009 and 2010) with snow lying on the ground at 09 am in the morning, during this period. The maximum depth of snow ever recorded on Christmas Day was 27cm at Casement Aerodrome in 2010.

There were 200 days with snowfall in December over a 70 year period at Dublin Airport. Snow fell on Christmas day on 12 days (1950, 1956, 1962, 1964, 1970, 1984, 1990, 1993, 1995, 1999, 2000, and 2004) since records began there in 1941. The maximum depth of snow recorded at Dublin Airport on Christmas day was 20cm in 2010.

This was also the only Christmas Day with snow lying on the ground at 9am in the morning since records began in 1941.

For many people, a White Christmas means a complete covering of snow on the ground. However, sometimes the definition used by bookmakers is for a single snow flake to be observed falling during the 24 hours. The statistical likelihood of snow falling at Dublin Airport is approximately once every 5.9 years, as snow fell on Christmas day 12 times in the last 71 years. The accuracy of forecasting snow falling on Christmas Day falls significantly beyond about 5 days.

This satellite image from Met Éireann on Christmas Day 2010, one of the coldest days ever recorded, shows Ireland almost completely covered by snow and ice:

Severe winters in Ireland of the past:

There are many historical references to severe winters in Ireland. A huge snowfall which lasted three months is reputed to have occurred around 764 A.D., and in 1433/1434 Ireland suffered another severe winter. There was a great snow in 1635 (Boate, 1652). From the late 17th century onwards weather diaries and newspapers provided information on the weather. From 1800 onwards meteorological observations were made regularly at an increasing number of locations. Daily observations commenced at the Phoenix Park, Dublin in 1829.

The following is a record of only the most outstanding snowfall events in the past two centuries. Many other events may have merited inclusion but records are scant.

Outstanding storms of the 19th century:

1807 On 19th and 20th November, a disastrous blizzard swept the country and many people were killed. Two transport ships were wrecked on the east coast. Heavy snow prevented the crews from realising how close they were to land. Records at the Phoenix Park detail heavy falls of snow during the winter and many people died.

1831, 1836-8 Records at the Phoenix Park detail heavy falls of snow during these winters.

1853 In a violent snowstorm on the 14th February a ship, the “Queen Victoria”, struck rocks off Howth Head with a loss of 55 lives.

1855 February was a cold month at the Phoenix Park, with snow on the ground from the 7th to 23rd.

1881 The records at the Phoenix Park, Dublin recorded remarkable snowstorms in January (O’Reilly, 1981).

1886 A great blizzard with snow depths up to 60cm struck Northern Ireland. Later between April 7th and 10th there was heavy snow, especially in the Tipperary area.

1891/92 This winter saw snowfalls which were greater than those previously recorded. Railway traffic was seriously disrupted in the third week of February. Snow to a depth of 46cm was recorded in Cork, the greatest fall since 1855.

1895 Heavy falls occurred in February, particularly in the West and South.

During the last 25 years of the 19th century the winters of 1878/79, 1880/81 and 1894/95 were very cold. Over the Ireland and Britain the winter of 1878/95 was one of the most persistently cold and snowy in fifty years (Bonacina, 1928)

Outstanding storms of the 20th century:

1908 Leinster was affected by heavy snow in late April.

1909/10 The bulk of this winter’s snowfall over the British Isles came in two severe spells, the first between 15th and 22nd of December and the second between 25th and 31st of January. A depth of 33cm was reported in Sligo in the December snowfall. The January spell was more severe and counties in Munster, notably Cork, Kerry and Clare were snow covered to a great depth for several days.

1917 The most severe snowfalls of this century and probably of the last two centuries  occurred. On the 24th January large quantities of rain, sleet and snow accompanied the south easterly gale in the south of Ireland. At Ballinacurra near Cork the measurement (of snow when melted) on the 24th was 52mm and on the 25th 19mm.  At Seskin the total amount of snow on the 25th and 26th yielded, when melted, 47mm of water.

On the 25th, the wind strengthened to a gale in the south of Ireland, when there were heavy falls of snow covering the ground to 30cm or more, with drifts of 300cm or more. Over a large area of Ireland railway traffic was stopped owing to the heavy snow.

During the period 28th January to 3rd February, the low maximum temperatures prevented the snow which had fallen during the preceding week from thawing to any considerable extent. Little fresh snow fell during the week.

East Clare experienced a great snowstorm on 1st April. Snow on level ground lay to a depth of 46cm. The greatest previous snowstorm remembered in the area occurred on February 19th 1892 when depths of snow measured 13cm.

1933 On 23rd February a small depression appeared in the polar current over the extreme north of Ireland. This disturbance moved south and increased in intensity. Snow began in the west on the 23rd and spread eastwards during the 24th. Widespread and heavy snowfalls were accompanied by strong squally winds. Snow depths of 30 to 60cm, with deep drifts, were reported. Heaviest falls were in the south and midlands.

At 4pm on the 24th in Broadford, Co Clare, the snow was 30cm deep where it had not drifted. At Hacketstown, Co Clare, drifts of up to 300cm were reported.

1947 The early months of 1947 saw one of the most persistent cold spell of the century, with snowfalls affecting all parts of the country from late January until mid-March. Although heavier individual snowfalls have been recorded, notably in January 1917, at no other time in the recent past has there been such a period of continuous cold weather.

Following the disastrous harvest of 1946 and the extension of wartime rationing of food and fuel, the severe weather caused hardship for many people and disrupted the country’s communication and transport facilities for several weeks. By the beginning of February there were reports of skating on frozen ponds and the unrelenting cold continued until the middle of March.

1951 Considerable snow fell on the 8th March in midland and eastern areas and was succeeded by a spell of cold easterly winds. Mullingar recorded a depth of snow of 15cm.

1955 A very cold northerly or easterly airstream dominated the country from the 10th to 25th February giving wintry showers and outbreaks of snow with prolonged periods of icy roads. There were 10 consecutive days with snow lying at Dublin Airport from 18th to 27th February where a depth of 13cm was recorded on the 22nd and 25th February.

1958 A cold north-westerly airflow set in on the 19th January, giving wintry showers, especially in the Northwest and west Munster. Malin Head recorded a depth of snow of 20cm on the 21st February. A depth of 17cm was recorded at Belmullet on the 24th,  the greatest depth of snow recorded at this station.

1960 Snow fell countrywide on a large number of occasions in February. Dublin Airport had 9 days with snow lying from the 11th to 19th February where a depth of 11cm was recorded on the 13th February.

1962/63 This winter was one of the most severe in recent times. The winter of 1963 was the coldest of the twentieth century. The second coldest was 1947, when more snow fell, but average temperatures were not as low. Bitterly cold weather set in around the Christmas period and persisted with only brief milder periods until early  March.

During this period easterly winds were directed over Ireland by a large Scandinavian anticyclone, with occasional depressions bringing falls of snow, some of which were heavy. On the morning of the 31st December 1962, a depth of 45cm of snow was recorded at Casement Aerodrome in an area where there was no significant  drifting.

1973 Widespread snow fell during the period 14th to 17th of February, heaviest in the Midlands. A snow depth of 25cm was recorded at Clones, Co Monaghan.

1977/78 This winter had some notable snowfalls. Snow fell in most places in the period 8th–20th February. The south and southeast were most affected, particularly on the 18th and 19th February when heavy falls of snow accompanied by strong winds contributed to the formation of large drifts. A depth of 26 cm was recorded at Cork Airport, the greatest depth recorded at this station.

1978/79 Appreciable falls of snow between 28th and 31st December 1978 were followed by frosts of unusual severity. This cold spell ended on January 6th but there were further snowfalls later in the month. The highest depths of snow recorded during this spell were Casement Aerodrome 26cm, Claremorris 16cm and Cork Airport 15cm.

1982 On 8th January there was widespread snow, heaviest in the East, where there was considerable drifting due to strong easterly winds. A severe cold spell followed and snow remained on the ground until 15th January. Dublin was badly affected. Snow was reported at most synoptic stations with the greatest depths as follows: Dublin Airport 25cm, Casement Aerodrome 16cm and Kilkenny 16cm.

1987 This spell started on the 11th January. By the 14th, appreciable depths of snow were reported particularly in the East and Midlands. Moderated north- easterly winds caused drifting. Temperatures rose a little above zero on the 15thand a slow thaw set in. Highest snowfalls recorded were as follows:  Dublin  Airport 19cm; Casement Aerodrome 12cm; Birr 12cm; Mullingar 12cm.  Roches Point recorded its highest  ever depth of snow at 12cm and a minimum temperature of -7.2 degree Celsius, the lowest there since records began in 1867.

Outstanding snowfalls since 2000:

2000 On the 27th December a shallow polar depression crossed the north of the country, bringing outbreaks of snow, heavy in parts of the west and north. Snow showers were widespread in all but the southeast on the 28th, giving significant accumulation of snow in many places. A depth of 19 cm was recorded at Knock Airport.

2001 Bitterly cold northerly winds brought falls of snow on the 26th–28th February, heaviest in the north and east. Snow depths up to 10cm were recorded in the east and northwest, 75cm of snow was measured in the Mourne Mountains on the 27th February.

2009/10 This was the coldest winter since 1962/3, temperatures were around two degrees below average. There were between 20 and 30 days with snow in many places, mainly in the form of showers, but snowfall accumulations were generally slight except on high ground.

2010/11 Following the middle of November 2010, the weather turned progressively colder. By the end of the month, there were accumulations of snow over most of the country, accompanied by extremely low temperatures. Both Dublin Airport (-8.4°C) and Casement Aerodrome (- 9.1°C) had their lowest November temperatures on record on the 28th. The very cold weather continued into early December with further sleet and snow, accompanied by daytime temperatures close to freezing and night-time values dropping below -10°C (-16°C at Mount Juliet on 3rd).

After an improvement in temperatures for 5 or 6 days, although still cold, it became extremely cold again from 16th with snow at times leading to significant accumulations and record low December temperatures. Snow depths of between 10 and 25cm were recorded at many locations. Casement Aerodrome recorded a depth of 27cm.

Ireland in the big freeze of 2010:

A frozen Sandymount Strand. Pic: Julien Behal/PA Wire

Grand Canal Dock froze over. Pic: Julien Behal/PA Wire

Coping with black ice. Pic: Julien Behal/PA Wire

Chaos in rush hour traffic. Pic: Julien Behal/PA Wire

Civil Defence members helping to release a family who had been trapped in their Wicklow home by snow for three weeks. Pic: Julien Behal/PA Wire.

One of the MacDhomhnaill children who the Civil Defence were helping in the previous photo. Pic: Julien Behal/PA Wire.

Treacherous conditions in Cavan town. Pic: Niall Carson/PA Wire.

Building an igloo by the frozen Grand Canal. Pic: Niall Carson/PA Wire.

Copse of trees, The Curragh. Pic: Eamonn Farrell/Photocall Ireland.

Athgarvan village. Pic: Eamonn Farrell/Photocall Ireland.

A Victorian graveyard at Connellmore Crossroads outside Newbridge. Pic: Eamonn Farrell/Photocall Ireland.

A Dublin Airport at a standstill. Pic: Sasko Lazarov/Photocall Ireland.

Struggling through the snow. Pic: Sasko Lazarov/Photocall Ireland

Postman Michael Deegan carries on with letters. Pic: Eamonn Farrell/Photocall Ireland.

A woman walks her dog on the snow covered embankment overlooking the Bogside in Derry on Christmas Eve. Pic: Laura Hutton/Photocall Ireland

References
G.O. Reilly (June 1988). Snowfall in Ireland.
P.K. Rohan (1986). The Climate of Ireland, Meteorological Service, Dublin.
F.E. Dixon (1953). Weather on Old Dublin, Dublin Historical Record.
L.W.C Bonacina (1928). Snowfall in the British Isles.
Peter Lennon. Editor of Monthly Weather Bulletin, Met Éireann.
Snow Loading Calculations for Ireland (March 2010), produced by Liam Keegan, Met Éireann

See here for news reports on the 2010/11 snow and ice>

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