Many Irish winters are free from major snowstorms, but because of its infrequent and irregular occurrence, snow in large quantities causes serious disruption. It can completely disrupt traffic, close airports and seriously damage overhead power lines and communication lines.
Many Irish winters are free from major snowstorms, but because of its infrequent and irregular occurrence, snow in large quantities causes serious disruption. It can completely disrupt traffic, close airports and seriously damage overhead power lines and communication lines.
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 (Appendix 1). Extreme western and south western areas have only about 10% 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 in Appendix 2.
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 meters 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.
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 crystals
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
