The occurrence of snow is linked closely with temperature, with falls rarely occurring if the temperature is higher than 4 °C. The numbers of days with snow falling and lying show an increase with increasing latitude and altitude, so values reflect topography. Snow is comparatively rare near sea level in Northern Ireland, but much more frequent over the hills. The average number of days each year when sleet or snow falls varies from around 10 near the east coast to over 35 in the mountains of Sperrin, Antrim and Mourne. Snow rarely lies on the ground at sea level before December or after March, and the average annual number of days with snow lying in Northern Ireland varies from less than 5 around the coasts to over 30 in the mountains. These averages can be compared with parts of the Scottish Highlands, which have about 60 days with snow lying on average and with the coasts of SW England, with less than 3 days per year.
The monthly averages of days with sleet/snow falling and lying at Aldergrove and Parkmore Forest are shown below (a day of lying snow is counted if the ground is more than 50 % covered at 0900). Days of sleet/snow falling for Parkmore Forest have been estimated from 1 km grids (to the nearest day), as this station is not manned 24 hours a day.
The number of days of snowfall and snow cover varies enormously from year to year. At many places in the last 50 years it has ranged from none at all in several winters to in excess of 30 days during the winters of 1962/63 and 1981/82. Even places near the coast experienced prolonged snow cover during these two winters. In heavy snowfalls there can be quite extensive drifting of the snow in strong winds, especially over higher ground, resulting in severe dislocation to transport and power supplies. Fortunately such occasions are comparatively rare. One recent example was 27 February 2001 when strong NE winds and heavy snow caused travel disruption for up to 5 days and brought down power lines (resulting in power cuts to 70,000 homes), mostly in Counties Antrim and Down.
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