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 snow lying increase with latitude and altitude, so values reflect topography. Snow is comparatively rare near sea level in Wales, but much more frequent over the hills. The average number of days each year when sleet or snow falls varies from 10 or less in south-western coastal areas to over 40 in Snowdonia. Snow rarely lies on the ground at sea level before December or after March, and the average number of days with snow lying in Wales varies from 5 or less around the coasts to over 30 in Snowdonia. 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 Swansea and Loggerheads 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 Swansea and Loggerheads have been estimated from 1 km grids (to the nearest day), as these stations are 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 very severe winter of 1962/63. Even places near the coast experienced prolonged snow cover during this winter.
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 rare, but one of the worst snowstorms in 100 years occurred in South Wales on 8-10 January 1982. Depths in excess of 30 cm were commonplace even at low levels, with severe drifting in the strong to gale force easterly winds resulting in power lines being brought down and some roofs collapsing due to the weight of snow.
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