The occurrence of snow is linked closely with temperature, with falls rarely occurring if the temperature is higher than 4 °C. For snow to lie for any length of time, the temperature normally has to be lower than this. Over most of the area, snowfall is normally confined to the months from November to April, but upland areas often have brief falls in October and May. Snow rarely lies at lower levels outside the period from November to April.
On average, the number of days with snow falling varies from less than 40 per year along the west coast to over 100 days over the Grampians. The number of days with snow lying has a similar distribution, with less than 6 over the westernmost islands, about 20 in Shetland and more than 50 days over the higher ground. On the highest summits, such as Ben Nevis, snow cover typically persists for 6 or 7 months each year.
The relatively plentiful supply of snow over the highest ground has led to the development of skiing areas such as Glencoe and the Nevis Range (Aonach Mor), both near Fort William, and the Cairngorms near Aviemore.
The monthly averages of days with sleet/snow falling and lying at Stornoway and Aviemore are shown below (a day of lying snow is counted if the ground is more than 50% covered at 0900).
Heavy snowfalls can cause travel and power supply disruption. Examples include 28-29 January 1978 when strong NE winds and persistent snow lasting over 24 hours resulted in 40-60 cm of level snow over a wide area with drifts several metres deep. This brought severe disruption to road and rail transport and to power and telephone services, with some loss of life.
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