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 then 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 December to March.
On average, the number of days with snow falling is less than 10 per winter in the Hebrides and about 20 near the Ayrshire and Galloway coast. Over 60 days is typical over the highest ground of the west Highlands. The number of days with snow lying has a similar distribution, with less than 5 in the islands and along the coast but over 35 days over the higher ground. These averages can be compared with the coasts of SW England where less than 3 days per year with lying snow.
The monthly averages of days with sleet/snow falling and lying at Eskdalemuir and Tiree are shown below (a day of lying snow is counted if the ground is more than 50% covered at 0900).
Occasionally, a combination of snow and strong winds can cause transport disruption. For example, frequent snow showers on 13 January 1984 in a very cold westerly airstream resulted in undrifted snow 20-30 cm deep in Argyll and over 40cm at higher altitudes in the Southern Uplands. Strong winds and further snowfalls later in January 1984 led to considerable drifting with road transport and trains stranded. Level snow lay 25-30 cm deep at lower levels inland whilst the greatest depth reported was a remarkable 112 cm at Leadhills (393 metres) in the Southern Uplands.
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