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 may have occasional falls in October and May. Snow rarely lies on low ground outside the period from November to March but over higher ground lying snow can also occur in October and as late as May.
On average, the number of days with snow falling is about 15-20 per year on the Isle of Man and from 20 to 30 days in lower-lying parts of the mainland but as much as 60 days over the highest ground. An average increase of about 5 days of snow falling per year per 100 m increase in altitude has been found typical.
The number of days with snow lying is also mainly dependent upon altitude but partly upon proximity to the sea. The number therefore varies from less than 5 days per year on the Isle of Man to around 3-7 days on the Cumbrian and Lancashire coasts and over 30 days in upland areas such as the Pennines and Lake District. 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. In most places, January is the month with most days of both snow lying and snow falling closely followed by February.
The monthly averages of days with sleet/snow falling and lying at Ringway and Malham Tarn 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 Malham Tarn have been estimated from 1 km grids (to the nearest day), as this station is not manned 24 hours a day.
Snowfall is, however, highly variable from year to year. For example, at Carlisle there were 26 days with snow lying in the unusually cold winter of 1962/63 but none in winter 1963/64.
The depth of undrifted snow does not often exceed 15 cm at low altitudes but on occasions depths of 30 to 60 cm may occur over a wide area. When depths exceed 15 cm in association with strong winds, serious drifting may occur, especially in hilly areas, leading to widespread travel disruption. A notable example was the blizzard of 5-6 February 1996 when more than 15 cm of snow fell in low-lying parts of Lancashire and drifts were up to 2 metres deep.
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