The occurrence of snow is linked closely with temperature, with falls rarely occurring if the temperature is higher than 4 °C. For snow to lay 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 brief falls in October and May. Snow rarely lies outside the period from December to March.
On average, the number of days with snow falling is about 20 per winter in the lower lying areas, particularly the lower Severn valley. An average of about 35 days is typical of upland areas in the north and near the Welsh border. An average increase of about 5 days of snow falling per year per 100 metres 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 cold (easterly) and warm (south-westerly) air masses. The number therefore varies from about 6 days per year in the lower Severn valley to over 20 days in upland areas such as the Peak 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.
The monthly averages of days with sleet/snow falling and lying at Brize Norton and Keele 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 Keele 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 Birmingham there were 75 days with snow lying in the unusually cold winter of 1962/63 but only 2 days in winter 1960/61.
The depth of undrifted snow does not often exceed 15cm 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 8/9 January 1982 when 36 hours of snow accompanied by easterly gales gave 30 to 50 cm of level snow and drifts that were locally over 1metre deep. Another example was 7/8 December 1990 when much of the Midlands had snow 20 cm deep, with over 40 cm in the Birmingham area. Strong NE winds whipped the snow into drifts 2 metres deep, resulting in considerable transport disruption.
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