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 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.
The degree of exposure to northerly winds is critical, and the North York Moors can receive nearly as much snow as the higher ground of the Pennines. Often this is of a showery nature, triggered by the passage of a cold airstream over the North Sea. On average, the number of days with snow falling is about 20 per year on the coast and low lying areas inland and as much as 50 days over the higher Pennines. An average increase of about 5 days of snow falling per year for every 100 metres increase in altitude has been found to be 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 about 10 days per year on the coasts to over 40 days in the higher Pennines. 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 Leeming and Whitby 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 Whitby have been estimated from 1 km grids (to the nearest day), as this station is not manned 24 hours a day.
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. Notable examples affecting NE England include the prolonged, heavy snowfalls of 3-5 February 2001 and 25 February 2005.
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