The occurrence of snowfall 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. In the south west snowfall is normally confined to the months from November to April, though upland areas may have brief falls in October and May. Snow rarely lies outside of the period from December to March.
On average, the number of days with snow falling is less than 10 per winter in the islands and coastal districts of Devon and Cornwall, though slightly higher near to the upper Severn Estuary. About 8-15 days is typical of inland areas with the higher figure in the north east near to the border with Wiltshire. In upland areas such as Exmoor and Dartmoor more than 25 days per year are possible. A rough average increase of about 5 days of snow falling per year per 100 metres increase in elevation has been found typical in the south west.
Lying snow is not a frequent occurrence in the south west, and in lowland areas about a third of years do not record any snow lying. In the Scillies and coastal Cornwall and Devon less than 3 days per year is a typical average. This increases to 5-10 days per year inland with the higher figure applying to north eastern areas. Altitude is significant and a rough guide is an increase of about 5 days per year with snow lying per 100 metres increase in elevation. Dartmoor and Exmoor have more than 20 days per year. Compare this with Balmoral, Grampian which has about 60 days with snow lying on average.
The monthly averages of days with sleet/snow falling and lying at St. Mawgan and Yeovilton are shown below (a day of lying snow is counted if the ground is more than 50 % covered at 0900).
The graphs show that the coastal site at St Mawgan has a similar incidence of falling snow to Yeovilton which is inland and considerably further east. This demonstrates the effect of altitude since St Mawgan is the higher site.
Despite being the mildest region of the British Isles, the south west peninsula has experienced some of the most severe blizzards to affect the country. This is because of a set of circumstances that affect the UK only rarely. If bitterly cold easterly winds from Europe meet a slow moving Atlantic depression situated off south west England then a prolonged snowstorm accompanied by high winds is possible. An example is 18/19 February 1978 when fine snow accompanied by 25 knot easterly winds at temperatures as low as -2 °C deposited around 50 cm of snow in inland Devon. On 8/9 January 1982 conditions were especially severe in the Bristol area where drifts were locally 1 m deep. Another event, this time from convective showers, on 12 January 1987 produced snow depths of 35 cm (Falmouth), 39 cm (Penzance) and even 23 cm on the Scillies. There was further snow in November 2005, which resulted in traffic disruption on the A30 across Bodmin Moor.
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