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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, 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 five
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 Isles of Scilly and coastal Cornwall and
Devon less than three 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 five 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,
Aberdeenshire, which has about 60 days with snow lying on average.
Table 2 gives some station data for 1971-2000.
Map of annual days of snow lying at 0900 (1971-2000)
Table 2. Monthly averages (days) of snow falling and lying
for 1971-2000 at Long Ashton (altitude 51 metres), Yeovilton
(altitude 18 metres) and St Mawgan (altitude 103 metres).
|
Snowfall
|
| |
St Mawgan
|
Long Ashton
|
Yeovilton
|
| |
Falling*
|
Lying**
|
Falling*
|
Lying**
|
Falling*
|
Lying**
|
| November |
0.4
|
0.0
|
0.3
|
0.1
|
0.4
|
0.0
|
| December |
1.4
|
0.1
|
1.0
|
0.6
|
1.3
|
0.2
|
| January |
2.9
|
0.5
|
2.6
|
1.9
|
2.0
|
1.3
|
| February |
3.0
|
0.4
|
2.8
|
1.2
|
3.0
|
0.6
|
| March |
2.1
|
0.1
|
1.9
|
0.2
|
1.5
|
0.0
|
| April |
1.2
|
0.1
|
0.7
|
0.0
|
0.7
|
0.0
|
| May |
0.2
|
0.0
|
0.2
|
0.0
|
0.1
|
0.0
|
| Annual |
11.2
|
1.2
|
9.2
|
4.0
|
9.0
|
2.1
|
|
*Includes any incidence of snow or sleet falling.
**A day of lying snow is counted if the ground is more than 50% covered at 0900.
Table 2 shows 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 and over 90 cm on Dartmoor and Exmoor. On 8/9 January
1982 conditions were especially severe in the Bristol area where
drifts were locally one metre 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
Isles of Scilly.
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section - Wind
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