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The south-west peninsula forms the most southerly and westerly
part of the British Isles. The sea which surrounds the area
on three sides has the highest annual mean temperature of any
sea area near to the UK, close to 11 to 12 °C. With winds
mainly blowing from the sea the annual mean temperatures are
close to this in coastal areas of Cornwall and the Isles of
Scilly. Compare this with the lowest UK annual mean temperature
for low-lying areas of about 7 °C in the Shetland Isles.
The annual mean temperature tends to fall towards the north-east
of the region, where 10 °C would be more typical.
The strong maritime control of temperature is best seen in
the annual temperature range, or the difference between the
mean temperature of the warmest and coldest months. In west
Cornwall this is about 9 °C, which is similar to that found
in western Ireland or the Outer Hebrides, but it increases to
about 12 °C in the north-east of the region, and about 14
°C in the English Midlands.
Away from the coast, altitude is the main factor affecting
temperature. The mean temperature decreases with height with,
for example, Princetown at 414 metres on Dartmoor having an
annual mean temperature of about 8 °C.
Temperature shows both a seasonal and a diurnal variation,
but due to the modifying effect of the sea the range is less
than in most other parts of the UK. The sea reaches its lowest
temperature in February or early March so that on average February
is the coldest month in Cornwall and Devon. In this month the
mean minimum temperature varies from rather more than 5 °C
in the Isles of Scilly to around 1.5 °C in inland Devon.
Further north-east in Somerset and north Somerset, where the
influence of the sea is less, January is the coldest month with
mean minimum temperatures between 1 and 2 °C.
The strong maritime influence normally prevents very low temperatures,
but surprisingly low minima have been recorded occasionally.
In coastal regions and in the islands the lowest temperatures
are found when a strong, cold easterly wind ousts the normal
south-westerlies. Such an occasion was 13 January 1987, which
was probably the coldest day of the 20th century in the south-west.
For that day minima of -9.0 °C at St Mawgan (Cornwall) and
even -6.4 °C on the Isles of Scilly were records for the
sites.
Away from the coast, spells of frosty weather with clear
and calm nights have produced records such as -15.0 °C
at Exeter Airport on 24 January 1958 and Bastreet (Cornwall)
on 1 January 1979. Yeovilton (Somerset) reported -16.1 °C
on 14 January 1982.
July and August are the warmest months in the region with mean
daily maxima ranging from around 19 °C in coastal Cornwall
to 21 °C in inland areas of Somerset and North Somerset.
This is to be compared with 22.5 °C that is typical of the
area of highest UK temperature around London. Instances of extreme
high temperatures are rare and are associated with hot air brought
from mainland Europe on south-easterly winds accompanied by
strong sunshine. Records include 35.4 °C at Saunton Sands,
a site which is next to a large dune/heath area in north Devon,
and 34.5 °C at Yeovilton in Somerset, both on 3 August 1990.
The highest UK temperature stands at 38.5 °C at Faversham
(Kent) on 10 August 2003.
The variation of mean daily maximum and minimum temperature
for the months together with the extreme highest and lowest
temperature is shown in Figure 1 for Yeovilton and St. Mawgan.
Yeovilton has the lower minimum temperature throughout the year
because it is inland. It also has higher mean maximum temperatures
except in mid winter when St Mawgan is slightly higher. St Mawgan
has lower maximum temperatures in summer because of the cooling
influence of the sea. The inland site at Yeovilton has higher
extreme maximum and lower extreme minimum temperatures compared
to St Mawgan.
Map of January mean maximum temperature (1971-2000)
Map of July mean maximum temperature (1971-2000)
Map of January mean minimum temperature (1971-2000)
Map of July mean minimum temperature (1971-2000)
Figure 1. The monthly variation of mean daily maximum and minimum air temperature (1971-2000) and the site extremes for St Mawgan and Yeovilton (extremes are for 1964-2003 for Yeovilton and 1957-2003 for St Mawgan).
Frost frequency is highly variable across the region and despite
its reputation for mildness can be relatively high in some places.
The Isles of Scilly will experience an air frost only very occasionally,
usually when cold continental air arrives on an easterly wind.
Here the mean frequency of air frost is less than two per year.
It rises to around 15-20 in coastal areas of Cornwall and Devon.
Frequencies exceed 50 per year in inland low-lying spots in
Somerset and North Somerset. The first air frost of autumn can
be expected about mid October inland in Somerset and north Somerset.
The continuing warmth of the sea usually prevents coastal autumn
frosts. Air frost in April is common and even in May is not
unknown. Ground frosts follow a similar pattern to air frost.
Most coastal locations have between 35 and 60 days per year,
but in some eastern inland districts this rises to over 100
days per year. Figure 2 shows monthly mean air and ground frost
frequencies at Long Ashton, near Bristol and at St Mawgan in
Cornwall.
Map of annual average number of air frosts (1971-2000)
Map of annual average number of ground frosts (1971-2000)
Figure 2. Frost frequencies at St Mawgan and Long Ashton.
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section - Sunshine
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