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South-west England

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About south-west England - Temperature - Sunshine - Rainfall - Snowfall - Wind
Temperature

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).

Image: temperature extremes

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.


Image: St Mawgan frost

Image: long Ashton frost

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