The south-west peninsula forms the most southerly 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 Scillies. 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 where 10 °C C would be more typical around Bristol.
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 near Bristol 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 Scillies, to around 1.5 °C in inland Devon. Further north-east in Somerset and the Bristol area, 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 were recorded of -9.0 °C at St Mawgan (Cornwall) and even -6.4 °C on Scilly. Away from the coast, spells of frosty weather with clear and calm nights have produced records such as -15.0 °C at Exeter 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. 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 temperatures month by month, together with the highest and lowest temperatures recorded, is shown for St. Mawgan and Yeovilton. 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.
Frost frequency is highly variable across the region and despite its reputation for mildness can be relatively high in some places. An 'air frost' occurs when the temperature at 1.25 metres above the ground falls below 0 °C, whereas incidence of a 'ground frost' refers to a temperature below 0 °C measured on a grass surface. The Scilly Isles 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 2 per year. It rises to 15-20 in coastal areas of Cornwall and Devon. Frequencies exceed 50 per year in inland low-lying spots in Somerset. The first air frost of autumn can be expected about mid October inland in 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.
The graphs show the average frequency of air and ground frost at St. Mawgan and Yeovilton. Although the summer months are usually free of air frost, ground frost may occur at any time of the year, especially at sites in inland valleys.
