Temperature
Over England the mean annual temperature range at low altitudes is about 8.5°C to 11°C, with the highest values occurring around or near to the coasts of Cornwall and lowest values in inland northern regions. On Exmoor the range is increased by altitude differences: from about 7.5°C to about 11°C. The mean annual temperature decreases by approximately 0.5°C for each 100m increase in height so that there is a great range from Porlock, at sea level, to Dunkery, at 519m, within the short distance of 5km.
The higher specific heat of sea water renders it slow to heat up, but equally reluctant to cool down. These differences affect the climate in a number of ways. Firstly, the sea can warm the air flowing over it therefore making parts of the British Isles warmer. It can, of course, happen the other way, when the sea is cooler than the land, the wind from the sea is cooler. This explains why sea-surface temperatures reach their lowest values in late February or early March. Around the West Country coasts February is thus normally the coldest month. In this month the mean minimum temperature varies from over 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. 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.
The air temperature changes in the Atlantic are known as the North Atlantic Oscillation (NAO), and they are governed by the behaviour of the Azores high pressure and the sub-polar low pressure zones. When air pressure is unusually low over Iceland and high over the Azores, the NAO is said to be positive. When both pressure areas are not significantly or poorly developed, the NAO is negative. In positive mode, the westerlies are strengthened so that warm air is drawn towards the British Isles. The mild winters of the 1980s and early 1990s were a time of significantly positive NAO. In the negative phase, the Icelandic low becomes displaced south-westwards and the polar high pressure penetrates southwards. The exceptionally cold winters of 1962/63 and 1941/42 are examples of this negative phase. On a larger scale, temperature differences can occur as a result of periodic fluctuations in sea-surface temperatures. These are smaller in scale compared with the El Niño in the Pacific. Even so, they are still important as far as temperatures in Britain are concerned.
The sea that surrounds the West Country on three sides has the highest annual mean temperature of any sea area around Britain, close to 11 to 12°C. The water in the Bristol Channel can be particularly warm in summer, as it is less influenced by ocean currents. With winds mainly blowing from the sea the annual mean air temperatures are close to those of the sea in coastal areas. This temperature tends to fall towards the north-east of the region, where 10°C is 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. This increases from about 9°C in west Cornwall to about 12°C in the north-east of the region, with Exmoor somewhere in between. Away from the coast, altitude is the main factor affecting temperature.
July is normally the warmest month on Exmoor, as it is in the rest of England, and the highest temperatures of all have occurred in central England furthest away from the cooling influence of the Atlantic. The climate of Exmoor is generally more equable: less extreme in temperature than further east. Parts of Somerset to the north and east of the moor, however, have some of the highest average temperatures in Britain whilst having more frost than other parts of the region.
Minimum temperature normally occurs shortly after dawn and the maximum temperature two to three hours after midday. In urban areas the minimum temperatures experienced tend not to be so low as those recorded in rural areas such as Exmoor, although maximum temperatures are often higher. Urban areas can heat up more quickly than rural areas to produce their own urban heat island. It is not uncommon for the minimum temperature for London to be 2 or 3 degrees higher than comparatively rural locations. Some sheltered and low-lying areas have a greater incidence of frosts, and more severe frosts, than the surrounding areas. These are known as frost hollows.
Frost frequency is highly variable across the West Country and, despite its reputation for mildness, can be relatively high in some places. The Isles of Scilly 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. Inland, 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.