Rainfall is caused by the condensation of the water in air that is being lifted and cooled below its dew point. Rainfall tends to be associated with Atlantic depressions or with convection. The Atlantic depressions are more vigorous in autumn and winter and most of the rain which falls in those seasons in the south west is from this source. In summer, convection caused by solar surface heating sometimes forms shower clouds and a large proportion of rainfall falls from showers and thunderstorms at this time of year.
The air humidity is an important factor determining rainfall and the sea temperature largely controls this. The sea temperature off SW England is at its maximum in late summer and autumn and is coolest in late winter and spring and as a result rainfall tends to be most in autumn and least in spring.
A final factor which greatly affects the rainfall distribution is altitude. Moist air which is forced to ascend hills may be cooled below the dewpoint to produce cloud and rain. A map of rainfall looks very like a topographic map.
Annual rainfall totals are about 850-900 mm in the Scilly Isles. Most coastal areas of Cornwall and Devon have 900-1000 mm, but up to double this amount falls on upland such as Dartmoor, Bodmin Moor and Exmoor. Areas to the lee of high ground have lower totals e.g. 800 mm near Exeter and even 700 mm in the low-lying parts of central Somerset. Further east the Mendip Hills have annual totals exceeding 1100 mm, while the Bath-Bristol area has totals around 800-900 mm. These figures can be compared to annual totals around 500 mm typical of the driest parts of Eastern England and over 4000 mm in the western Scottish mountains.
The course of mean monthly rainfall for 1971 - 2000 for 4 sites is shown below. The highest rainfall is in December and January when the sea is relatively warm still and the Atlantic depressions are most vigorous. The months from April to July are the driest when the sea is relatively cool and the Azores high pressure system exerts more influence. August shows an increase of rainfall over July and starts the inexorable rise in rainfall into the autumn and early winter.
The effect of altitude is seen by comparing the records for Plymouth and Princetown which are about 23 km apart, but differ by 403 metres in altitude. The Princetown rainfall is twice the Plymouth rain on average.
Monthly rainfall is also very variable. Most months of the year have recorded totals below 20 mm in coastal districts and many below 10 mm. Even at Princetown one May recorded only 7 mm. The highest monthly totals tend to be in the autumn and winter months. At Plymouth for example every month in the year has had more than 100 mm, but totals in excess of 200 mm have only been recorded from December to February.
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The numbers of days with rainfall totals of 1 mm or more ('wet days') tend to follow a similar pattern to the monthly rainfall totals. In coastal areas in winter about 15 or 16 days is the norm, but this decreases to 9 or 10 in late spring and summer. In Somerset and the Bristol area there are fewer days having 1 mm or more throughout the year. In winter about 12 or 13 days is normal with about 7 to 9 in summer. The numbers of days increase with altitude and at Princetown for example there are over 18 days in the winter months and 12 to 13 days in summer.
The south west peninsula is prone to rare, but very heavy rainfall events lasting from about 5 to 15 hours. The famous storm which devastated Lynmouth in north Devon on 15 August 1952 was one of these, when one place on Exmoor had 228 mm in 12 hours. Other similar events are the 200 mm at Otterham near Boscastle in Cornwall on 16 August 2004, 203 mm at Camelford in Cornwall on 8 June 1957 and 243 mm in 13 hours at Bruton in Somerset in June 1917. The highest recorded daily rainfall total in UK was at Martinstown in Dorset when 279 mm was recorded on 18 July 1955.
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