| 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.
Air humidity is an important factor determining rainfall,
and the sea temperature largely controls this. The sea temperature
off south-west 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 dew point 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 Isles
of Scilly. Most coastal areas of Cornwall and Devon have
900-1,000 mm, but up to double this amount falls on uplands
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
1,100 mm, while the Bath-Bristol area has totals around
800-900 mm. These figures can be compared to annual totals
around 500 mm for the driest parts of eastern England and
over 4,000 mm in the western Scottish mountains.
The course of monthly mean rainfall for 1971-2000 is shown
for four sites in Figure 3. All the sites have a
'semi-Mediterranean' rainfall regime. The highest rainfall
is in December and January when the sea is still relatively
warm 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 some 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.
Map
of average spring rainfall (1971-2000)
Map of average summer rainfall
(1971-2000)
Map of average autumn rainfall
(1971-2000)
Map of average winter rainfall
(1971-2000)
Map of average annual rainfall
(1971-2000)
Map of average annual rain
days with >=1 mm (1971-2000)
Figure 3. Mean monthly rainfall (1971-2000) at four
sites.
The numbers of days with rainfall totals of 1 mm or more
tend to follow a similar pattern to the monthly rainfall
totals. In coastal areas, about 15 or 16 days is the norm
in winter, but this decreases to nine or 10 in late spring
and summer. In Somerset and the north-east of the area there
are fewer days having 1 mm or more throughout the year.
In winter, about 12 or 13 days is normal, with about seven
to nine 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 five 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 203
mm at Camelford in Cornwall on 8 June 1957 and 243 mm in
13 hours at Bruton in Somerset in June 1917. The heaviest
recorded daily rainfall total in UK was at Martinstown in
Dorset when 279 mm was recorded on 18 July 1955.
Next
section - Snowfall
|
