The Exmoor cataclysm
On the evening of Saturday 16 August 1952, when news reached London of the sad disasters at Lynmouth, Dulverton, South Molton and other places in the Exmoor region, I remarked to a friend that some eight inches of rain must have fallen on the watershed, and next day there came the official announcement of a measurement of nine inches at Longstone Barrow. The Daily Weather Report showed a warm front, reported to be unstable to a great height, and a cold front not far behind it. The geographical location of these fronts, soon to be occluded, over the south-west of England was such that the air currents within the slow-moving depression saturated with vapour from the warm August seas, were being impeded and uplifted on their eastward course by the elevated ground of Dartmoor and Exmoor.
From the fact that four inches of rain were recorded at both Plymouth and Okehamp-ton it is fairly evident that there was excessive rainfall also over Dartmoor, though there is a chance that this tableland could have taken nine inches of rain, had this amount actually fallen, with less tragic conse-quences to the surrounding country. Dartmoor is larger than Exmoor with more rivers near their sources to share the discharge of storm waters, and none of its flanks bears down on the sea coast, as does one side of Exmoor, in high ramparts of grand precipitous cliffs, with villages at their feet obviously at the mercy of any streams which might become unruly. So it is against such a physical background that one must picture those lovely Exmoor rivers Exe, Barle, Lyn and Bray, all cradled near one another amid the exquisite glens of the Lorna Doone country suddenly roused by a gigantic storm to dispense death and destruction all around.
As I was in London on Friday 15 August (a day marked there by broken storms with singularly impressive rolls of thunder) I cannot give any personal impressions from the west country; but letters and newspapers sent to me from various points in Devonshire considered in the light of the synoptic situation have left me in little doubt as to the kind of storm which was so savagely let loose. I think it can best be described as a cyclonic warm front thunderstorm, prolonged and widespread, with the thunder and lightning developing at points of concentration within the larger area of thundery rain. The surface wind is generally described as coming from some easterly point, as is so often the case in intense rainfall of the kind. A lady who was travelling by bus from Minehead to Ilfracombe through Lynmouth on the afternoon before the flood rather significantly speaks of a strong chilly wind blowing from the sea during a torrential downpour. Climbing Porlock Hill the bus became enveloped in hill fog, and it became so dark that the lights had to be turned on.
At Plymouth the rain began about 5 a.m. on the Friday morning with severe thunder and lightning, and Chudleigh and Ottery St. Mary both had two inches about 9 a.m., also with a sharp thunderstorm; but at Ilfracombe it is said not to have begun till 11 a.m. and then without thunder which, as at Barnstaple, developed heavily later, when the storm was waxing to a climax at nightfall just before the Lymouth deluge. In south-west Devon the rain appears to have ceased during the afternoon without any clearance of the sky about the time when the conditions in north Devon were beginning to get critical. An impressive and rather sinister feature of the storms, was the amount of low cloud which intensified the gloom and hid the thun-dery cumulus clouds from view.
Some newspapers, particularly in the west of England, commented upon the curious manner in which this disastrous August storm in north Devon was foreshadowed by the severe thunderstorm in the same region of 16 April 1952 when Barnstaple and the country towards Ilfracombe were seriously flooded. It is further a curious fact that the only three instances of rainfall over nine inches in 24 hours recorded in Britain all belong to this same part of the country, namely 15 August 1952 on Exmoor along the Somerset-Devon border, 18 August 1924 at Cannington in the Quantock country in the heart of Somerset, and 28 June 1917 at Bruton in the Mendip country likewise in the heart of Somerset. This, of course, may be nothing more than pure coincidence; nevertheless, I am inclined to suspect geographical influences behind the coincidence for this reason. The south-western counties of England lie open to abundant supplies of summer moisture from the Atlantic, and within these counties orographic influences are strong. Hence these factors may have sufficed on three occasions of very rare rainfall to enhance the intensity sufficiently for the nine-inch limit to be passed – the limit for England so far as official records go, but far below that for France.
L. C. W. Bonacina
An Exmoor journey
We left Lynton (by motor coach) on Friday, 15 August 1952 at 2.40 p.m.; rain was falling fairly heavily, having commenced soon after mid-day, but this lessened and eventually ceased after we had ascended the hill beyond Barbrook on the road towards Simonsbath. The cloud ceiling was low over the intervening moors but no rain fell until we approached this village. Proceeding over Holnicote there was intermittent drizzle from low cloud but again there was no rain on arrival at Tarr Steps on the River Barle, our first stop at 3.35, but it recommenced soon afterwards. Continuing our journey at 4.5 we ran into moorland fog for a short distance but this had cleared before we reached Exford at 4.35 where we stopped for tea, the rain again holding off. The level of the Exe river here was not higher than usual after a wet spell. Fog was again encountered on the road to Porlock, after we had left Exford at 5.20, and was accompanied by intermittent rain. Low clouds becoming increasingly dark were visible especially in the valleys seen from the higher levels. Very heavy rain was experienced shortly before reaching the junction with the coast road near Porlock Hill and continued all the way back to Lynton. Crossing the Lyn at the fork of Countisbury Hill about 6.20, the level of the stream was not abnormal but it was certainly running fast. The heavy rain lasted till about 7.30 when it became lighter out at sea; whereas only Countisbury Foreland had been visible through the rain from the North Walk at Lynton, a lighter patch had spread back to the top of Countisbury Hill by 8 p.m. There was a short break in the rainstorm at that time but it soon resumed and continued till late at night with occasional lightning, the resulting thunder following at intervals of 10 or 15 seconds. On the following morning the sky was overcast but little rain was experienced until the time of departure from Lynton, 9.30 a.m., when local drizzle was falling. Much flood damage could be seen on the road thence to Barnstaple.
A. H. Ratcliffe
The Lynmouth floods
As a preliminary to examining the meteorological situation directly connected with the exceptional rainfall in north Devon on 15 August 1952, it is relevant to survey the weather experienced during the preceding fortnight.
A fine, warm and anticyclonic spell in the second half of July, unusually free from thunderstorms, resulted in an ‘absolute drought’ over a large part of the south of England, including the area in which we are now particularly interested. These conditions broke down at the beginning of August, to be followed by a period of very changeable weather over the whole country. Rain was frequent and often heavy. Thunderstorms occurred somewhere in Great Britain almost every day, the centre of activity changing daily from place to place, as it often does in thundery weather. On the 6th, for instance, the most badly affected places were to the west of London where the rainfall intensity at Wembley – 2.1 inches in half an hour – was officially described as ‘very rare’. A few days later there was prolonged rain and some flooding in the Isle of Man, Northern Ireland, the Lake District and parts of south-west Scotland. Severe thunderstorms elsewhere on other occasions gave a normal month's rain in one day, and so on – sometimes here, sometimes there. Although north Devon and west Somerset did not have any outstandingly heavy falls up to that time, rain fell there on all but two of the first fourteen days, and not in small amounts. The 14-day total of just under three and a half inches at Weston-super-Mare included six days each with more than a quarter of an inch of rain, while at Ilfracombe and Chivenor, slightly to the west, rainfall aggregates were only slightly less. In the prevailing type of weather the rainfall up on the high ground at Exmoor could be expected to be at least as much as down on the coast, and probably more. Immediately before the Lynmouth floods, therefore, the ground was unusually wet for the time of year, especially in the large boggy area on Exmoor itself near the source of the West Lyn, where a layer of rock only a little below the surface effectively prevents any appreciable percolation of water.
With that background of very disturbed meteorological conditions in mind, the earth already soaked with rain and the generally acknowledged tendency for any long established type of weather to persist, we come to the synoptic situation existing on 15 August 1952, as shown in Figure 1. The depression centred at midday over the western English Channel can be traced back to three days earlier when it formed in mid-Atlantic, subsequently moving south-east and then north-east in the general air stream. Although this depres-sion initially had no frontal structure, warm air from France seems to have been drawn into its circulation as it approached Brittany. The characteristics of the air masses of which the depression was composed are indicated by the upper air soundings at Larkhill and Camborne respectively, plotted in con-ventional form in Figure 2. The fairly steep temperature lapse rate and the large moisture content of the air over southern England combined with the fact that thunderstorms occurred widely in France along the warm front on the previous day, clearly point to a strong risk of thunderstorms breaking out anywhere near the track of the depression. In fact, they occurred in most parts of south and south-east England during the twelve hours following the time to which the chart relates.
Three-hourly weather reports from Meteorological Office observing stations in south-west England (Table 1) show the approximate time of onset and duration of the main rain belt associated with the depression as it moved north-east and then east-north-east. The track of the centre is indicated in Figure 1 by means of a broken line.
Continuous rain which began at the Scilly Isles and at Culdrose, near The Lizard, in the early hours of 15 August had spread to all parts of Cornwall, Devon and Somerset by midday. At Chivenor, the nearest synoptic reporting station to Lynmouth, and at St. Eval, in north Cornwall, the rain was almost incessant for 18 hours or more. Rainfall measurements at representative low-lying places between 9 p.m. on the Thursday and 9 a.m. on the Saturday are given below [Table 2].
|Date/time gmt||14/2100 to 15/0900||15/0900 to 15/2100||15/2100 to 16/0900||14/2100 to 16/0900|
|Scilly||30||7||—||37 (1.46 inches)|
|Culdrose||14||8||1||23 (0.91 inches)|
|Plymouth||12||61||16||89 (3.50 inches)|
|St. Eval||9||14||3||26 (1.02 inches)|
|Chivenor||0.2||35||32||67 (2.64 inches)|
|Bristol||3||13||1||17 (0.67 inches)|
Rainfall amounts varied with locality even where orographic influences were not applicable, the largest falls being on the left-hand side of the track of the low – the position normally favourable for heavy rain. But this was not merely a case of orthodox rainfall distribution around a frontal depression with the ordinary orographic intensification of precipitation on high ground. It was complicated by thunderstorms all over the area of heavy rain, and one set of storms seems to have come up from Brittany in the south-south-east wind which was observed at 700 millibars over Brest at 0300 gmt on the 15th. The long duration of rain may have been due to slow movement combined with elongation of the rain area along its direction of movement. The rain had reached Manchester and Anglesey by midnight.
In addition, the continuance of north-east to north winds in the lower layers along the coast of North Devon and West Somerset throughout the afternoon and night of 15 August could be regarded as having been an important supplementary factor in the convectional processes which led to the tragic sequence of events. The consequent sustained ascent of relatively cold but moist and unstable air up the northward facing slopes of Exmoor introduced a region of continuing orographic ascent in an already existing area of heavy rain, which might have been decisive in producing the excessive rainfall there.
On-the-spot investigations which have been made by the Climatological Branch of the Meteorological Office indicate strong orographic influence on rainfall distribution over the area. The general public, including holiday-makers, have since been invited by radio and in the Press to lend the Meteoro-logical Office any photographs they may have taken of hillsides and the upper reaches of streams running down from the hills as they appeared before 15 August. It is thought that a comparison of these views with photographs which were taken of the newly eroded stream beds and landslides would be valuable in supplementing the few actual rainfall measurements in the area. Persons who may have been on Exmoor were similarly asked to send detailed reports of the time of occurrence and duration of the heaviest rain.
All things considered, the exceptional rain and resulting damage seem to have been brought about by a specially favourable combination of factors. The rainfall observer at Longstone Barrow, situated at the headwaters of streams draining down to Lynmouth, recorded nine inches of rain between 11.30 a.m. on 15 August and 9 a.m. on 16 August, of which it is estimated that about seven inches fell between 5 p.m. and midnight, and six inches between 7 p.m. and midnight. A concentrated fall of that kind would be sufficient to cause flooding in any normally situated position. In this instance it was seriously aggravated by the topography of the northern edge of Exmoor lying immediately behind Lyn-mouth, where a wide stretch of country falls quickly from a height of 1500 feet to the Bristol Channel in a distance of only four miles. From all accounts, the rain – representing more than half a million tons of water per square mile – came pouring down every hillside and valley with a rapidity only possible in such steep surroundings. Local residents described the sudden onrush as resembling a ‘wall of water’ or a ‘tidal wave’. In these circumstances it is scarcely surprising that the cumulative effect of these swollen torrents was to break the banks of the River Lyn, and so bring death and destruction to Lynmouth ‘in the darkness of a single night’, as, the national relief appeal so vividly summarised the disaster.
W. A. L. Marshall