SOME WEATHER PATTERNS IN SNOWDONIA

SOME WEATHER PATTERNS IN SNOWDONIA

By D. E. PEDGLEY Centre for Overseas Pest Research, London

T is widely accepted that weather among the mountains is fickle and un- I predictable. The rapid changes in space and time can be bothinteresting and a nuisance to mountain dweller and casual visitor alike; to the meteorologist they provide subjects for serious study. Climatological charts often vividly portray the influence of mountains on the weather, and this is particularly so with rainfall. Thus, among the wetter highlands of Britain, annual rainfall is 5 to 10 times that over the drier plains. On the other hand, the distribution of rain on individual days is less well known, and so are the local patterns of wind in a mountainous area.

For convenience, mountain influences on the weather can be divided into two types:

(I) intensification or redistribution of weather patterns already existing on a scale much larger than the mountains - for example, rain areas associated with depressions and fronts, and broad airstreams covering the whole country;

(2 ) local phenomena that would not occur a t all in the absence of the mountains .

Both types often coexist and it can be difficult to distinguish between them. Moreover, these influences are not always clearly shown on weather maps because the spacing of observations on which they are based is often comparable with the size of a mountain area. One way of discovering these influences is to have observations more closely spaced, using the help of observers in a dense network. This article presents an analysis of the records taken by such a dense network of volunteers who kept a log of the weather during February 1971 among the mountains of Snowdonia in North Wales (Fig. I).

THE OBSERVATIONS

At the start of the study, in October 1970, letters requesting assistance were sent to many schools, field centres and youth hostels, and to organizations already involved in keeping local weather records. In addition, many private

Fig. I . Location map showing places mentioned in the text. The 800-ft contour has been added to show the positions of the principal valleys

individuals who had raingauges, both official and unofficial, were asked if they could help. The two principal aims of the study were explained, namely:

(I) to examine the distribution of rainfall accompanying individual atmospheric disturbances ;

(2) to determine the nature of distortions of the broad-scale wind flow caused by the mountains acting as a bamer.

(I) Wind and weather at 10 am (BST), together with a tabulation of which summits in a short list were in or out of cloud;

(2) times of start and stop of precipitation; (3) depth of lying snow, or the lowest altitude at which it could be seen

on nearby mountains. Some observers volunteered to keep similar records at I pm and 4 pm,

whilst some pupils kept a record in their homes at 8 am and 6 pm. School

Volunteers were asked to make a note each day of:

Fig. z. Location map showing (a) places where 10 am observations were made for the study (circles) and (b) places where rainfall data were available (dots)--see list on p.443

observations were made by staff members or by pupils under supervision, but records were not always available at weekends nor during the mid-term holiday, zz and 23 February.

Many observers recorded temperatures, sometimes at the fixed hours, more often only maxima and minima. Wind directions were noted on an 8-point compass (16-point a t some places) and speeds according to the Beaufort scale. Some places were poorly sited for wind observations because of the presence of trees, buildings or sloping ground. Lack of time prevented a personal examin- ation of all sites.

Observers were asked to note the occurrence of fog, drizzle, rain, snow, hail and thunder, and also the visibility in yards when fog was present. Observers were not asked to distinguish between showers and ' general ' rain ; in the event, it was possible on most occasions to do this in retrospect by consulting the Daily Weather Report. Even so, such was their enthusiasm, many observers did in fact make the distinction, and they often made note of the cloud amount and

also the occurrence of frost. Routine reports were available from four synoptic stations in the area, whilst records of bases and tops of clouds measured from aircraft were kindly made available from the Royal Aircraft Establishment at Llanbedr. The positions of all places recording at 10 am are shown in Fig. 2, and the names of observers are given in Table I (p. 443).

Many existing raingauges were used in the study, both official and private, and 13 supplementary gauges were installed for the month, largely with the aid of a grant from the Scientific Activities Fund of the Royal Meteorological Society. The positions of all these gauges are shown in Fig. 2.

In any project such as this, one is at the mercy of the weather! To ensure the sampling of a reasonable variety of weather types, several weeks of records are needed. A longer period is too demanding on volunteer observers; one month seemed to be a reasonable period and the choice of month was almost automatic. It had to be:

(I) in term time, so that schools could participate, but not so as to conflict with examinations;

(2 ) in winter, to reduce the complexities of convective effects which would be compounded with the barrier effects being sought;

(3) in mid-winter, to ensure a reasonable chance of snow. February was the only month that complied with these restrictions.

It was also a month when field centres and youth hostels were comparatively quiet and, conveniently, it had exactly four weeks starting, in 1971, on a Monday !

THE RESULTS

February 1971 had more anticyclonic weather than usual. Despite this, there was a great variety of events; almost every day provided evidence of a different mountain-effect. From this whole, 10 days have been selected for detailed discussion on the following pages. The presentation is much the same for each example:

(I) A map for 10 am showing winds and weather over Snowdonia, together with an inset map of the British Isles at the same time. The symbols used are explained in Fig. 3. Wind speeds have been converted from the Beaufort scale into knots; also some maps show: (a) 24-hour or 48-hour rainfall totals in hundredths of an inch (to

the right of the place of observation) ; (b) minimum temperatures during the preceding night in O C (to

the left of the place of observation, and enclosed in a box). (2) A discussion of the principal events in Snowdonia and their relation-

ship to the large-scale weather. (3) Additional maps or illustrations where these make events clearer. The inset maps are based on routine weather charts prepared in the

Meteorological Office and they show isobars and fronts, as well as winds and weather at a selection of places to illustrate the broad-scale patterns over the British Isles.

3 February I971 A north-westerly airstream covered Snowdonia at 10 am, flowing around

a large anticyclone off south-western Ireland (Fig. 3). On exposed western coasts of Lleyn and around the Conway estuary (for area names see Fig. I), the north-westerlies reached 15-20 kt. These places were on the flanks of the mountain mass relative to the direction of the approaching airstream; on the upwind side, over Arfon, winds were much lighter, mostly 5 kt or less, and there was some indication of a bifurcation in the flow - one part being deflected to the left as a south-westerly along the north coast, and the other part to the right as a northerly towards Eifionydd. This deflection was asymmetric; the point of bifurcation was not in the middle of the upwind side of the mountains but well to the right, near Pen y Groes (Station 16). Part of the deflected flow on the south side turned into the Vale of Ffestiniog as a 5-10 kt south-westerly, but there was no well-marked funnelling into windward-facing valleys. Westerly winds down the middle and lower Conway valley suggest a flow descending in the lee of the Carneddau. Of special interest are the 10-20 kt westerlies along some of the central valleys.

The airstream was very cloudy; most places reported a complete cover a t 10 am, and only in parts of the upper Conway valley were there any breaks. On the windward sides of both Lleyn and Ardudwy, the base was at 700-1000 ft, falling to 300-500 f t over northern Arfon and the lower Nant Ffrancon, where Gerlan (stations 58 and 59) was in hill fog. The base was near 1500 ft along the north coast, whilst over the centre and east of Snowdonia it was a t 2000 ft or above, although hill fog was present a t the high-level station of Llyn Conwy Treatment Works (station 85, 1280 ft). Drizzle was falling widely over windward slopes (from the northern side, through west to the south) and over the

Fig. 4. Progress of the first rain on 2 February 1971: times of start and isochrones (lines of equal timing)

Fig. 3. Weather map for 3 February 1971 showing wind and weather at 10 am. The figures indicate rainfall in hundredths of an inch during the previous 24 hours; T refers to a trace (<0.005 in.). The inset map showswind and weather at 10 am for a selection of places over the British Isles, and isobars have been added to show the broad-scale pressure pattern. Conventional symbols used on this and subsequent maps are explained on the right

weather wind symbols. symbols

8 calm = tog

9 drizzle -0 1-2 ht

rain % 3-7 1 sleet ‘-a 8-12

3~ snow ‘CO 13-17

A hail h 1 8 - Z

0 shower ub 23-27

thunder % 28-32 ] stowed in past hwr

-

central valleys; eastern valleys were dry. A notable point was the absence of drizzle over Ardudwy, Lleyn and Arfon, and over Anglesey except the north- east. There is a strong impression that drizzle was able to develop in the cloud only along the windward slopes of Snowdonia.

This cloudy, drizzly airstream had spread across the area during the previous night behind a weak warm front that had passed over late on the 2nd

and by 10 am on the 3rd had cleared the eastern side of England. Ahead of the front, during the afternoon and evening, there were outbreaks of light rain, preceded by snow in the upper Nant Ffrancon (and probably more widely a t places above about I ooo ft before milder air spread in). The first rain arrived as a well-defined, short-lived burst coming from the west; its time of onset is shown in Fig. 4. Although this rain was widely reported over Lleyn, Arfon and the western mountains, it did not reach the eastern side of Snowdonia. The 24-hour rainfalls associated with this front and the succeeding drizzle up to 10 am are shown in Fig. 3. Well to windward, falls were only 0-o1-o.02 in. and very uniform, but over the south-western mountains there was an area with dimensions about 20 mi by 10 mi where falls were greater than 0.1 in., and locally greater than 0.2 in. By contrast, there was little or no rain over the central and southern Conway valley. Notice that the greatest falls were not over the heart of Snowdonia.

4 Februury 1971 Yesterday’s anticyclone had moved north to become centred over Ireland

so that winds over Snowdonia were northerly. On exposed coasts they reached about 10 kt, but a t many places winds were light and variable or calm (Fig. 5) . This northerly stream brought an extensive thin layer of cloud, the base of which was 2000-2500 ft, but 1500 ft at places on the windward (northern) coast. Aircraft from Llanbedr found that this cloud sheet had a top at about 2500 ft, i.e. below summit levels of the central mountains. As a result, the cloud sheet was blocked by the mountains and on their leeward (southern) side an extensive area of clear skies had developed (Fig. 6). This area was wedge-shaped with its apex near the Carneddau; it is visible in the picture taken by the ESSA 8 satellite at 1001 GMT (Fig. 7, p. 431. one hour later than the time of Fig. 6), from which the clearance can be seen to extend downstream across the

Fig. G. lieports of clout1 amounts at 10 am j February

b bc = partlycloudy c o = overcast

1971: = little or no cloud

= large amounts of cloud

Fig. 5. Weather map for 4 February 1971 at 10 am. The figures in boxes are minimum temperatures during the previous night, in degrees centigrade

whole of western Wales as far as the entrance to the Bristol Channel. By 4 pm, the clear area had changed orientation to NE-SW because the broad-scale wind flow had by then veered to north-easterly.

Under the clear area, night-time temperatures fell much lower than those at persistently cloudy places. Minima were +I to -1°C (i.e. there was a slight air-frost in places), compared with +5 to +7OC in cloudy areas (Fig. 5) . Note, in particular] the strong contrast between Llanrwst (+ 7"C, station 76) and Cape1 Curig (0°C at 64 and -1°C at 63). and the many reports of +5"C over Arfon contrasting with 0°C at Beddgelert (station 37). The cold, dense air near the ground within the clear area tended to drain slowly down-valley giving, for example, light winds from between west and south along the upper tributaries of the River Conway. These down-valley, or ' katabatic I , winds had directions strongly contrasting with that of the broad-scale flow.

By contrast with the 4th. the 25th (not illustrated) was cloudy over the whole of Snowdonia because, although the cloud sheet was similar, tops were at about 4000 ft (reported by aircraft) so the summits were not high enough to give a downwind clearance.

Fig. 9. Similar to Fig. 6, but for 10 am 5 February 197'

On the next day, the anticyclone was centred over north-eastem England so that winds over Snowdonia had veered to south-easterly. The airstream still had an extensive cloud-sheet with a base near 1500 ft (but only 1000 ft over Lleyn), and tops at about 2500 ft (reported from aircraft and by climbers; see Fig. 8). Blocking and a leeward clearance was still present, but now it was to the north-west, corresponding to the change in direction of the broad-scale flow (Fig. 9). This clearance shows as a V-shaped dark area in the satellite picture taken at 1052 GMT (Fig. 10, p. 431). Again the clearance of cloud allowed local air frost to form; minimum temperatures during the previous night were + I to -1°C in central valleys, and + I to + 2°C over Arfon and nearby Anglesey. By contrast, minimum temperatures were + 5 to +7"C over Lleyn. and mostly +3 to +4"C elsewhere. Fog reported from Rhiwlas (29) and Menai Bridge (23) was probably a result of this night-time cooling.

8 February 1971 The anticyclone that had dominated the weather during the first week of

February was now receding south-eastwards to the English Channel and, as a result, winds were south-westerly over Snowdonia. Speeds were still low - even on exposed coasts they were only 5-10 kt. A southerly flow across Arfon is well shown in Fig. 11, turning to south-west along the north coast as far as the Conway estuary.

Light south-westerlies also drifted along the Vale of Ffestiniog, but elsewhere there were calms or light and variable winds. However, there were two important exceptions: 10 kt west to south-westerlies a t Cape1 Curig (63) and Dolwyddelan (81 and 82), and 10 kt south to south-westerlies at Llanberis (50)

and Betws Garmon (34). Clouds in this airstream were very low - on the windward (western and

Fig. 11. Weather map for 10 am 8 February 1971

southern) sides the base was widely reported at between 300 and 500 ft. As a result, many places higher than this were in cloud, and fog was widely reported (most observers called it ' mist '). Fig. 11 shows that this ' hill fog ' was widespread, with visibilities generally between IOO and 500 yards. Moreover, it is likely that some sea fog was present over Cardigan Bay; it was reported coming inland at both Criccieth (10) and Harlech (41). Fog was present at all places around the Vale of Ffestiniog, but not at Maentwrog (45) in the valley bottom. Likewise, it did not reach valley bottom along Nant Gwynant (53, 54 and 55; see Fig. 12 (b), p. 432), but it was present to the north of Beddgelert (35 and

Away from the very low cloud and hill fog in the south and west, there was a higher layer of cloud with a base around 1500 ft, widely reported from the north and east. At Ekthesda (57), the lower layer was reported as a valley fog, separated from the cloud above.

There were no aircraft reports of cloud tops on this day, but since there were no leeward clearances it is likely that the top exceeded 3000 ft. This is supported by the reports of light drizzle at some places in Lleyn, on the western and southern slopes, and in the central valleys. Such drizzle usually develops in

36).

clouds a few thousand feet deep, and it is significant that the drizzle had stopped by mid-afternoon, when aircraft reported a base at 1800 ft and tops at 3800 ft (i.e. only zoo0 ft deep).

12 February 1971 By this day the weather had changed with a vengeance! Strong winds and

rain were widespread as a deep depression moved eastwards near Iceland and a south-westerly airstream covered the British Isles. Speeds reached near gale force in Lleyn (Fig. 13) and also a t places in some leeward valleys, particularly at places along the Conway valley. By contrast, there was 'a considerable reduction of speed at places on the windward side of the mountains, particularly south of Caernarfon, in Eifionydd and in Ardudwy. Very light winds were reported from Dolgarrog (74) in the Conway valley and near Beddgelert, both places being under a steep lee slope. The flow along the Nant Ffrancon was deflected to a south-easterly, contrasting strongly with the westerlies towards Cape1 Curig (64).

At 10 am, rain was falling at almost every place of observation, and it was heavy at many. Periods of rain fell throughout two days as a warm front, followed by a cold front with a wave, swept across the British Isles. Amounts falling in the 48 hours ending at 10 am on the 13th are shown in Fig. 13. Totals of 2 in. or more fell over most of the mountains, covering an area of about 20 mi by 20 mi; in the centre, totals exceeded 5 in. within an area having aradiusof a mile or two. Around the sides of the mountain mass, rainfall gradients were I in. over z to 4 miles. Fig. 14 reproduces the charts from two recording raingauges; it shows that rain started as a short burst on the 11th, around 1230 GMT

(I 30 pm) at Botwnnog (2) and 1330 GMT (2 30 pm) at Cwmystradllyn (13). The passage of this rain-burst across Snowdonia is shown by the times of onset in Fig. 15, the isochrones indicating a steady north-eastward progress. However,

Fig. 15. Similar to Fig. 4 but for the first burst of rain on 1 1 February 1971

Fig. 13. Weather map for 10 am 12 February 1971. Rainfall amounts refer to the 48 hours ending 10 am on the 13th

0.2

0.1 in.

0.2

0.1 in

0

Fig. 14. Charts from the recording raingauges a t Botwnnog and Cwmystradllyn showing variations in the rate of rainfall during the period 12 GMT on the 11th to 23 GMT on 12 February 1971. The slope of the line AB shows the lower limit of rates of fall described conventionally as ' heavy ' (>0.16 in. per hour)

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Fig. 14 shows that most of the rain fell between 3 30 am and 12 am on the 12th in association with the warm ikont of a wave depression moving rapidly north- eastwards in the circulation of the depression near Iceland. Further bursts of rain occurred in the warm sector and again on t4e cold front which, judging by the rainfall records, passed Botwnnog about g 45 pm and Cwmystradllyn about 10 30 pm. The observer at Bryn Gwynant (54) noted the passage of the cold front at 10 45 pm when the wind veered to westerly and gusted to gale force.

13 February 1971 This was another day of strong winds, but they had veered to westerly. On

exposed coasts they reached 30-40 kt (Fig. 161, but speeds of 15-25 kt were widely reported and there was noticeably less reduction of speed on the wind- . ward (western) side of the mountains compared with the 12th. Directions were mostly westerly and there was no sign of bifurcation similar to that of the 3rd, but the wind turned to south-westerly in the Vale of Ffestiniog, and odd directions were reported from Llanberis (51), Beddgelert (36) and Dolgarrog

In contrast to the widespread layered clouds and prolonged heavy rain of the Izth, the weather of the 13th was showeay with broken clouds of the cumulus and cumulonimbus types. At 10 am, a few light rain showers were reported, with hail at Yspyty Ifan (86). During the day, showers of rain, hail and snow became frequent over and downwind from the central mountains. By contrast, there were only a very few light showks over Anglesey and the north coast, and none over Lleyn. The 24-hour rainfdl ending at 10 am on the 14th (Fig. 16) illustrates very well the way showers formed over the mountains and were then carhied downwind. Totals of about 0.2 in. fell along a strip about 6 mi wide lying east-south-eastwards from the central mountains of Snowdon and the Glyders. Falls of this magnitude did not occur over the central valleys, probably because there was not enough time for the clouds to develop before the droplets in them could grow and fall out as rain, hail and snow. It is interesting that the greatest falls on this day were in the lee of the mountains - the exact opposite of the distribution on the 3rd.

15 February 1971 With a deep depression to the north of Scotland a west-south-westerly

airstream covered Snowdonia (Fig. 17). Although speeds reached 10-20 kt on exposed coasts, most other places recorded remarkably light winds, 5 kt or less. This may have been due to the low temperatures, for minima during the previous night had fallen to between +I and -1°C away from windward coasts. With a layer of cold air near the ground, particularly in the valleys, wind speeds were reduced.

Showers of rain and snow were falling at 10 am. Using the recorded times of onset and cessation, it is possible to show that some of these showers formed a definite line extending east-north-eastwards from near Caernarfon, contrasting with the apparently more irregularly-distributed showers elsewhere over the

(74) -

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mountains. Grouping of showers into bands is sometimes found in showery airstreams, and it was confirmed on this day by the satellite picture taken at 0950 GMT (Fig. 18, p. 434, nearly one hour later than Fig. 17) which shows a narrow belt of cloud lying almost east-west at 54"N, and probably identical with the clouds that gave the showers in Fig. 17.

During the previous 24 hours, a cold front within the showery air-stream had swept across Snowdonia. The total rainfall (Fig. 17) exceeded I in. widely over the mountains, but falls were much more erratically distributed than in Figs. 3 and 13, suggesting that convection played a big part in the growth of the rain. This suggestion is supported both by the variability in time, as shown by the irregularity of the traces on the charts from the recording raingauges (Fig. rg), and by tKe map (not reproduced) showing the times of onset of the rain (mid-afternoon of the 14th). On the map, it was difticult to find any smooth progression of the isochrones, and it seems almost certain that rain started over the mountains earlier than at places well to windward. Thus, rain was noted as starting between 2 pm and 3 pm at places from Bethesda (57) to Capel Curig (63), whereas the main rain-area did not reach low ground to the west until

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\ \ I /r


10 am

15 Feb 71

Fig. 19. Similar to Fig. 14 but for the period 12 to 23 GMT 14 February 1971; Cwm- ystradllyn above, Botwnnog below


3 30 pm (Fig. 19). The cold front swept across Snowdonia late on the 14th; from Fig. 19 it appears to have passed Botwnnog at about 1030 pm and Cwmystradllyn about 11 pm and the observer at Bryn Gwynant (54) noted its passing at 11 15 pm. Rain turned to snow at 11 30 pm at Yspyty Ifan (86), presumably as temperatures fell behind the front. The showery weather near sunset is illustrated by the panoramic view in Fig. 20, p. 433, taken from a point near Harlech (41).

16 February 1971 A small depression, or ' polar low ', had been moving south-eastwards

from west of Scotland where it lay yesterday afternoon. After crossing Northern Ireland, it was lying over north-eastern Anglesey at 10 am (Fig. 21). To the south-west of the centre, north-westerly winds of 20-30 kt were blowing over Lleyn, whilst to the south-east much of the flow was lighter and channelled along the valleys. However, strong south to south-east winds were blowing along the north-western parts of Nant Ffrancon, Nant Perk and the Gwyrfai valley. These strong lee-side winds were apparently similar to those, in Figs. 3 and 13. Another interesting wind pattern in Fig. 21 occurs along the Conway

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valley and its upper tributaries. Fig. 21 shows that snow was falling almost everywhere at 10 am (see Fig.

12 (d), p. 433); only in Lleyn was there rain or sleet. Temperatures had fallen widely to below 0°C during the previous night, with minima of -3 to -4°C along the Conway valley, so the snow was able to settle easily. Fig. 22 shows the depth lying during the late morning when it had stopped falling at most places. Amounts were greater than 2 in. over most of the mountains, and locally there was 5 in. or more. In Lleyn, snow lay only at altitudes above about 200 ft, whilst in Arfon and along the north coast depths were mostly less than I in.

Snow started to fall in the extreme west before 7 am and had spread to most of Snowdonia by 8 am; only around the estuary of the Conway was it as late as 8 30 am. A clearance reached the extreme west around 10 am, spreading to the east by about noon. Thus, the period of snow lasted three hours at most places.

The centre of the polar low crossed Snowdonia later in the morning, and by 4 pm it had moved further south-east to southern England. The few observations available at 4 pm showed a complex wind-flow - strong north-westerlies still over Lleyn, but directions mostly between north-east and south-east among the mountains, with a tendency to bluw along the valleys. This was probably because with little cloud by then and still much snow lying after a slow thaw, temperatures were near 0°C and the cold, dense air was constrained to flow slowly along the valleys or lie stagnant in the valley bottoms. Thus, the observer at Betws Garmon (34), enjoying a hill walk after the snow, at 4 pm noted a light north-westerly wind on the hillside south-west of the village at an altitude of 1000 ft, i.e. above the restricted flow in the valley bottom. Fig. 23 (p. 434) is a view of Snowdon from Llanberis a t I 30 pm.

Fig. 22. Snowdepths, inches, late morning February 1971

, in 16

Fig. 24. Weather map for 10 am 17 February I971

17 February 1971 An occlusion had crossed most of Ireland, and ahead of it a southerly

airstream was blowing across Snowdonia. Speeds reached 15-20 kt on exposed coasts (Fig. 24), but almost everywhere elese they were much less; in the Conway valley and its tributaries directions were somewhat variable. Of particular interest were the strong winds reported from some western valleys. At Nantlle (IS), the observer noted " a very stormy morning ", and it can be seen that winds in the area reached 25-35 kt, much stronger than the surface broad-scale flow on exposed coasts! By contrast, speeds were less than 10 kt on the windward (southern) side of the mountains. The directions in Nant Ffrancon, and around Cape1 Curig (63), are strange.

Rain was widespread; places that were dry at 10 am were largely confined to the north coast, i.e. on the leeward side. Sleet was still falling at two high places in the south-east - Llyn Conwy Treatment Works (85) and Trawsfynydd village (42) ; sleet and snow was probably more extensive over high ground earlier in the day.

429

Fig. 2 5 . Weather map for 10 am 20 February 1971

20 February 1971 A deep depression lay to the north-west of Scotland. Its cold front, with a

belt of rain and drizzle, had almost crossed Scotland and was approaching the west of Wales (Fig. 25) . Ahead of it, winds were south-south-westerly over Snowdonia, reaching 25-30 kt in exposed places, but speeds were much lighter on the windward side of the mountains. Along valleys lying approximately parallel to the broad-scale flow, south-westerlies reached 15-25 kt ; but in valleys lying across the flow, winds were more light and variable. The westerlies to the west of Cape1 Curig (62 and 64) were distortions similar to that noted on the 12th (Fig. 13).

Rain was falling at many places at 10 am, and the 48-hour totals up to 10 am on the zIst (i.e. the sum of the falls accompanying the warm and cold fronts of the depression) are shown in Fig. 25. Totals were generally 0.10 to 0.15 in. at places well to windward of the mountains (south-western Lleyn and Anglesey), but they increased over the mountains, exceeding 0.5. in among the highest summits, and exceeding 1.0 in. over thecentral parts. By contrast, little or no rain fell over the lower Conway valley where there was a typical ' rain shadow '.

430

5o"N

Fig. 7 Fig. 10 Crown Copyright

Fig. 7. Cloud picture taken by satellite ESSA 8 at 1001 GMT 4 February 1971. The arrow indicates a wedge-shaped dark area - a region of clear skies in the extensive sheet of white cloud. The pointed end of the clear area lies over the centre of Snowdonia

Fig. 10. Similar to Fig. 7, but for 1052 GMT 5 February 1971

Photograph by J . R. Noble Fig. 8. View of Snowdon summit from Crib Goch - above a layer of cloud with its top

near 2500 f t

Fig. 20. Panoramic view at 5 1 5 pm 15 February 1971 , looking from Harlech along directions between south-west and north, showing an old, decaying shower cloud coming across the Irish Sea to the south of Bardsey. and an active shower cloud newly-growing over the central mountains

. .

Fig. 12. Four views of the same scene: looking north-westwards to Lliwedd, a summit in the Snowdon group, from Bryn Gwynant in Nant Gwynant (a) 10 30 a m I February 1971 (b) (c) (d)

10 30 am S February 1971 ; cloud base about 500 f t 10 00 am 1 2 February 1971 ; cloud base about 1400 f t with heavy rain 10 30 am 16 February 1971 ; cloud base obscured in falling snow

432

Photograph by R . H a q

(4 Photographs by E . W . Hughes

433

55"

50"N

Crown Copyright Fig. 18. Similar to Fig. 7 but for 0950 GMT 15 February 1971. The arrow shows a narrow

E-W belt of clouds probably corresponding to the clouds that gave the showers over northern Snowdonia seen in Fig. 17

Photograph by S . W . Oweit

Fig. 23. View of Snowdon from Llanberis at I 30 pm 16 February 1971

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I


28 February 1971 An old occlusion was lying stationary down the eastern side of Ireland, and

a light south-south-easterly airstream covered Snowdonia (Fig. 26). All places reported calm or only light winds. The airstream was full of low clouds giving much hill fog, particularly at places above about 500 ft (even lower in Arfon), and the visibility was less than IOO yd in places. This cloud was giving some light drizzle whilst higher clouds were giving outbreaks of rain. Ahead of the occlusion; rain had been falling intermittently for most of the previous 24 hours, and totals for that period are shown in Fig. 26. A point of some interest is that amounts were between 0-2 and 0.3 in. over awide area,including bothmountains and lowlands. Only among the central mountains did falls approach 0.5 in., whilst over much of Arfon and in the north-western valleys falls were between 0-1 and 0.2 in. This comparative uniformity in distribution, contrasting with, for example, Fig. 13 and Fig. 25, can be accounted for by the light winds and consequent weak lifting of the lower atmosphere, with most of the rain falling from higher clouds in air relatively undisturbed by the mountains. The occurrence of relatively light falls immediately in the lee of the highest summits,

435

rather than further downwind, probably had R. similar cause. The lower falls over the northern Conway valley compared with south-western Lleyn (0.15 in. as against 0.40 in.) reflect a decrease eastwards from the occlusion.

RAINFALL PATTERNS

The rainfall of the IIth-Izth illustrates a classic situation for the development of heavy falls over coastal mountains. %ch falls have been discussed for Britain (Sawyer 1956) and Norway (Spinnangr and Johansen 1954). For their production it is necessary to have a prolonged spell of mild, moist and strong south-westerly winds such as can be found on the southern side of a deep depression moving eastwards near Iceland. The airstream already has widespread clouds with rain or drizzle before reaching the mountains, but the subsequent forced ascent can be expected to increase the water content of the clouds, particularly in the lower atmosphere, so that the falling drops are able to scour the clouds of their added water and thereby grow larger than they would have in the absence of the mountaim (Bergeron 1965, Storebij 1968, Pedgley 1970). When these upwind rains themselves produce substantial falls (for example, 0.5 - 1.0 in. on the 12th, Fig. r3), then comparable falls can be found in the lee of the mountains and there is no clearly-defined ' rain shadow '. On other occasions, when upwind rains are light, a good rain shadow is possible. For example, on the 20th (Fig. 25) the upwind rains amounted to 0.1 - 0-2 in. over western Lleyn and Anglesey, whilst on the leeward side amounts were less than 0.05 in. over the northern Conway valley. At the same time, falls exceeded 1.0 in. over the central mountains. A good tain-shadow is particularly likely when falls are largely confined to windward slopes and upwind falls are negligible (for example, see the 3rd, Fig. 3). By contrast, a poor rain-shadow is likely when there is little lifting of the lower atmosphere because winds there are light. As a result rainfall amounts are not then closely related to topography; the 28th (Fig. 26) was an example.

When the rising air contains convective elements, rainfall distribution is more erratic and the contribution released by the mountains acting as a barrier is more difficult to distinguish than on occasions when the rain falls from pre- dominantly layered clouds. Such was the case on the 14th. the totals for which are shown on the map for the 15th (Fig. 17). On this occasion the rainfall was the result of a combination of the three influences that are usually distinguished in text books: frontal (cyclonic), convective and orographic.

When convective showers are triggered off by forced ascent of an airstream that otherwise has few showers, they can be carried downstream, depositing rain in a strip on the leeward side of the mountains, i.e. in a position iust where a rain shadow is to be expected when rain falls from layered clouds. The 13th (Fig. 16) was a very good example because sbowers were light and very well scattered except in a strip downwind (i.e. oriented WNW-ESE) from the central mountains, and in which totals from showers of rain, hail and snow exceeded 0-2 in. With an observed distance on this day of 10-15 mi between the upwind side of the mountain mass and the, nearest reports of about 0-2 in.,

436

and taking a speed of movement of the showers as about 30-40 kt, the approximate time for the initial development of the showers is easily calculated - a quarter to half an hour. These times are consistent with observations by radar of the growth of rain and hail inside shower clouds, and also with theoretical calculations (see, for example, Mason 1962, pp. 88 and 91).

Widespread rain associated with vigorous fronts usually forms in clouds that are deep compared with the height of Snowdonia. Such rains occurred on the 12th and 20th (Figs. 13 and 25) and although no measurements of cloud tops were available on that day it is likely that they exceeded 10 ooo ft, and possibly 20 ooo ft. Upcurrents due to forced ascent were probably greatest in the lower atmosphere so that the majority of the extra water in the clouds can be supposed to have been present in a layer between cloud base and 5000 or 10000 ft . Downwind from the mountains this layer would have been relatively free of low clouds because the airstream would have been warmed by compression as it descended in the lee, and the small amount of cloud water left unscoured by the rain falling over the mountains would easily evaporate. As a result, rain drops falling from the more persistent, higher clouds would have much less chance of growing at lower levels on the leeward side, where the rainfall would be correspondingly less intense than over the mountains (Fig. 27) and also usually less intense than on the windward side where much more low cloud is likely to be present. Moreover, any drops still forming in the higher clouds would have a good chance of evaporating before reaching the ground because they would be falling through a deep layer of relatively dry air.

Fig. 27. Formation of a rain shadow in the mountain barrier

lee of a

An important factor which determines the ability of a cloud to give rain is its depth. For layered clouds, some idea of the minimum depth can be gained by examining the clouds on days when drizzle did or did not occur. For example, drizzle was falling at 10 am on both the 3rd and 8th, when the cloud base was at about 500 f t and tops were probably above 3500 ft . (There were no measurements, but in the absence of significant leeward breaks the tops were probably higher than the highest summits - contrasting with the 4th and 5th when the tops were well below these summits and extensive leeward clearances developed). Thus, a depth of at least 3000 f t seemed to be necessary to give drizzle. By contrast, no drizzle was present on the 4th and 5th (when bases were about 1500 f t and tops at 2500 ft), on the 10th (base 4700 ft, tops 5800 ft) and on the 25th (base 2500 ft, tops 4000 ft) - i.e. when depths were 1000-1500 ft. These depths are consistent with observations elsewhere, and also with theoretical calculations (see, for example, Mason 1962, p. 77).

437

CLOUD PATTERNS

On both the 4th and 5th there were remarkably well-defined leeward clearances in an extensive thin layer of cloud whbse top was below the altitude of the highest summits. The reason for these clearances was that a strong temperature-inversion existed above the cloud top (Fig. 28). As a result, the atmospheric layer between cloud top and the ground was dense compared with the clear air above, and it can therefore be expected to have resisted forced ascent over the mountains. Instead, there was a tendency for the airstream to flow around the mountains. Wedge-shaped clearances, well shown in Figs 7 and 10, have been observed at other places where mountainous islands protrude through a layer of cloud (see, for example, the photographs on pp. 360-1 of Weather in 1969).

A remarkable feature of both the 12th and 20th, when there was widespread rain, was the presence of a cloud base rising downwind across the mountains. On both days at 10 am the clouds had a base of about 500 ft on the windward slopes whilst over the mountains as a whole, even among the higher peaks, the base was generally around 1500-2000 f t (see Fig. 12 (c), p. 433). This downwind rise in base is not usual in the absence of widespread rain; the reasons for its development in the presence of rkin are not obvious but it may well be quite common.

WIND PATTERNS

It is to be expected that the windflow across irregular terrain will be complicated, and the sample charts from this study readily c o n k the

Tem,perature (deg C)

Fig. 28. Temperature soundings by radiosonde at IZ GMT 4 February 1971 from Aughton (near Liverpool) and Long Kesh (near Belfast) showing strong temperature-inversions at about 2500 ft. Continuous line: temperature; broken line: dew point

438

expectation. Several types of distortion of the large-scale flow were demon- strated, but perhaps one of the most unexpected was a deceleration on the windward side. The charts for the 3rd, 12th and 13th all show lighter winds there than might be expected. Two mechanisms can be suggested to account for this:

(I) an increase of friction as the wind blows from sea to land; (2) a blocking of the lower atmosphere similar to that more clearly

shown on the 4th and 5th. On the evidence available, it is difficult to distinguish between these two

effects. In either case, the deceleration will result in upward motion, and a consequent increase in cloud water-content, to windward of the mountains; Fig. 29 illustrates this, and the rainfall distributions in Figs. 13 and 25 (12th and 20th) are particular examples. Evidence in support of blocking comes from the tendency for the flow to bifurcate (especially on the 3rd; also on the Ioth, 21st and 23rd, not illustrated). Moreover, a number of repogs of unexpectedly light winds came from places well exposed on windward slopes. Blocking and bifurcation is more likely when a temperature inversion is present in the lower atmosphere. Conversely, a showery day, when low-level inversions are usually absent, is less likely to show a windward deceleration; the 13th (Fig. 16) was such an occasion.

1- . . . . . . . . . .

Fig. 29. Expected increase in cloud density (water content) as a result of deceleration of the wind on the upwind side of a mountain barrier

An expected result of the study was the occurrence of strong decelerations, or sheltering, on the leeward side of mountains. This sheltering led to some places reporting light and variable winds, or even calms on days when the broad-scale flow was strong or gale-force (Figs. 13 and 16). Such extreme sheltering was usually found to the immediate lee of steep slopes, particularly at the bottoms of deep valleys such as the Nant Ffrancon, Nant Perk and north of Beddgelert. On the other hand, leeward valleys sometimes had remarkably strong winds, particularly when they were oriented parallel to the direction of the broad-scale flow. Occasions illustrated were: 3rd, 8th, 12th and 20th, but they also occurred (not illustrated) on the and, Ioth, 14th, 18th and 21st. Speeds of these winds were often comparable with those on exposed windward coasts, and it is reasonable to suppose that they represent the descent of air into the valleys from higher levels, probably comparable with summit levels (Fig. 30). Unexpectedly, there was little evidence for funnelling being the cause of these strong winds.

439

Places in the bottoms of deep valleys lyhg across the direction of the broad-scale flow had winds that vaned greatly speed and direction. Sheltering was dominant at some, whereas eddies seemed to develop with horizontal axes at others so that there were some reports of directions opposite to the broad- scale flow, e.g. Llanberis (51), Beddgelert (36) and Dolgarrog (74) in Fig. 16, and the Nant Ffrancon and around Capel Cung (63) in Fig. 24. Sometimes the flow was down-valley, in a direction perpendicular t o the general flow; the Nant Ffrancon on the 12th was an example (Fig. 13). The flow near O p e n Cottage (61) seemed particularly complex: a south-westerly airstream gave south- easterlies down Nant Ffrancon and westerlies towards Capel Cung.

Fig. 30 Acceleration of the wind on the leeward side of a mountain barrier

Sometimes a strong down-slope flow develaped; examples: Nant Pens on the 12th, Nant Gwynant on 13th, Nantlle on 17th particularly. In the absence of more detailed studies it is not possible to be sure of the mechanisms leading to these winds, but they may be similar in origin to the strong winds observed along leeward valleys lying parallel to the broad-scale flow. Such local winds are known in many mountainous parts of the world (Brinkmann 1971).

On nights with clear skies and light winds, valley-bottom temperatures fell rapidly, and cold, dense air tended to flow down-valley as gentle katabatic winds, in directions bearing little relation to the broad-scale flow. Katabatic winds were present at 10 am on the 4th, 5th and 15th (Figs. 5, 8 and 17). and also on the Ist, 2nd (when minimum temperatures in the bottoms of central valleys were -4 to -5"C, irrespective of altitude above sea level), 11th and 22nd. These shallow, weak, drainage winds are usually destroyed during the day as temperatures rise, but changes may be slow in deep valleys during winter when some parts are permanently shaded. A brief discussion of various mountain winds has been given by Pedgley (1967).

The wind pattern at 10 am on the 16th (Fig. 21) deservesparticular comment because it was the most complex. Over the northern part of the Conway valley, the light southerlies probably represented part of the broad-scale flow ahead of the advancing depression; so did the easterlies near Capel Curig (62, 64 and 65) feeding into the south-easterlies of the Nant Ffrancon (57 and 61). However, the light northerlies around Llanrwst (75,76 and 77) can be looked upon as feeding the easterlies, whilst the westerlies at Dolwyddelan (81) and Penmachno (84) may have been a feeder into the southerlies since the valleys here are closed at their western ends. The south-westerlies from Baxigor (30~23 and 24) to Penmon (25) show the flow south-east of the depression's centre.

440

Apart from barrier effects, the complexity of the flow on this day probably

(I) a depression over north-eastern Anglesey, with an accompanying cyclonic circulation;

(2) a pressure-pattern changing quickly with time (as the depression moved south-eastwards) so that the winds were unable to remain in balance with the forces producing them;

(3) a strong temperature-contrast between cold south-easterly winds over the snow-covered land and somewhat warmer north-westerlies over the sea.

arose largely from the presence of three factors:

Some points of particular interest were: (I) the strength of the north-westerlies compared with the south-

easterlies (a temperature inversion at low levels was almost certainly present in the latter airstream, thereby enhancing blocking) ;

(2) locally strong south-easterlies (e.g. Nantlle and Nant Ffrancon - possibly an effect of blocking) ;

(3) the apparent circulation centred a few miles south of Caernarfon - this was probably unreal, being due to the coexistence of the leading edge of the north-westerly stream over the area and opposing strong south-easterlies blowing from the direction of Betws Garmon.

A schematic representation of the flow pattern on this day is shown in Fig. 31.

Fig. 31. Schematic flow-pattern at 10 am 16 February 1971, based on Fig. 21

CONCLUSIONS

The aims of this study were modest, and it was not expected that anything startlingly new would be discovered. The study was not aimed at ex$hining local weather effects, but at demonstrating their nature and describing them as completely as the observations would allow.

Through the diligence and sustained effort of many people over this one month, it has been shown that a variety of local effects can be clearly outlined by a network of observers, many unused to such work and largely without the use of instruments other than raingauges. Thdr records fill in a remarkable amount of detail that would otherwise have gdne unrecorded. Detail such as that examined during February 1971 is of considerable interest to people living, working or holiday-making among the mountains. For them, these results should amplify their knowledge of the complexities of mountain weather, and perhaps indicate that these complexities do conform to recognisable patterns. For the professional weather-forecaster, charged with helping people in the mountains, the results presented here should be food for thought.

Perhaps the greatest benefit from a study such as this comes from its educational potential. Using their own simple observations, school pupils can construct maps of the local weather and the results may then be used for class- room discussion. Surely this is better lesson material than models illustrated in a text book!

ACKNOWLEDGEMENTS

This study would not have been possible without the time and energy given freely and in abundance by many people of different professions and activities, but all with a common interest -the weather. I am deeply indebted to them all, and to the organizations that assisted in many ways - from giving permission for their observers to help with the study, to supplying data gathered routinely. I wish to thank the Director-General of the Meteorological Office, the Directors of Education for Caernarvonshire and Merioneth, the Chief Engineer of the Gwynedd River Authority, the Engineer of the Eryri Water Board, the Director of the Nature Conservancy Research Station at Bangor, the District Forest Officer of the Forestry Commission at Llannvst, the District Officer H.M. Coastguard at Holyhead. the Area Agent of the National Trust at Betws y Coed, the Central Electricity Generating Board and the Senior Air T r a c Controller at RAE Llanbedr. Those who kept the daily weather log are listed in Table I ; I hope this account will be in some measure a reward for all their efforts.

REFERENCES

BERGERON, T. 1965 On the low-level redistribution of atmospheric water caused by orography. Sup#l. Pvoc. Int. Conf. Cloud Plays., Tokyo 1965, pp. 96-100

BRINKMANN, W. A. R . 1971 What is a foehn? Weather, 26 (6) , pp. 230-239 MASON, B. J. 1962 Clouds, rain and rainmaking. Cambridge University

Press.

442

PEDGLEY, D. E. Weather in the mountains. Weather, 22 (7), pp. 266-275 Heavy rainfalls over Snowdonia. Ibid., 25 (8). pp. 340-

SAWYER, J. S. The physical and dynamical problems of orographic

SPINNANGR, F. and 1954 On the distribution of precipitation in maritime JOWANSEN, H. tropical air over Norway. Met. Ann., 3 (14),

1967 1970

1956 350

rain. Ibid., 11 (IZ), pp. 375-381

- . .. PP. 351-424

Tellus, 20 (z), pp. 239-250 STORES~, P. B. 1968 Precipitation formation in a mountainous coast region.

TABLE I. List of places where observations were recorded specially for the study period

Station number (see Fig. 2) I 2 3 4 5 6

i 9 I 0 I1 I2 I3 14 IS 16 17

I8 I9 20 21 22 23

24 25

26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

41 42 43

Approximate altitude (feet above mean sea level) 280 I80 50 150 150 I 0

I00

100 320

50 50 30 670 460 350 380 500

750 320 50 140 50 150

80 30

50 400 700 450 270

40

520 600 450 50 440

220

200

100 20

20 800 700

Place of observation

Rhoshirwaun, Llidiardau School Botwnnog, Ysgol Botwnnog Porthdynllaen, H.M. Coastguard Nefyn, Primary School Bodfuan Pwllheli, Ysgol Granymor Trefor, Primary School Llangybi, Primary School Chwilog, Primary School Criccieth Borth y Gest Tremadoc, Primary School Cwmystradllyn, Water Works Pant Glas Nantlle, Primary School Pen y Groes, Ysgol Dyffryn Nantlle Groeslon

Rhosgadfan, Primary School Caeathraw, Ysbyty Reservoir Caernarfon, Ysgol Segontium Caernarfon, Ysgol Sir Hugh Owen Newborough Menai Bridge

Llanfaes Penmon, H.M. Coastguard

Port Dinorwic, Primary School Llanrug, Ysgol Brynrefail Deiniolen, Ysgol Gwaun Gynfi Rhiwlas, Primary School Bangor, Pen y Ffridd Field Station Bangor, Friars School Llanfairfechan, Primary School Penmaenmawr, Capelulo School Betws Garmon Rhyd Ddu Beddgelert, Hafod Ruffydd Isaf Beddgelert, Aberglaslyn Hall RhYd Penrhvndeudraeth. Primarv School

Name of observer

Mr. C. Rowlands Mr. R. D. Moms Mr. E. Robins Mr. I. M. Evans Forestry Commission Mr. R. Hughes Mr. H. Parry Jones Mr. H. D. Jones Mr. E. Thomas Mr. C. A. Gresham Mr. E. Davies Mr. R. D. Jones Mr. D. G. Humphreys Mr. J. R. Thomas Miss E. Williams Mr. G. Jones Mr. 0. Griffith and Mr. H. R. Williams Mr. E. W. Evans Mr. C. Jones Mr. D. W. Thomas Mr. J. A. Williams Forestry Commission Major C. Fanning- Evans Mrs. A. R. Oswell Mr. S. Williams and Mr. R. N. Kirby Mr. G. Jones Mr. J. 0. Lloyd Mr. W. J. Eames Mr. W. I. Hughes Mr. C. Ellis Mr. H. R. Griffiths Mr. A. H. Evans Miss G. Jones Miss B. Breeze Mr. R. G. Humphreys Forestry Commission Mr. S. Glass Mr. G. R. Jones Mr. E. F. Williams

Llanbedr, Royal Aircraft Establishment Mr. A. H. Smith, Mr. R. Harper and Mr. A. Pearcy

Harlech, Ysgol Ardudwy Mr. T. R. Moms Trawsfynydd Miss D. Owen Trawsfynydd. Atomic Power Station Mr. P. R. Jones

443

44 700 45 50 46 600 47 750 48 600

49 so0 50 510 51 400 52 360 53 310

54 3 1 0

55 I 8 0

56 550 57 530 58 650 59 750 60 670 61 I 0 0 0

62 680 63 640

64 780

66 450

69 70 70 20 71 I 0 0

72 170 73 700 74 30 75 30 76 30 77 30 78 I 0 0 79 50 80 450

81 460 82 460 83 500 84 500

86 850

65 600

2 0 2 I 0 0

85 1280

Maentwrog Dam Maentwrog. Plas Do1 y Moch Ffestiniog, Bontnewydd Ffestiniog, Cwm Teigl Blaenau Ffestiniog. Doppog

Blaenau Ffestiniog, Ysgol y Moelwyn Llanberis, Llwyn Celyn Youth Hostel Llanberis, Dolbadarn School Nant Peris Nant Gwynant, Cwm Dyli Power

Station Nant Gwynant, Bryn Gwynant Youth

Hostel Nant Gwynant. Bryn Dinas Llanllechid, Primary School Bethesda, Ysgol Dyffryn %wen Gerlan, Primary School Gerlan, Water Works Ty’n y Maes, Snowdonia Park Motel Ogwen Cottage Idwal Cottage Youth Hostel Nant y Gwryd, Dyffryn Cottage Capel Curig, Plas y Brenin,

National Mountaineering Centre Capel Curig, Haulfryn Capel Curig, Plas Curig Youth Hostel Capel Curig, The Towers Llandudno, Urban District Council Deganwy, Primary School Conway, Ysgol Aberconwy Conway, Shellfish Culture Unit Llangelynin, Henryd School Roewen. Primary School Roewen, Rhiw Farm Youth Hostel Dolgarrog, Primary School Trefriw, Primary School Llanrwst, Ysgol DyfTryn Conwy Llanrwst Llanrwst, Gwydyr Uchaf Betws y Coed, Rhyd y Creaau Pont y Pant, Lledr House Youth

Dolwyddelan, Lledr Forest Office Dolwyddelan, Elens Castle Hotel Penmachno, Primary School Penmachno, Machno Forest Office Yspyty Ifan, Llyn Conwy Treatment

Yspyty Ifan, Bryniau Hywel

Hostel

Works

Mr. G. T. Jones Mr. T. Roberts Forestry Commission Mr. R. E. Jones Ffestiniog Power Station staff Mr. D. Morrow Mr. D. Glass Miss M. Butler Mr. A. W. Strudwicke

Mr. T. S. Roberts

Mr. E. W. Hughes Mr. J. Rogers Mr. A. H. Puw Mr. R. G. Williams Mr. A. A. Williams Mr. K. Dart Mr. N. W. Turner Mr. J. Cheesmond Mr. J. Roycroft Mr. A. Clark

Miss R. E. Walmsley Mr. 0. W. Owen Mr. J. Pope Mr. H. Drasdo Mr. G. A. Roberts Mr. I. P. Williams Mr. J. Whiteley Mr. P. F. Millican Mr. R. J. Thomas Mrs. M. Roberts Mrs. B. E. Hanson Mr. E. W. H. Lunt Mr. G. G. Williams Mr. B. P. Knowles Mr. N. A. Selkirk Forestry Commission Mr. P. J. Herlihy

Mr. A. Evans Forestry Commission Mr. H. R. Valintine Mr. J. H. Hughes Forestry Commission

Mr. A. V. Roberts Mr. N. E. Morgan

444

Documents

SOME WEATHER PATTERNS IN SNOWDONIA