SNOWDRIFT PATTERNS O N VERTICAL SURFACES

By K. CLARK Portstewart, Co. Londonderry

The effect upon snow accumulation of wind moving over topographic irregularities is well-known and the creation of windscoops and snow drifts round buildings has frequently been described. There are fewer descriptions of how air movements around other forms of structure have been demonstrated by snow patterns. The wind damage to cooling towers a t the Ferrybridge, W. Yorkshire, power station recently highlighted the need for knowledge of the ways in which air passes through and around groups of towers.

Some interesting and relevant observations were made on a group of oil storage tanks at the disused whaling factory at Deception Island, South Shetland. During the passage of the warm sector in a number of winter depres- sions, northerly blizzards brought heavy-flaked snow which not only drifted but adhered to vertical surfaces. Consequently it was possible to see something of how the air had moved in depth around the tanks.

The shapes of windscoops (Fig. I) reflect the movement near the ground, but the adhered snow on the iron plates showed both in its thickness and in a close pattern of ridges and grooves the lines of air motion along the sides of the tanks. Two major patterns were recorded (Fig. 2): one had a build-up starting at the base of the tank and prominent updraughts, the other had a nucleus of accumulation high on the sides of the tank with attendant downdraughts.

The faces of windscoops also disclosed movements in eddies. A particularly clearly divergent motion is seen in Fig. 3. This appears to be a minor superficial pattern imposed on a drift face created in an earlier storm. Faces of snow drifts determined by the positions of eddies may also reveal the build-up of a drift, as in Fig. 4 where the variations in speed, direction, and snow supply during one snowstorm produced a structure reminiscent of dune sands.

Fig. I , Plan of wind scoop round group of oil storage tanks. ‘lank diameter about io m. Mean direction of associ- ated wind shown by arrow

Fig. 2. Drift patterns on windward faces of tanks. Height of tank about 10 m

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Fig. 3. Pattern of divergent striation on eddy face of snowdrift. Height of drift about 1.5 m

Fig. 4. ' Dune bedding' on face of snowdrift. Length of section about z m

SEASONAL CHANGES OF SNOW COVER O N THE CAIRNGORMS

By I;. H. W. GREEN The Nature Conservancy

HE series of photographs on pp. 212-213 was taken in 1962 and 1963 from T the front of the Speyside Research Station of the Nature Conservancy near Aviemore, Inverness-shire (grid reference NH 897142) looking SE towards Cairngorm and Braeriach. The summit of Cairngorm, 4084 ft OD, appears in the centre of the pictures. Two different cameras were used, and this explains slight differences in the field of view.

Photographs (a) , ( b ) and ( c ) show the gradual disappearance of snow after the intermittently cold and rather snowy winter of 1961-2. They clearly show the ' perennial ' drifts, including (centre) ones which have been exploited by the development of the Coire Cas ski-lift, the new road to which can be seen.

The fourth photograph (d ) shows the pattern after the first snowfalls of the season; signs of accumulation in the perennial drift areas can already be seen. The fifth picture ( e ) shows the snow-cover after a heavy fall; the snow had melted quickly on the lower ground, but cover was almost complete above 2000 feet. The sixth picture (f) is the scene after another heavy fall in latgewinter; the snow-line was lower, and snow covered the low ground as well as the high. The seventh picture (g) shows how much had disappeared 15 days later, and the final picture (h), taken exactly one year later than ( c ) , shows that remnants were in almost the same positions at the same dates in the two successive years.

Readers will find it interesting to refer to the note in the May 1968 issw of Weather (Green 1968), and also to the well-illustrated report prdduced by the Scottish Development Department (1967), which includes air photographs of the snow cover in the area.

REFERENCES

GREEN, F. H. W. 1968 Persistent snowbeds in the western Cairngorms. Weather, 23, pp. 206-208

SCOTTISH DEVELOPMENT 1967 Cairngorm area: report of the Technical Group on the Cairngorm area of the eastern Highlands of Scotland. H.M.S.O., Edinburgh

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