The Editor,
British Glaciological Journal
Sir, Wind Slab Avalanche
During a ski-ing expedition in the Ben Lawers range above Loch Tay I had the interesting experience of observing the formation of a wind slab. It was a January morning, a calm, dear day with a bright winter sun. The whole hillside facing south-west was covered to a depth of approximately 2 feet with perfect powder snow. We were congratulating ourselves on the wonderful ski-ing conditions when quite suddenly a strong north-west wind sprang up sweeping the snow in clouds across the slopes. Almost at once we detected the difference in the surface of the snow. We halted under a slight bluff and examined the surface closely. There appeared to be innumerable small pellets of snow the size of small shot rushing across the slope. In about half an hour the slab was over 
inch thick and in approximately 50 minutes I crossed the slope on my skis, breaking the surface into small slabs fully 
inch thick. With a little persuasion the whole slope avalanched. There did not appear to be any air space between the slab and the underlying powder snow. Doubtless the latter had not had time to settle down.
By the time our observations were finished the whole hillside was practically unski-able and all prospects of a satisfactory day’s sport completely ruined
Yours truly,
H. MacRobert.
26, West Nile Street, Glasgow, C.1.
[It is interesting to note that Mr. F. S. Smythe has described wind-driven snow, just before it consolidates into wind slab, in exactly the same way as Mr. MacRobert—small pellets rolling over the snow surface. The point is that they appear to be to some extent rounded. In a film taken by me at the Jungfraujoch these grains can be very distinctly seen to disappear as they consolidate on the snow surface to form wind slab.—G.S.]
