Loose scree guarantees the ski-runner against ground avalanches, but, as the winter advances, loose scree is soon covered with deep snow, from which later layers of snow can slide uninfluenced by the underlying scree. In fact, as the winter advances, the original underlying surface plays a smaller part in the problem of avalanches. Hard-crusted snow covered by soft snow is especially dangerous, and of course ice, as is so often met with in the High Alps, is the worst under-surface of all. Fortunately, snow often attaches itself firmly to ice, transforming an ice slope into a snow slope.
The conditions necessary for this transformation will be explained on pp. [426-427].
So far we have dealt with primary conditions, the nature of the ground before the snow has begun to fall and the gradient of the slope from which the avalanche slides. An important factor is the quantity of snow on the slope. It often happens that a shallow superficial layer detaches itself and carries a ski-runner down the slope. If the slope ends on gentle ground, no damage is done beyond the loss of height and the consequent waste of time and effort in reascending to the spot from which the avalanche started. But if the snow slide carries the ski-runner over a cliff or into a bergschrund or crevasse, it is clearly immaterial to the ski-runner whether his original snow slide was 1 inch or 6 feet in thickness.
I propose to use the word SNOW SLIDE for such small avalanches as are only dangerous where they carry the ski-runner on to dangerous ground, such as the edge of a precipice, and to reserve the word AVALANCHE for avalanches deep enough in themselves to overwhelm and possibly to kill a ski-runner.
Though a very small layer of snow—an inch or even less—is enough to produce a snow slide, especially if the shallow layer rests on ice, the amount of snow necessary to produce a real avalanche is much greater, and varies very much with the quality of the snow.
Classification of Avalanches
The old writers divided avalanches into ‘Grundlawinen’ (ground avalanches) and ‘Staublawinen’ (dust avalanches)—a misleading classification, for a ground avalanche may be composed of dry powder snow, and produce all the appearance of a ‘Staublawinen’: the clouds of white snow dust, once supposed to be peculiar to avalanches of powder snow, are really common in almost every type of big avalanche, especially where the avalanche falls over steep cliffs. I prefer to divide snow avalanches into four main classes:
I. Dry powder avalanches.
II. Wet new snow avalanches (i.e. powder snow which has begun to thaw as differentiated from old wet snow which is formed by crust which has been thoroughly melted).
III. Snow-slabs.