Falling snow is usually accompanied in the High Alps, and often accompanied in the lower regions, by wind. If the wind is powerful, the falling snow is driven over exposed ridges in whirlwinds, and comes to rest on the lee-side and in sheltered hollows. In this way the snowy avenues leading to glacier passes and the more sheltered snow-fields receive more than a fair share of snow. This action of the wind, denuding the exposed ridges and feeding the hollows and lee-sides, takes place on both a large and a small scale: on a large scale, when snowy valleys are fed from the snow blown off the exposed ridges that rise out of them, and on a small scale on any slope exposed to wind which is divided by ridges, however small. Any tributary ridge on a slope across which a wind is blowing will have a wind-side and a lee-side, and will accumulate snow on the lee-side and give off snow on the wind-side. A stone wall or even a hedge provides an example of this on a small scale.
Snow driven by wind and settled on lee-sides may either be more or less powdery, a denser, heavier powder than normal powder, or it may assume one of the many forms of wind-caused crust. We have described on p. [407] the various forms of winded powder, such as ripplemark, caked powder, etc., and on p. [408] the various forms of wind-formed crust, such as windboard and Skavla.
In general, the greatest caution should be exercised when crossing any slope which has accumulated much wind-driven snow. Wind-driven powder snow is heavier and more adhesive than ordinary powder. It is more detached from the underlying surface, and is much more conducive to avalanches. The ski-runner should keep his eyes open for traces of wind action, and when he finds snow which has obviously been exposed to severe wind, he should exercise the greatest possible caution on crossing over to the lee-side of a ridge dividing slopes, whence the snow has obviously been blown away, from slopes where the wind-driven snow may have fallen back to earth. Wind-driven powder may in turn be covered by a new snowfall, which adds to the difficulties of diagnosing the avalanche risk.
So far we have been dealing with wind-driven snow which still retains some suggestion of powder, which is soft and dense and caky. A still more treacherous and dangerous wind formation is the wind-slab, or ‘Schneebrett,’ of Continental authors.
Windboard, as already explained (p. [407]), is a hard, slippery crust formed by wind. Windboard is common on glaciers in the winter months, and though disagreeable to ski on, it is safe enough in most cases. Sometimes, however, this windboard, instead of being homogeneous with the underlying snow, is loosely attached, and in places forms a vault with a hollow space between the windboard and the snow beneath. The windboard is, then, properly speaking, a wind-slab. The wind-slab, or, as some writers prefer to call it, the snow-slab, is formed by wind-driven snow, which eventually settles into a hard crust. As the wind-drifted snow is of a different density to the snow on which it settles, it tends to form a distinct stratum from the underlying snow. In winter the temperature is usually considerably below freezing, so that snow which falls, or snow which is drifted by the wind, cannot bind with the snow below. In order for two strata of snow of very different density to form a homogeneous whole, there must be a period when the temperature is just above freezing in order to produce the melting followed by frost, which is a necessary factor in the fusing together of two successive layers of different types of snow.
The wind-drifted snow, ultimately transformed into a hard crust, is then of a different density to the snow below, to which it is loosely attached. This underlying snow may be either soft snow or hard crust; in either case the wind-slab forms a covering layer insecurely attached to the foundation snow. This superficial layer is subject to different strains from those which affect the underlying snow; for the tension due to the expansion and contraction which follows changes of temperature affects the layers formed of snows of different density and character in varying ways. The surface layer, or wind-slab, may contract more obviously than the snow below, so that if the slope is concave in shape, the wind-slab, in contracting, tends to form an arch above a more or less shallow vault.
There is little if any surface indication to betray the fact that the wind-slab is not homogeneous with the underlying snow; the ski-runner may cross some such slope without the least suspicion that the hard, slippery crust is not quite so solid as it appears. Suddenly he will hear a sharp cracking noise; the hard crust will settle under him and cave in; the crust cracks along the line made by his ski, and the whole slope comes down on top of him in a cataract of tumbling blocks. The strata formed by the wind comes away, tearing with it much of the soft underlying snow, and pours down in a floor of hard, icy blocks of snow.
The wind-slab is the most dangerous and deceptive form of avalanche. Its hard polished surface gives a false sense of security. The temperature is no guide, for wind-slabs can avalanche at any temperature. Indeed, extreme frost tends to make the wind-slab more brittle. It can avalanche after days or weeks of fine weather when all the more obvious avalanches have fallen. Lastly, this wind-slab is to be found in the natural line of approach to glacier passes, in the long sheltered avenues that collect the snow blown off the exposed ridges.
It is of primary importance to distinguish most carefully between the crust formed by sun action and the crust formed by wind. A south slope crusted by sun followed by frost will never avalanche so long as the crust remains unmelted. Sun-formed crusts never avalanche. A careful study of south slopes will soon teach the ski-runner to recognize crust formed by sun and to distinguish it from crust formed by wind. The wind-slab is usually patchy, granulated, and often betrays the action of wind by a slight rippled appearance. The expert can detect wind-formed crust and can distinguish it from sun-formed crust.
The contrast between crust formed by sun (or by any process of alternate melting and frost) and crust formed by wind is instructive. The fact that the former is safe and the latter often dangerous is due not to any surface differences but to the difference in the nature of the connection between the under-surface of the crust and the underlying snow.