In general, therefore, the direction of the prevailing wind can give us no certain guidance as to whether or not to expect the existence of cornices. If a snow-bearing wind has blown against the visible side of our ridge or summit, we shall see them. If upon the invisible side, we must assume their presence until we can disprove it. But a wind-shift or variable current may always contradict our calculations. A cornice is very quickly built.
If we have not been following the performances of the prevailing wind, or if we mistrust its portent in any case, we have to rely upon visual evidences when we are prospecting the approach to any snow summit or the traverse of any snow-crowned ridge. In the case of a single summit it is less trouble to assume the presence of a cornice, until we fail to find it. In the case of a continuous ridge it may save us much tribulation to ascertain its condition in this respect before we start out for a long day upon it. If there are cornices, and they project towards us, they will at once be recognizable; if away, it is not always easy to make certain, without getting a sight of some part of the ridge in profile. Their detection at one point of a ridge will dictate the necessity of precaution along the rest. A telescopic examination can often discover the thin dark line of junction or strain, running parallel to the crest in the snow on the near side, where the projection of a cornice towards the far side starts its inclination. (In more considerable snow ranges, such as the Andes, this crack has, I am told, on occasion been proved to be due to a longitudinal crevasse on the ridge. In the Alps, however, I know of no case of this having been observed.)
An alternative and rarer indication will be the discovery of a band of shadow or duller tint, seen in the right light, running along the snow wall just below some part of the crest. This will be due to a relative steepening of the snow wall, sometimes even taking the form of a concavity, where the back of a high, steep cornice, facing towards the far side, hummocks up off its supporting ridge. The appearance I believe to be occasioned by a fall or a shift in the prevailing wind,—a fall permitting the snow to accumulate upon the head of the cornice and form a sort of bulbous whale-back over it, or a shift of wind to the near side beginning a projection which may ultimately grow into a ‘double’ cornice, with the higher of the two facing towards us.
Failing either of these signs, any two projecting points of rock or snow, near together on the suspected ridge, should be examined. If the connecting line of snow between them shows sharp and continuous against the sky, and ascends at its either end in a continuous single curve to the points selected, the cornice on the far side, if any, will be slight and local. If, however, the snow rim seems to merge indefinitely on the skyline, and its curve ascends to the points at either end in a variable arc of different centres, so that we seem to see, as it were, round the edge of a fold where the snow-curve hangs to the rock points, a cornice is indicated. If the breadth of the points or towers is sufficiently ascertainable to enable us to estimate the average thickness of the connecting rock ridge hidden below the snow between them (as it usually is unless the towers are seen absolutely flat in face), then the look, almost a reflection, in the sky immediately above the snow curve, and the character of the snow curve where it wavers over against the sky, will indicate to an expert eye whether the cornice is large or small.
Where, again, sufficient rock points projecting from a snow ridge are visible, as we see it in face, to suggest what must be the actual line of the snow-covered rock connecting them, any wayward sweep away from us in the visible line of the snow between them is an indication that the snow rim, where the skyline thus unaccountably retreats from the eye, is superintending a cornice on the far side.
The presence of a double cornice—the fatal cornices that face two ways, one built up above the other—is not difficult to establish. A band of shadow or low light on the near side of a snow crest, below a band of higher light, means a lower cornice facing towards us under an upper cornice curving away. A band of markedly higher light, below a band of shadow or obvious concavity, means an upper cornice facing towards us above a lower cornice facing away.
For all such observations on snow, which find their opportunity in the relative positions of light and shadow, it is apparent that a time must be chosen when the sun shines from the right quarter. Their accuracy depends upon their being continued over a time sufficient for the sun to travel past, and so indicate to us dimension, by the change in, or the disappearance of, the shadows cast.
Wind and Snow Signs.
Wind is not ‘seen,’ but its immediate effects upon snow are, more especially on high ridges, corniced or otherwise. The quality and quantity of the surface snow on a ridge, which it is important to know beforehand, can be discovered in times of wind from the amount and direction of the snow particles, which are seen to be blowing off the ridge, and appear as a film or puff of vapour upon the sky just above. The absence of any halo to the ridge, in spite of the manifest action of wind upon adjacent clouds, is equally valuable evidence of the stable quality of the snow.
The direction and force of the wind will often suggest the side of the ridge to which we shall prefer to limit our passage on the morrow. On the windy side, if the ridge be snow-covered, according to the quarter and character of the wind, the snow will be crusted or hard, or we may have to encounter a bare sheet of ice. If it be rock, again according to the wind, we shall find the holds either cleared or ice glazed in the morning. On the side sheltered from the wind, on a high exposed ridge the snow should be just of good quality. But even on such ridges it may be spongy and cantankerous if the wind has had Föhn in it.