Snow may be expected to slide, under the usual conditions, not only after hot days or days of wind. Days of warm mist and diffused sunlight may be even more deleterious.
Certain states of snow may start sliding irrespective of all changes of temperature, at any time and on calm days, merely as the result of some slight disturbance. First among these is ‘powdery’ snow. This may be either fresh snow or old snow which has never been melted and refrozen. Older snow in a powdery state we shall only expect in summer to find at greater heights, where the daily sun has little power. Since we shall recognize by sight that it is older snow, and since it has neither blown off nor as yet fallen, we shall be reassured about its holding quality, provided that we can also make certain that it is not lying in great masses or at a minatory angle. Sometimes, although we may not have seen it, we shall get evidence of its presence above us, in the appearance on gusty days of cascades of thin powdery snow streaming like dust down the lower ice or snow slopes. In the Alps these wind cascades of old powdery snow are sensational to see or to feel pouring round us on a steep wall, but they are too light to be harmful. The difference in weight between a flying veil of dry snow and one of wet snow must be felt on the shoulders or ankles to be appreciated. Cascades, of a more threatening sound, betray to us the presence above of old granular or crystal snow. The grains are easily set in motion; but in the Alps snow of this sort rarely lies at a depth to be worth regarding. Old powdery or granular snow, lying at a good angle and in light mass, we shall expect to be laborious but not to avalanche:—at least until we have put it to its last test of touch.
New snow, as a dry powder, if it lies in large masses, must be expected to avalanche on all slopes of steeper angle. In lesser quantities, it may be started by wind or by an incautious passage. On ice or rock it is never stable, until it has been cleared away or melted and refrozen.
New snow, as a wet powder on rocks, is desperately chilly for the hands, and has the disagreeable quality of transforming itself into an ice glaze under the pressure of the boots or the glove. If we are caught out by a fall of new wet snow on rock, ice-claws will protect our foothold, but nothing will save the hands. I have known very few men whose hands would stand more than a very short spell of clearing out rock holds in wet snow. Gloves are soon saturated, and the ice glaze forming under the hand still further reduces the effective grip of the numbed fingers. It is this that makes climbing on rocks impossible for days after a snowfall; while from snow or ice slopes, where we are not so concerned with handhold, the same storm will restrain us by its threat of avalanches.
After a storm of any length, two or three fine days are required in summer for the rock to clear. The first day of sun should melt the snow; the night following will refreeze it into a glaze: the second hot day will melt the glaze; and, with luck, the third day may find the rocks clear.
All wet snow, melting in heat or rain, in Föhn or after wind, is liable to avalanche, according to the angle at which it lies and in proportion to the amount of water which it contains. Only experience and touch can tell whether snow is or is not overcharged for the angle at which it is lying. It is astonishing what a quantity of water snow will hold without moving; and again, what a trifle will set it falling or arrest it. I have seen a whole quiet mountain face break into avalanches under the extra snow weight discharged during a few minutes of after-squall; and I have watched water-logged snow streaming like thin milk over the slabs of a north precipice, stopping suddenly as heavy, cold grey clouds crept over the rocks, only to begin again the instant the clouds passed and the sun broke through.
Frothy snow, neither powdery nor wet, but of the texture of dry sea-foam, is not so frequent in summer in the Alps as in some other great ranges. But I have found it occasionally near big summits or high up on northern snow faces. In spite of its intimidating quality, it is safe at almost any angle if it lies on a good sub-surface. But it is infinitely laborious. A step takes us up to our waist or shoulders. Progress is only possible by flogging a furrow up it with forearms and shins, which leaves a trail like an ecstatic sea-serpent. Some relief can be got by slapping down the shaft of the axe flatwise above us and crossways to the line, and pulling up on it as on a horizontal bar.
Before venturing upon snow of definitely unstable type, such as powdery snow or wet snow, we must make certain of its mass, its angle and the nature of the surface below it. We then adapt our methods accordingly.
For instance, if we find that wet or powdery snow is lying upon ice at anything but a very low angle, we start by finding out if it is of the same quality for all its depth. If it is, we must clear it away at each pace and cut a step in the ice below. If the poor quality is only superficial and improves below, or if we notice that where the lowest skin of snow rests on the ice it adheres either in a film or in measle-patches,—which signifies that the ice and the snow surfaces are going through a gradual process of peaceful interpenetration,—we can count on this lower snow surface for sure footing. We need not cut steps, but only stamp through the snow to a safe depth for a sound step.
If we have any doubt as to the security of a slope, which we shall have when the angle of the slope and quality of the snow are varying and the resultant stability is indeterminable, we must ascend or descend it vertically, if possible, or on very steep zigzags. We must take care to cross incipient cleavages, or traces of horizontal strains on the surface, at right angles, and never encourage them by letting the weight of our party tramp parallel to their length.