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.
IV. Wet old snow avalanches—the Grundlawinen of the older authors.
I. Dry Powder Avalanches.—Newly fallen snow, which has not been subject to thaw or sun, contains a great deal of air until it settles, and even when it has settled it still imprisons a considerable quantity of air. This makes for stability, for snow is less likely to avalanche when it lacks cohesion. I have often experimented on newly fallen snow at a low temperature, and I have found it almost impossible to start an avalanche on any slope below about 35 degrees and less than about 200 feet in height. Small snow slides are common enough if the underlying surface is hard; but as a rule even snow slides come to a standstill after a few yards. The really dangerous dry powder avalanches only occur on very long and steep slopes, where the amount and mass of the snow is sufficient to produce the necessary momentum for a big avalanche. Dry powder has a strong internal friction, and, as a rule, some powerful external impact is necessary to start an avalanche. Of such impacts wind is the most dangerous. A sudden blizzard may convert a valley, safe when the ski-runner entered it, into a veritable death-trap. Further, the fall of an avalanche on one side of a valley may precipitate other avalanches on the opposite side. Partly owing to the air imprisoned in dry powder, and partly owing to the momentum of the avalanche itself, the wind caused by a big fall of snow is extremely powerful and destructive. Houses and trees are torn away by the blast, even though they may be beyond the track of the avalanche. The force of the wind is multiplied manifold when the avalanche falls into a constricted space, such as the floor of a narrow valley. I have seen a bridge just below Gletsch destroyed by a spring avalanche, or rather by the wind caused by an avalanche on the opposite side of this very narrow valley. A large part of the bridge, weighing several tons, had been thrown upwards to a height of about 150 feet!
After a heavy snowfall the danger of dry powder avalanches may last for a day or two, or even more; but as a rule two or three days of settled weather and keen frosts render most northern slopes—in winter, though not in spring—safe enough. When the powder snow has passed into the stage known as Crystal powder—i.e. when the small light dry powder has been converted into crystals of an appreciable size—the danger of avalanches is very remote. On a windless day with a temperature in the shade below freezing, I should not hesitate to cross almost any slope up to 35 degrees which was covered by genuine crystal powder snow, provided that the slope petered out gradually on to the level and did not overhang a cliff.
II. Wet New Snow Avalanches.—Directly the powder snow is exposed to surface thaw, either owing to a rise of temperature or to the sun, its weight and cohesiveness increase, and the danger of avalanches is consequently much greater. Sometimes the snow falls with a temperature above freezing. This wet new snow is dangerous, but as it is also extremely unpleasant for ski-ing, few ski-runners are likely to be abroad. On the other hand, ski-runners are often tempted to cross a southern slope where the powder is beginning to melt. Snow on a steep southern slope soon gets thawed through the bottom, so that ground avalanches are quite normal in winter on south slopes. As a rule, south slopes in winter get rid of their superfluous snow in the first two or three days of fine weather. The snow that remains is thawed by day and frozen by night, so that at the end of four or five days the south slopes have got rid of their avalanches, and the snow that remains is a crust more or less hard and slippery. This crust by day may become soft breakable crust, but once it has crusted, a slope in the winter is not likely to avalanche until there is a new snowfall. A very marked rise in temperature may make a south slope that has been crusted dangerous again; but such sudden and marked rises of temperature are rare in winter.
In general, therefore, south slopes in winter are safer than north slopes. They give rise to more avalanches, but such avalanches as fall off south slopes generally fall within two or three days after a snowfall, after which a south slope is crusted and safe in winter; and though after a few days of settled weather most north slopes are absolutely safe, very steep and very long north slopes, or short steep slopes overhanging a cliff, are always dangerous. Of course, whenever the wet Föhn is blowing, or whenever there is a general thaw, all steep slopes, and a great many moderate slopes, become very dangerous indeed. The effect of the thaw is to give the snow the cohesiveness and weight which it lacks in its pristine dry condition. The dampness in the air saturates the snow with moisture and increases its weight. During a severe Föhn you will often see huge ground avalanches almost as destructive as those that fall in spring. When the Föhn is blowing ski-ing is always extremely dangerous. Fortunately, it is also extremely unpleasant, or fatal avalanche accidents would be more frequent.
The dry Föhn (see p. [418]) is much less dangerous. Unless it is very pronounced, it will hardly affect northern slopes in winter, though it may convert a south slope, usually covered by hard crust, and therefore safe, into soft wet and dangerous snow.
In spring northern slopes usually hold powder snow for a few days at high altitudes, and even at moderate altitudes in the early spring, such as March. This powder snow soon loses the dry, light, powdery characteristics of winter powder. Though it continues to yield excellent running, spring powder is very liable to avalanche. It is damper and more cohesive than dry powder, and therefore more dangerous. Dry powder often rests on the ground below. A northern slope will often be covered with a homogeneous layer of powder some feet in depth, but spring powder (see p. [413]) invariably rests on a hard-crusted slope below. It therefore tends to slide away during the warm hours of the day, and should be treated with very great caution.
Avalanches composed of spring powder are, properly speaking, new wet snow avalanches. They must be carefully distinguished from old wet snow avalanches, for old wet snow is formed by the melting of crust, whereas spring powder is formed by the melting of powder snow, i.e. ‘new snow.’ Snow may be defined as ‘new’ before it has been crusted, and as ‘old’ when it has been through the crusting process. Thus powder snow is always ‘new,’ however long it may be since it fell. On the other hand, a snowfall in June may be turned into crust within twenty-four hours, and thereby become ‘old snow.’ Crust and soft snow formed by the melting of crust are both ‘old’ snow.
Spring powder is all the more dangerous, because it yields wonderful ski-ing at a time when other slopes have been spoiled by the sun. Furthermore, as spring powder is found on north slopes, ignorant ski-runners underestimate its danger; for it is a common fallacy among the inexperienced that south slopes are more dangerous than north. In spring the reverse is usually the case, for avalanches in spring are occasioned by the general air temperature just as much as by the sun. (See also below, p. [438])
III. The Wind-slab.—The wind-slab is the most treacherous of all avalanches, the most difficult to foresee, and the most incalculable in effect.
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.
Sun-formed crust always merges gradually into the underlying snow. There is no sharp plane of cleavage. The hard crust merges into softer crust; the softer crust into soft snow. There is often, of course, a plane of cleavage between two successive falls of snow—the upper layer may be soft snow resting on crust; or it may be snow which is superficially crusted resting on crust. And directly the sun melts the superficial crust there may be danger. But so long as crust formed by alternate melting and frost remains unsoftened by the sun, it may be deemed to be absolutely safe so far as avalanches are concerned, for this sun-formed crust will merge gradually into the snow immediately below it.
Wind-formed crust is, however, often sharply separated from the snow underneath it. Wind-swept crust may overlie powder snow with no intervening and softer crust to act as a binding influence. The crust may be absolutely separate, susceptible to different strains and tensions, and forming the shallow vault described above.
Should you suspect a wind-slab, sound with the ice-axe, and try to discover whether the snow is homogeneous or rests on a soft streak of snow below. If, at the border of the dangerous slope, a sharp stamping with your ski produces a settling noise, followed by the breaking away of detached fragments of snow-slab, you will know that the slab is probably insecurely poised on a shallow vault below.
Wind-slabs are, fortunately, not very common. They can only exist under winter conditions, heavy snowfalls, severe wind and comparatively weak sun action. After April, for instance, the formation of a wind-slab would be impossible, for the May sun is strong enough to melt any crust formed by wind or by any other action. In summer they are uncommon excepting under unusual conditions. They sometimes occur in late summer, when the sun has lost much of its strength and is no longer powerful enough to thaw snow which has been converted by wind into a wind-slab.
IV. Old Wet Snow Avalanches.—For the distinction between old wet snow and new wet snow, see p. [427].
Old wet snow avalanches are very common in spring. The snow, which has been melted and frozen, and remelted again and again, gradually becomes denser and heavier. As the spring advances the power of the sun becomes very great. In the afternoon, and at lower altitudes long before midday, most snow slopes are saturated to a greater or a lesser depth by the melting power of the sun. Such old wet snow is of course extremely dangerous.
The great spring avalanches, the ‘Grundlawinen’ of Continental writers, usually select well-known tracks. Some of them have local names, and their annual occurrence is as regular as the return of spring. The long tongues of bare spaces between forests mark their track. Incredible quantities of snow are torn from the mountain side; trees are uprooted and boulders carried downwards. The avalanche comes to rest far below, and spreads out a discoloured tongue of snow-blocks, dark with the earth rooted from the mountain side, and strewn with small trees and shrubs. Sometimes, after an unusually severe winter, these big spring avalanches extend their domain, and destroy chalets, and bridges, and even villages. Roads that cross the line of these spring avalanches must be ensured against destruction by tunnels.
Superficial avalanches of old wet snow are more common than these big ground avalanches. These superficial avalanches occur daily in spring weather. The snow is saturated with water, which acts as a lubricant between one layer of snow and the harder crust beneath. Sometimes avalanches are started by the snow thawing from the ground upwards, for the ground in the late spring is warm enough to thaw the snow immediately above it. I have seen a vault one foot in height between the ground and the overlying snow.
The power of avalanches is best appreciated by those who have visited the Alps in May. It is an interesting, if annoying, experience to be confined to some high alpine club hut in May by a sudden invasion of Föhn. If the club hut can only be approached over steep ground or up a steep and narrow valley, there is nothing to be done but to wait till the Föhn disappears. Hardly a minute passes without an avalanche falling off some near or distant slope. The roar of big avalanches is varied by the hiss of the smaller snow-slides. Thousands of tons of snow are removed from the steeper slopes every hour.
Old wet snow avalanches are much more deadly than avalanches formed of new snow. Newly fallen snow weighs about 1½ cwt. the cubic yard. Old wet spring snow weighs about 15 cwt. or ¾ of a ton the cubic yard—in other words, ten times as much as newly fallen snow.
Furthermore, if you are overwhelmed by old wet snow, you will find the very greatest difficulty in freeing yourself, even if you are only covered by a layer a foot or so in depth. Powder snow contains a great deal of air, so that you can live for some time even if buried in a powder avalanche, but the wet spring snow contains nothing but water, and suffocation is a matter of minutes.
Whenever the Föhn blows in spring, all slopes above a very moderate degree of steepness immediately become extremely dangerous. In normal clear weather there is a frost at night, so that any slope, however steep, can be crossed without fear of avalanches between sunset and dawn. As soon as the superficial soft crust begins to form on the wet snow all danger of avalanches disappears.
On the lower slopes in May, the interval after the dawn during which a steep slope may be crossed with safety varies greatly. In May hard crust softens with surprising speed, and after 9 a.m., or even earlier, the risk of avalanches below the glacier level soon becomes formidable.
A vital distinction must be drawn between the kind of softening that is produced when a solid homogeneous crust softens superficially, and the melting of a superficial layer or crust resting on an older crust below. The second case occurs when a layer of soft snow, or of crust, rests on the older strata of crust. Once this new layer has melted it is very liable to slide off from the older layer below. On the other hand, a homogeneous crust softening superficially is usually safe enough so long as the underlying crust remains hard. Telemark crust, which is crust softened superficially so that Telemarks are easy (p. [415]), is usually safe.
The great danger is the existence of a layer of crust formed by a recent snowfall resting on an older layer. I was once climbing the steep slopes that lead from Zinal to the Mountet glacier. It was on the last day of April, and the sun had just struck the slope. The local guide was leading, and I ventured to suggest a detour to avoid a traverse across a slope that had begun to soften. He ignored the risk, and proceeded. I remained behind and watched him. Suddenly a layer of snow about six inches in thickness, which had softened down to the old hard crust beneath, slid away with startling rapidity. The guide gave a small jump, and got his ski into the old layer, while the softened snow slid away and disappeared over the cliff below. The guide’s top ski had cut through to the old layer before the snow slipped, otherwise he would have been killed.
In May in the High Alps the risks of such avalanches is small on all save very steep slopes. Most of the big spring avalanches fall below the limits of the summer snowline. They slide off slopes which are bare of snow in summer. Once the region of the névés is reached the danger is very much less, though of course by no means non-existent, especially when the Föhn is blowing. The May ski-runner must often time his ascent to a club hut to arrive in the early hours of the morning, and wait for his descent from the glaciers to the lower valleys for the hour after sunset.
As the winter advances the danger from avalanches increases, not only because the quantity of snow increases and because the sun is more powerful and the temperature higher, but also because the inequalities on the underlying surface gradually disappear. Scree, small boulders, shrubs and other natural checks to the flow of an avalanche vanish in the ever-deepening snow. Roads and small shelving plateaus, which break up a steep slope, get buried. Each succeeding avalanche leaves some of its burden on all protruding shelves, and thereby tends to smooth out the mountain side, creating, in place of a slope broken by inequalities, one long, even flow which presents no hindrance to the avalanche. Thus big avalanches tend to take the place of the smaller avalanches which fell down part of the slope, only to be arrested at some convenient terrace, such as a road or small plateau.
In the spring avalanches often fall right across rivers, which very soon form a tunnel beneath the snow-bridge of the avalanche. Such snow-bridges should be crossed with caution. More than one ski-runner has been killed by breaking through the remains of an avalanche into a river.
It is a common illusion among the inexperienced that north slopes are safer than south slopes in spring. They are not—in fact, north slopes are more dangerous than south slopes. In spring it is the general air temperature which determines the fall of avalanches. True, the south slopes avalanche first, and for this reason north slopes hold much more snow, so that when they finally get rid of their superfluous snow they produce far and away the most destructive avalanches. Of course in spring the sun shines on all slopes, and it shines with quite sufficient force even on due north slopes to produce an avalanche. In fact, the really great spring avalanches are those which fall from northerly slopes.