Chapter IX: Mountaineering on Ski (2)

Spring snow is quite common in winter when dry Föhn follows wet Föhn, and the wise ski-runner will seize every chance of securing spring conditions once he has despaired of proper winter conditions. He will choose south slopes instead of north slopes, low altitudes instead of high altitudes, and time his descent for the sunny rather than the shady hours. “The milder the frost the better the crust.” From which it follows that the lower the altitude and the drier the Föhn the better the ski-ing--once normal winter conditions have been interrupted.

It is most interesting to observe how the same slope will be composed of typical spring crust one day and of impossible slippery winter marble crust the next day. The difference is solely due to the fact that, in the first case, the night’s frost had been mild, and in the second case severe.

After Föhn you will often, for instance, find a queer kind of surface, called Foam crust, composed of innumerable overlapping edges, miniature cornices formed by a little trickle of water, a mere drop, which has run off a thin small eave of snow. Now hard frozen foam crust is very unpleasant, but directly the dry Föhn gains the mastery, and directly the hard foam crust is exposed to a hot sun and a hot dry Föhn atmosphere, it immediately softens, and yields very fine ski-ing not unlike the best Telemark crust.

To summarize the effect of Föhn in WINTER:

Wet Föhn followed by frosts produces a crust on all slopes which have been exposed to thaw. If the Föhn is very pronounced, and is accompanied by rain, which is followed by frost, all slopes will be covered by a hard solid crust.

If a period of normal cold winter weather sets in, this crust will be very hard and very slippery, and will only yield good ski-ing on south slopes exposed to the sun.

If the wet Föhn is followed by dry Föhn, you will get spring conditions at any rate at low altitudes--a hard crust, smoother but not unlike perforated crust, in the early morning, and a soft crust, not unlike Telemark crust, on south slopes when the sun is shining on them.

[Sidenote: Föhn in Spring.]

The Föhn is less deadly in spring, because snow which has been crusted by Föhn and frost is remelted by the sun, and the sun and frost together will always produce the same surface, whatever has gone before. A wind-swept slope or a Föhn-crusted slope are affected in precisely the same way. In winter Föhn spoils all snow that it has affected, and, save for the lucky accident of dry Föhn, one has to wait for a new fall till normal winter conditions are restored. But in spring, once the Föhn has ceased, a single sunny day followed by a single cold night’s frost is sufficient to produce the normal spring conditions, hard crust in the morning passing through the normal transformations of the spring day.

Wet Föhn is, however, most unpleasant in spring while it lasts. If you are caught by wet Föhn in a club hut you are imprisoned till the Föhn passes, for the wet Föhn brings down the avalanches on every slope above 23 degrees (see p. 430). Ski-ing, while there is a touch of wet Föhn in the air, is always unpleasant. Snow which has been melted by the sun in a dry atmosphere never entirely loses its crystalline formation, excepting at low altitudes in the very late spring. And even then sun-melted snow is never so unpleasant as snow melted by the Föhn. The Föhn disintegrates the snow, destroys the crystalline formation, reduces the snow to one uniform heavy mass.

Such snow does not stick _in spring_, provided it has been through the usual process of melting and refreezing on previous days. But though the spring Föhn does not produce stickiness, it gives the snow a dragging, clogging grip. It may not ‘ball’ under the ski like sticky snow in winter, but on all but steep slopes it makes ski-ing desperately slow. Uphill work is most trying, for the friction between wet Föhn snow and the ski is very marked. The ski have to be thrust through the clinging surface, and the wet Föhn not only affects the snow, it affects one’s whole body and produces a general sense of lassitude.

SUMMER SNOW

Between July and the end of September the snowline climbs ever higher. The snow above this snowline obeys the laws that we have tried to explain in the preceding pages. Snow is transformed into crust by the action of sun or thaw or wind, and the crust itself affords good or bad ski-ing, according to the conditions under which it has been produced and the conditions that affect it when once it has been formed.

In the summer you will meet with every type of snow in the High Alps. After a fresh snowfall the snow will often remain powdery for days on northern slopes at great altitudes. Between July and the beginning of the winter you will find typical winter snow, powder snow or wind-driven snow: you will find typical spring snow, such as film crust or perforated crust, but you will not find any type of snow which can fairly be described as mainly characteristic of the summer. The conditions in July and the first half of August will approximate to those of June, though of course there will be far less snow in August than in June. Towards the end of September the snow conditions will approximate to those of the winter proper, with the important difference that the snow at the end of September approaches a minimum.

Once the ski are left behind and the final climb begun on foot, there will be many complex and difficult problems of snow craft to solve which, however, hardly come within the scope of pure ski-ing. Of these the most important is the problem of safety. Once the ski are left behind, the climber’s interest in the snow is reduced to two main problems: Is the snow hard enough to make going easy and yet not so hard as to need step-cutting? Is the snow safe or will it avalanche?

The avalanche question will be treated in its proper place. Summer Ski-ing will be dealt with on pp. 468-470.

I have tried to condense this section, and, like all condensed and theoretic writing, it will no doubt prove rather dull reading. In the later sections I shall try to provide concrete illustrations of the principles here explained, and to show how the laws of snow craft may be applied in order to get good ski-ing in the months of the alpine calendar.

SNOW AVALANCHES

Snow avalanches may be classified either as _Ground avalanches_ or as _Superficial avalanches_.

GROUND AVALANCHES--the ‘Grundlawinen’ of Continental writers--may be defined as avalanches in which the entire snow surface is stripped off a slope, revealing the underlying earth, grass or rock.

SUPERFICIAL AVALANCHES may be defined as avalanches in which a layer of snow, more or less deep, slides off an underlying layer of snow or ice.

Before proceeding to discuss avalanches in general, and to analyse more exactly the various subdivisions of these two principal categories, it is essential to analyse the primary conditions that produce avalanches. _Primary conditions_ may be defined as those which exist before the snow has covered a slope, originally bare of snow. These primary conditions are the _contour_ and _gradient_ of the slope and the _nature of the surface_ that underlies the snow.

No avalanche has yet been observed on slopes whose gradient is less than 23 degrees, though, of course, even level slopes have often been overwhelmed by avalanches falling from steep slopes above.

Other things being equal, the stability of a snow slope depends not only on its gradient, but also on the gradient of the slopes just below and the slopes just above.

A concave slope, for instance, which has an even outflow so that there is no sudden change of gradient and so that the steeper slopes merge gradually into gentle slopes and these gentle slopes into a level outrun, is infinitely safer than a convex slope the higher portions of which are more gradual than the lower portions below. Slopes that steepen suddenly below a comparatively safe gradient should always be treated with great respect.

A slope, whose gradient would be perfectly safe if the slope petered out gradually, may be highly dangerous if the gentler slope ends suddenly in a steep slope, for the snow on the gentle slope is, so to speak, ‘in the air.’ It has lost the natural support which is afforded by a gradual concave base leading out on to the level, and there is a reasonable chance of the weight of the snow on the safer slope proving just too much to stand the strain at the point where the slope steepens. In general concave slopes are safer than convex slopes, and slopes where the gradient steadily diminishes towards the base are safer than those in which the gradient increases before the base is reached. Of course, any slope overhanging a cliff is always to be treated with very great care, as even a superficial snow slide which would be quite innocuous if the slope ended on easy safe ground may prove fatal if it carries the ski-runner to the edge of a cliff below.

The chance of surviving an avalanche depends greatly on the nature of the ground where the avalanche comes to rest. Many ski-runners have escaped unhurt after being carried down several hundred feet because they have managed to keep on the surface of the avalanche and because the avalanche has gradually spread out fan-shaped on open, gentle slopes. But an avalanche falling into the bed of a narrow V-shaped valley with steep sides is almost certain to prove fatal, for the victim of this avalanche will be buried by the snow falling above, and this snow will fill up the narrow bed of the valley and freeze solid instantaneously by pressure. Thus all narrow valleys such as the Urbachthal, or the upper Rhone valley between Gletsch and Oberwald, should only be ascended when the snow is thoroughly safe.

An analogous case is where a tributary ridge runs across a hillside. An avalanche falling down this hillside will pile itself up against the tributary ridge and a ski-runner will probably be crushed below the avalanche, squeezed in between the tributary ridge and the main slope. Often a large moraine fulfils these conditions, so that an avalanche falling from a neighbouring slope is arrested at the moraine and piled up against it.

Similarly, if you are caught by an avalanche while ascending a gully, your chance of escape is much greater if the gully widens below the point where the avalanche overwhelms you. If it contracts, the pressure of the snow forcing its way through a narrow space may crush you to death. Compare the account of the avalanche that killed Bennen quoted in _Scrambles in the Alps_.

The bottom of a valley is not only dangerous for reasons just stated, but also because the stream at the bottom of a valley often exercises, especially in spring, an undercutting effect on the snow slopes that end in the stream bed.

_The nature of the underlying surface_, apart from its contour and gradient, is a factor of vital importance, especially in the early winter and in the spring. At intervening periods most avalanches are superficial, and slide from an underlying surface of hard snow; but in the early winter and in the late spring the whole snow slope slides away, so that the nature of the underlying surface and the probable support that it affords is of great importance.

Steep grass-slopes form a dangerous under-surface, especially where it is never or seldom mown; for long unmown grass generally lies facing downwards, and offers a most slippery surface.

Grass which is regularly mown is usually short and stubby in winter, and gives better purchase to the snow. A slope covered by stony boulders, bushes or trees is usually fairly safe, though a big avalanche, once it is fairly under weigh, will sweep over shrubs and even over small trees. Fairly dense wood may usually be considered as safe, provided one avoids the long open clearings made by old avalanches, which so often run down the middle of a forest.

An elementary knowledge of geology is useful; the excellent geological maps published by the Swiss Survey can often be consulted with benefit. Rocks which suffer much surface disintegration provide a better purchase for snow than very hard and consequently very smooth rocks. The hard ‘Hochgebirgskalk,’ an alpine variety of limestone, which is very common, especially in high regions, is slippery, and instead of disintegrating gradually, as gneiss or granite disintegrates, has a habit of breaking away along vertical and horizontal joints.

The common rock known as ‘Flysch,’ common in the lower Alps, provides a much safer surface.

Glacier-polished rocks are, of course, especially dangerous, and the whole Grimsel region is consequently swept by avalanches throughout the winter.

The lie of the strata is an important factor. Where, as is usually the case, the strata are inclined, one slope of a mountain will usually be safer than the other. Diagrams I and II represent the north and south slopes of a ridge running more or less east and west. The ridge is formed by parallel but inclined bands of strata. Rock climbers know that the slope in Diagram I, though of the same gradient as the slope in Diagram II, is very much more difficult to climb. It is also much more liable to avalanche, as the outcrop of the strata provide a natural check to avalanches in Diagram II; whereas in Diagram I each outcrop forms a small steep snow slope quite unsupported. If the outcrops are of reasonable breadth, there will be belts running across the face of the slope at A’, B’, C’, D’ which will be inclined into the slope, and provide a safe line of traverse; whereas there is no safe line for a ski-runner desiring to traverse or ascend the slope A, B, C, D.

Geological maps indicate the ‘strike’ of the strata, and therefore provide useful clues as to the varying liability of slopes to avalanche.

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.

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.

SUMMER SNOW AVALANCHES

During the summer months, as already explained, every type of snow can be found in the High Alps from pure winter powder to the numberless varieties of spring crusts and spring soft snow.

It follows that avalanches in summer obey the same laws as in spring. There are far fewer avalanches in summer than in spring, for there is far less snow to fall. Quite enough, however, is left to make the avalanche problem of vital importance for the ski-runner and the summer climber.

The various types of avalanches described in this chapter are not confined to winter or spring, with the possible exception of the wind-slab avalanche. I should have said, a priori, that wind-slab avalanches were peculiar to winter and the early spring, for they depend for their existence on snow falling at a very low temperature, and on the snowfall being followed by strong and cold winds before the sun has time to melt the snow and to bind it into the underlying surface. I am told by Mr. Young that he has seen wind-slab avalanches in summer--probably, I should imagine, in the very late summer or early autumn, when the conditions begin to approximate to winter conditions. Apart, then, from wind-slab avalanches, which must be very uncommon in summer, all the other types are by no means unusual. Avalanches of powder snow, of old wet snow and new wet snow can occur at any month of the year.

It is not necessary to give separate rules for summer and for spring. The important factors in the problem are not so much the season as the amount of snow that has fallen, the temperature, the angle of the sun, etc. etc. If the reader thoroughly understands avalanche craft in winter and in spring, he should be able to cope with the same or with similar problems in summer.

The majority of fatal avalanche accidents in summer are due to snow slides. I have for convenience’ sake adopted the arbitrary distinction of avalanches into avalanches proper--those which are dangerous owing to the weight and quantity of the snow that falls--and snow-slides, which are only dangerous in so far as they carry the climber or ski-runner with them over a cliff or into a crevasse. Most summer avalanche accidents are due to snow slides, and occur in places which the ski-runner could not reach on ski. Snow resting on ice in gullies, or snow resting on smooth slabs, etc., are frequent causes of fatal accidents. Such avalanches or snow-slides have been described by Mr. Young.

The classical device for testing whether snow is resting on ice--i.e. throwing a big rock down the suspicious slope--is often of use to a ski-runner who is in doubt whether to descend a doubtful slope on ski or on foot.

TACTICS ON AVALANCHE GROUND

The simplest rule is to avoid avalanche ground. Unfortunately, this is not always possible. The limits of danger are so wide that one may occasionally find oneself on a slope which might conceivably avalanche. Such a slope may provide the only possible means of getting down to the valley, so that the choice is not merely between giving up an expedition or risking an avalanche, but between the certainty of a night out if one recrosses the pass and the possibility of an avalanche.

It has sometimes been asserted that a ski-runner could escape an avalanche by turning his ski downhill and making a sudden dive downward. Of course this is wildly absurd. I have only once been caught in an avalanche, and, long before I could have turned my ski downhill, the avalanche had carried me some twenty yards downhill. An avalanche does not start by a kind of snowball action. It starts with a sharp crack, and the sudden sliding away of a deep layer of snow. Watch snow sliding off a roof and you will understand that an avalanche is very sudden and overwhelming. Almost every avalanche leaves a clean line of cleavage behind--a wall of snow which is exactly as deep as the avalanche at its birth.

The chance of escaping an avalanche by flight is infinitesimal if you are near the point where the avalanche starts. You are lucky if you have time enough to kick your ski off, and you will only be able to do this if you have unloosened the bindings previous to crossing the dangerous slope. There will be no chance of unstrapping them once the avalanche is upon you. If you cannot kick them off instantaneously, they will remain attached to you.

Your chance of surviving is very much greater if you can get rid of your ski, for the ski drag you under, and prevent all hope of fighting your way to the surface of the avalanche. Once you are overwhelmed you should adopt a swimming motion. Above all, try to keep your head uppermost. A vigorous swimming motion with the hands is said to be useful by people who have survived.

Dangerous slopes should always be crossed as high as possible. An avalanche is much more dangerous if it overwhelms you from above than if it starts from the immediate neighbourhood of your ski. Steep slopes below a cliff usually yield a fairly safe passage just below the rocks, for there is often a little gap between the edge of the snow and the rocks which affords a secure route. Take advantage of every belt or shelf of gentler ground that may run across a steep slope. Traces of old roads or even of footpaths are better than nothing.

Clearly a man on ski is much more likely to start an avalanche than a man on foot. I have often been surprised to see chamois-tracks down slopes 40-50 degrees steep. It is true chamois occasionally get killed by avalanches, but they certainly possess a great immunity. The reason is obvious. The chamois’ slender hoofs sink in very deeply. They penetrate right through the snow to any hard underlying crust that there may be beneath. Further, the chamois does not, like the ski-runner, cut the snow by a continuous line which divides the slope in two, and deprives the snow above the ski-tracks of much of its support. Chamois-tracks form a row of small holes, and therefore have a much less unsettling effect.

A man on foot has some of the advantages of the chamois. He is at least much safer than on ski. His feet get down to the old crust below unless the soft snow is very deep. On ski it is extremely easy to start a small superficial avalanche or snow-slide; and on ski it is very difficult to check such a small or big avalanche once it is started. The ski may sink just into the superficial layer and help to detach it.

On dangerous ground you should therefore remove the ski and proceed on foot. If possible, tackle such slopes by a direct ascent or descent in single file. Traversing is much more likely to cut the snow slope and start an avalanche.

Never rope on avalanche ground unless one member of the party can remain on safe ground, i.e. a cluster of rocks, and secure the man who is traversing a short stretch of dangerous snow. If two or three ski-runners are caught by an avalanche while roped together, their chance of escape is slight, as the rope tends to get caught, to drag them under the snow and to suffocate them.

Dangerous ground should, of course, be crossed by only one man at a time.

It has been suggested by an experienced mountaineer that each member of the party should drag behind him a long thin red cord, which would provide a clue to the whereabouts of a man who had been buried by an avalanche. The ski-ing mountaineer’s kit is already so overcharged that few people would be likely to add to it for this purpose.

For the descent of dangerous slopes, if the party do not proceed on foot, they should put on their sealskins, so as to run the slopes as steeply and directly as possible.

The ice-axe should always be driven in as far as possible, so as to find purchase in the underlying snow. Ski-sticks that are provided with removable disks are very useful, as they can be driven in much deeper than ordinary ski-sticks.

When in doubt, sound with an axe or stick and try to discover whether the snow is homogeneous, and if not, what lies below the surface layer and what lies below the snow itself.

When in doubt, turn back if possible. If it is necessary to proceed, take off your ski. If speed is important, keep on your ski; and if you are descending, put on sealskins. If you cannot spare the time to put on sealskins, sit on your ski and descend by a sitting ski-glissade, which is, by the way, a knack in itself.

THE HIGH ALPS IN WINTER

[Sidenote: Weather Conditions.]

The weather after the New Year is usually more settled than in the summer. A spell of absolutely unbroken weather lasting from three weeks to a month or more is almost inevitable some time in January or February. As a rule, February is the finest month in the winter.

These typical fine-weather periods are often accompanied by mild weather. The temperature even by night is often surprisingly high on the glaciers, even when there is no touch of Föhn in the air. This has led some observers to claim that the temperature in winter--even in the shade--is often lower in the valleys than on the mountains. I have never seen any evidence produced for the phenomenon which has been romantically described as ‘inverted temperature,’ and have no reason to believe that any such violation of the laws of temperature really exists. It is often, of course, colder in the plains when the plains are covered by _nebelmeer_ than in sunny alpine stations, but this is very different to any general inversion of temperature. I have, it is true, sat on the summit of the Finsteraarhorn in midwinter stripped to the waist, and I have often been uncomfortably hot at great altitudes in the sun. But it is dangerous to generalize from such experiences, and though on a windless day, winter mountaineering may be as warm and comfortable as summer climbing, the ski-runner must always be prepared for sudden danger of temperature, and severe cold.

The variations in temperature are surprising. A cushion of cold air, several degrees below freezing, may exist near the surface of the snow, and three feet above the surface of the snow the air may be quite mild. This is a phenomenon well known to rink-makers. In the sun the heat may be quite intense, and yet, a few yards off, in the shade the temperature may be several degrees below freezing.

A suspicion of wind may transform a mild and equable into an unpleasantly cold atmosphere. The changes of temperature are very sudden. You may be basking in shirt-sleeves on one side of a ridge, and be frost-bitten within a few minutes on turning a corner into the wind.

The great danger of winter mountaineering is the risk of a sudden change of weather. Storms seem to blow up out of clear skies with a suddenness to which summer affords no parallel. The man who is caught in a big winter storm is lucky if he escapes without casualty. A driving wind makes ski-ing almost impossible. The snow is blown into one’s face, and in a few minutes one’s eyelashes are gummed up with miniature icicles.

The big storms that sometimes prevail for more than a week at a time may imprison the climber in a club hut until all his provisions are exhausted. From most club huts escape is completely impossible till the storm drops. Even when the storm has given place to fine weather, retreat may be very dangerous owing to the avalanche peril.