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.