Winter Snow
There are certain forms of snow which are characteristic of certain seasons. ‘Powder snow,’ for instance, is normal in winter and uncommon in the late spring; none the less powder snow is often found even in summer, just as certain forms of snow which are characteristic of spring or early summer are occasionally found even in midwinter. ‘Winter snow’ must then be taken to mean the kind of snow which is common and characteristic of, but not limited to, winter.
Powder snow.
In winter a normal snowfall is accompanied by a temperature below freezing; snow that falls with the temperature just above freezing usually descends in the form of sleet. Fortunately, such conditions are not common.
Snow will even fall when the temperature registers a degree or two of thaw. I have known dry snow fall with a temperature of two degrees above zero—centigrade, at a height of 5500 feet above the sea. Similarly, as icemakers know, water will freeze on a rink even when the temperature is above freezing-point.
The explanation for this anomaly may be sought in the well-known law that pressure lowers the freezing-point.[19] It seems a natural deduction that diminished pressure will raise the freezing-point. 0° centigrade is the freezing-point of water at sea-level. Under a pressure greater than that of the air at sea-level, the freezing-point is lowered so that the bottom of a glacier will be in a condition of liquefaction at a temperature below freezing. It should follow that an air pressure less than that of the air at sea-level should raise the freezing-point; or, in other words, that at any point above sea-level the freezing-point should be higher than zero. From my own observations I believe that water will freeze and rain turn to snow when the temperature is 2 degrees centigrade above zero at a height of 5000 feet.
Fortunately, the normal snowfall is accompanied by a temperature below freezing, for if the snow is on the point of turning to rain, it forms a crust when the temperature falls. A normal dry snowfall takes the form of small hexagonal crystals. Newly fallen snow is not compact, and contains a great deal of air, which slowly escapes as the snow settles, so that after a day or two the new snow has lost nearly a half of its apparent depth.
New snow is very soft and slow, but gives good running on steep slopes; though, of course, long steep slopes are liable to avalanche. Good running is also often obtained during a snowfall by choosing south slopes, previously covered by a hard crust; for there is nothing pleasanter than two or three inches of snow on hard crust, and during a gentle snowfall it is possible to enjoy quite good sport on such slopes. In general, expeditions—even quite short ones—are impossible till the snow stops, and unsafe for a day or two after a heavy fall. Newly fallen snow sticks badly in the sun even if the temperature is well below freezing.
Once, however, that the snow has settled it soon gives perfect running. One night’s really hard frost is sufficient to produce excellent snow. Powder snow remains good almost indefinitely in winter on northern slopes, provided it is not spoiled by wind. It steadily improves. In sheltered places the crystals gradually grow in size so that sometimes you find large leaf-like formations, which rustle under the ski like autumn leaves. Such snow is a dream of paradise.
Powder snow is the ideal running surface. It is equally good for straight running and for swings. In deep soft powder the Telemark is the best swing, but in fairly compact powder any turn or swing is easy and safe.
The ski-runner who visits the Alps in winter, and has good luck, may run for day after day on perfect powder. He is in danger of thinking himself a better runner than he is. If he concentrates on Telemarks, the first tour in the High Alps, where he will meet varieties of hard and soft crust, will find him out badly. He should therefore make a point of spending a day or two on south slopes so as to master the Stemming turn on hard snow. If he is getting pleased with his Telemarks, let him try a Telemark on the soft breakable crust to be found on south slopes at midday, and if he can do a series of fast-linked Telemarks in such crust, he may then return to his powder snow with a good conscience.
The Effect of Wind on Powder Snow.
Snow which is spoiled by the sun is never quite impossible, and is always curable by a hair of the dog that bit it; in other words, by another and stronger dose of sun. But snow which has been wrecked by the wind is the despair of the ski-runner.
The wind does not affect snow that has once been crusted, so that wind-swept snow is almost unknown in spring. The favourite victim of wind is precisely that light, dry powdery snow which is so common in winter.
The effect of wind depends on three factors, the strength of the wind, the time during which the snow is exposed to the wind, and lastly the type of snow exposed to the wind. Powder snow is the most sensitive, the quickest to spoil and the most trying when spoiled. Hard crust is entirely unaffected; heavy wet snow is very little affected.
A light wind produces on powder snow the markings which are known as ripplemark, a term which has been applied by scientists to the rippled effects which can be traced, not only in snow, but also in sand, and even in clouds which have been influenced by wind. Sand and snow behave very similarly under the influence of wind. A slightly stronger wind, or a wind blowing for a longer spell, produces caked powder, which is dense and compact, but has not quite lost its powdery quality. The ski sink in to a depth of an inch or more, and though fast running is dangerous and apt to produce broken ski points, a good runner can derive much pleasure from caked powder, and can force linked Telemarks without much trouble. The third stage in the deterioration of snow occurs when the wind produces a hard crust. Windboard is hard and slippery, but so long as it is not varied with soft patches of sticky snow or of breakable crust, windboard yields a surface which may annoy the novice but which should not prove, at any rate on slopes of moderate gradient, too troublesome to an expert who has mastered his hard snow turns. Windboard is common during the winter months in the High Alps; in appearance and texture it is not unlike the hard marble crust, which is found on south slopes after a long spell of fine weather, but it is vital to discriminate between windboard and marble crust, because the former may sometimes break away in slab avalanches whereas the latter never avalanches. Windboard betrays its origin by ripple markings, sometimes faint but seldom invisible to the practised eye. Windboard is often varied by pockets of sticky snow.
If wind produced nothing worse than caked powder or windboard, the ski-runner would have less cause for complaint. Unfortunately the wind often produces horrible surfaces which resemble each other only in their general unsuitability for ski-ing. Skavla is the generic Norwegian name for wind-swept snow, and skavla is common on the exposed fjords of Norway and Sweden, where it attains a degree of unpleasantness seldom matched in the Alps. Skavla varies. Sometimes skavla consists of patches of glittering ice varied by treacherous pockets of sticky snow; sometimes a breakable trap crust interspersed with windboard; sometimes waves of hard, icy snow which occasionally attain a height of two feet or more. The prevailing wind determines the direction of the waves and acts in precisely a contrary fashion to that of wind on water; for wind on snow has a burrowing effect, and forms these waves by excavating and eroding a slope, gradually forming a series of steps. In other words, whereas on the sea the wind will be found to be blowing up the longer slope of a wave, on snow the wind will have been blowing against the short steep side of the snowy waves.
To sum up: The effect of wind on powder snow is always bad. The stronger the wind and the longer that it blows, the worse the snow. Hence in winter avoid exposed slopes and seek out sheltered valleys. The summit slopes of winter mountains will usually be spoiled by the wind, which is one of many reasons why spring is so superior to winter as far as ski-ing in the High Alps is concerned.
The Effect of Sun on Powder Snow.
Powder snow acquires a crust if it is exposed to wind, to sun shining at a sufficiently direct angle, and to an air temperature above freezing. Wind alone has the power of forming crust without a preliminary melting of the surface. I do not know why this should be so, but no doubt the reason is to be found in the law that pressure lowers the freezing-point, so that the increased pressure caused by wind lowers the freezing-point, or, in other words, produces the effects of a thaw at the point where the wind acts even though the general air temperature may be well below zero centigrade.
At any rate, sun, wind and thaw are alike in producing a surface crust.
The effect of the sun depends mainly upon the angle of inclination. In midwinter, snow will remain unaffected by the sun on northerly slopes even though they receive several hours of sunshine in the course of the day.
In fact, in midwinter eastern and western slopes retain powder snow for a very considerable time, while a slope that is a few degrees north of west or a few degrees north of east retains powder snow throughout the winter, always providing that it is not exposed to wind or to thaw.[20]
I use the term ‘thaw’ to denote a general air temperature above freezing as opposed to the purely local melting caused by the sun’s rays, which may be, and in fact normally is, accompanied by an air temperature in the shade of several degrees below freezing.
The thermometer will often be registering more than ten degrees of frost in the shade and twenty or more degrees of warmth—centigrade—in the sun. It is, as icemakers know, quite common to find a belt of cold air, two feet in height, just above the snow and just below a strata of warm air several degrees above freezing-point.
In normal winter weather you may be uncomfortably warm in the sunshine, and yet the sun has no power to affect powder snow on any slope which has not a touch of south in it.
The steepest south slopes are the first to lose their powder snow. Afterwards, the gentler south slopes follow suit. Towards the end of February western and eastern slopes begin to crust. In March due north slopes still hold powder,—at any rate above 5000 or 6000 feet,—but slopes which are only a few degrees north of east or of west begin to crust. Finally, the level outruns get crusted, then the gentle due north slopes, and last of all steep north slopes.
Of course at high altitudes winter conditions prevail until well on into April.
In midwinter good ski-ing is usually to be had on south slopes for three or four days after a snowfall, though steep due south slopes soon crust. The time during which gentle south slopes or slopes which are just south of west or east will hold powder depends on the general air temperature and on the altitude. I have known pretty steep due south slopes hold powder below 4000 feet for three days of cloudless weather, but the temperature was about 15 degrees centigrade below freezing.
This is, however, unusual, and south slopes soon crust up even in midwinter. The sun melts the snow by day, and the melted snow refreezes at night, producing a crust. Such a crust will, at first, be soft and breakable.
Soft breakable crust is not a bad running surface. It is quite true, i.e. does not vary in pace from one place to another, and is usually soft enough to enable the ski-runner to make Telemark swings, either stop or linked, without much difficulty. Sometimes the crust is harder, and in this case the jump-round or quersprung is the only possible manœuvre.
A peculiarly villainous form of crust, fortunately rather rare, is known by the appropriate name of Trap crust. This crust is perfectly solid at one spot, and bears the weight of the ski without breaking, only to crack a few yards lower down. The ski-runner skids with great speed over the hard crust, only to be pulled up suddenly as his ski break through the soft crust below.
Trap crust is more usual in spring than in winter, especially on slopes which change in direction, so that the ski-runner passes from snow which has been sufficiently thawed to form a hard solid crust at night to slopes which have only been superficially melted. Trap crust is only found after fairly heavy falls of snow which require considerable time to form a solid crust. As a rule, trap crust covers a foot or more of powder snow.
To return, however, to more normal winter conditions. Under normal conditions the snow on south slopes in winter is gradually transformed into crust.
This crust is at first soft and breakable, but the continuous process of melting and refreezing gradually produces a solid unbreakable crust.
Unbreakable crust, as the ski-runner soon discovers, varies greatly in quality from hard crust which is so hard that the ski slip sideways without obtaining the least grip, crust which on any slope but the most gentle is almost unskiable, to the other extreme, crust which though quite hard and unbreakable is rough enough to permit even a Telemark and to make Christianias and Stemming turns delightfully easy.
The solid crust formed in winter is usually very hard and slippery before the sun strikes it. This crust is known as Marble Crust. You find marble crust on southern slopes in winter, and at high altitudes in spring. It is common in the High Alps. Even at the very end of April I have known a slope at a height of 12,000 feet remain hard, slippery and unsoftened for some little time after the sun had struck it because a bitter north wind had lowered the temperature several degrees below freezing.
In mid-winter a slope of marble crust will yield good ski-ing after the sun has struck it, unless the air temperature is several degrees below freezing. Marble crust, touched by the sun, yields a surface which will puzzle the beginner but which is pure delight to the man who has mastered Stemming turns and Christianias. The excellent running that may be obtained on south slopes is too little exploited, for winter ski-runners are firmly convinced that good ski-ing can only be found on north slopes where powder lies.
Mid-winter may be said to last till the end of January. In February you often find perfect spring snow, Telemark crust, etc., on south slopes, and in February, as often as not, the best running is obtained on southern slopes. This type of running will be dealt with later.
Summary of Winter Snow.
Winter snow may therefore be summarized as follows. The normal winter snow which is found throughout the winter on northern slopes and occasionally on southern slopes is powder snow which is perhaps the ideal ski-ing snow.
Powder snow deteriorates under the influence of wind and sun. Wind is the more mischievous. A little wind does no great harm, and the expert can even ski with pleasure on the hard crust formed by wind, but the extreme action of wind often produces varieties of snow which are all but unskiable.
The High Alps and lofty summits in general being most exposed to wind will usually yield inferior ski-ing to sheltered slopes. The best ski-ing in winter will therefore seldom be found on the glaciers.
Snow also deteriorates under the influence of sun, which forms a crust. This crust, however, once formed is often improved by a further dose of sun.
Though the best ski-ing is usually obtained on north slopes, excellent ski-ing is often obtained on south slopes by those whose knowledge of snow craft is sufficient to time their descent and to choose their route accurately.
Abnormal conditions produced by Föhn and thaw are dealt with on pp. [417-422].