Ski-runs in the High Alps

by F. F. Roget

There is no doubt that the nivometer will show every day in the year—though it will not be so often noticed—the height at which the snow stands against the face of that rock. But how much information can it give about the snowfall? Snow cannot find its true level on the face of a rock against which it is blown about by the wind and where it is interfered with by the temperature of the stone, sometimes heated by the sun and sometimes colder than the air surrounding it.

Snow is not like water or air. It is not an elastic consistent substance or a uniform fluid, like gas, seeking its own level or settling down upon a surface. It falls unevenly upon an uneven ground. It melts or accumulates, shrinks or flies about according to its local situation, and, within a given time, the nivometer will give very contradictory readings. A snow gauge is no easy thing to establish. When rain falls it is easily measured, because, in the course of nature, it is mere water. Not so with snow.

What is measured by the Alpine nivometers is the height of the snow lying at a certain place on a given day. Density cannot be checked. Yet it operates immediately after the snowfall. This mode of mensuration gives no reliable clue. Some of the snow was carried away by the wind that would have remained on a windless day. Some has been blown from elsewhere, in what proportion it is impossible to tell. How much has melted depends on the sun heat, and the amount of this deficiency no instrument is there to record. A storm may have intervened. Another may have blown the snow flat, concentrating the total mass within a smaller compass. Another may have piled it up in abnormal wreaths.

The science of snow measurement is quite in its infancy. When it is developed it will probably be on lines very different from those at present followed, and the results cannot be foretold.

Natural nivometers should be raised above the surface like dovecots and set up in wide-open spaces, in situations exposed to the four winds of heaven. They should be able to receive on all sides the snow moving in the air. They should be in the shape of a cone with long, gently sloping sides. And even then they would not prove much, unless the snow they had collected was gauged after every fall and the apparatus swept clean and prepared to receive the next fall on a smooth surface.

It would then probably be found that the amount of snow falling on the glaciers of the Alps is much smaller than we are apt to imagine. In any case, the depth of the snow that finds a permanent station upon the rock and ice surface of the Alps, till spring, is only a fraction of the depth of snow that would be obtained by adding together each volume of snow that might be gauged after each separate snowfall. Snowflakes form an aggregate which gradually passes into a conglomerate. They lie at first like the pieces of a game of spillikins, at different angles with one another. By degrees the crystals lose their shape. The edges of the prisms die out. The air that circulated between them is expelled. A hard texture takes the place of the flimsy structure of the first moment. In this process of reduction in volume and of increase in density, cracks are generated in the mass. They are at first potential and remain latent till wind-pressure, or the footfall of man, determines the bursting open of the surface, accompanied by a report which sometimes unnecessarily alarms the unwary, and at other times is a sure sign of a dangerous snow-quake.

The depth of the snow is also modified by a process of sublimation which causes it to shrink rapidly. The atmosphere while re-absorbing the air expired by the snow, also re-assimilates some of its moisture, even without the suggestion of a thaw.

The outcome of so many efficient causes may be summed up in one word: shrinkage. But, as snow almost always is wind-driven when it falls, a large portion of the quantity follows in the air a course parallel to the wind, and (when it strikes obliquely the smooth and slippery surfaces—old snow, ice, rock surfaces—over which it travels instead of locating itself upon them) it is impelled forward, and sweeps along till it can find a lodgement against a solid protuberance, or is dropped over the edge of some break in the surface, out of the reach of the wind, when it finds a resting-place and gets piled up. This is another reason why one meets with less snow on the wind-swept, high-lying surfaces than in the middle zone of the Alps.

A third effective cause is to be found in the clouds. Snow-laden clouds do not generally unload themselves at a very high altitude. They form themselves in belts on the lower flanks of each range and pour forth their contents nearer the grazing and forest zone than one would be led to expect when one looks up towards them from the bottom of a valley. We then see the basement and sides of the cloud masses. We project their vertical lines almost infinitely into space. This is the kind of delusion to which we are subject when we look at a house from the street-level or, vice versâ, when we look down from a roof on to the pavement. The actual volume of snow whirling above our heads is considerably thinner than we assume. This is the case particularly during the winter season in Switzerland, as winter balloonists may testify.