Let us consider the effect of wet Föhn in winter proper. The ski-runner is bound to detect the first hint of wet Föhn in the air by a slight stickiness in any powder snow that he may chance to be ski-ing on. If the wet Föhn is slight, copious waxing will eliminate its effects; but if it is pronounced, the snow will ball and stick atrociously.
Should the Föhn disappear, it leaves its legacy behind. All slopes of powder snow which have been affected are covered with a crust, which will be more or less thick according as the thaw has been more or less severe. I assume, of course, that the thaw had been followed by the frost at night, without which crust cannot be produced. Only a new snowfall can restore a slope of powder snow which has once been melted by Föhn to its pristine conditions.
Sometimes the Föhn is so slight that the surface of the powder snow is barely touched. It is not covered by crust, but there is a suspicion of resistance as the ski drive through due to an embryo crust. Telemarks are not quite so easy, and a very fast swing is apt to throw the ski-runner outwards, owing to the fact that the side-slip is less pronounced; for the snow gives less easily when covered by the first hint of crust. More usually the Föhn, if it once begins, is accompanied by rain. Often even in December heavy rain will fall on all slopes below 12,000 feet. Directly the Föhn disappears the usual winter frosts will convert such slopes into solid icy crust which is harder and more slippery than any marble crust produced by sun or wind. I have seen crust, formed by Föhn followed by frost, so hard that neither ski nor hobnailed boots left any perceptible trace; and on an easy slope which a beginner could take straight I have slid down helplessly on my back some hundred yards, only to be saved from a further and probably fatal fall by a guide, who thoughtfully received my head in the pit of his stomach, breaking a thumb in the effort to arrest me.
That was an extreme case of ice crust produced by Föhn, but it is not uncommon to find every slope below 10,000 feet covered by an icy crust, on which ski-ing is impossible. The wise ski-runner climbs on crampons, and selects a due south slope for the descent, and times his descent for midday. The slight surface softening produced by the sun is usually enough to produce superficial stickiness, which gives capital running, provided that you have mastered the Stemming turn and Christiania. But, except at or near midday, and except on south slopes, ski-ing is impossible during such periods.
Sometimes good ski-ing is obtained near stream beds when all other slopes are covered by hard crust. The mist that rises from the stream bed descends in hoar frost and covers the crust with a layer of crystals deep enough to take a turn. But such slopes are scarce.
Occasionally the wet Föhn is followed by a period of dry Föhn.
In this case the frost at night is much milder, and you will often find instead of marble or icy crust typical spring formations. I have found perforated crust and even film crust in January, when a period of dry Föhn with high temperatures by day and mild frosts by night had followed two or three days of rain.
For weeks together I have known the crust formed by dry Föhn to resemble perforated crust, but formed of a much smoother and more slippery texture. Imagine a hard surface pitted by numberless smooth little round hollows about the size of half-crown pieces. This type of hard perforated crust gives good running, but calls for a thorough mastery of hard snow turns.
Often the dry Föhn produced a surface-melting to a depth of two or three inches—typical spring conditions in midwinter.
This again is an illustration of the law stated on p. [414]: “The milder the frost the better the crust.” In other words, once the snow in midwinter has been thoroughly spoiled by thaw followed by frost, the ski-ing will be best when the conditions approximate as nearly as possible to those characteristic of spring.