To time a descent accurately, to forecast from a knowledge of the orientation and altitude of a given mountain that it will yield perfect ski-ing at a given hour, is to know one of the most satisfactory intellectual pleasures that the mountains afford.
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.