A first fall of dry winter snow upon dry slopes is extremely avalanchy, provided it be heavy enough. If it be a fall of wet snow on a porous surface—that is, neither frozen ground nor hard rock—the snow will as it were flop together in a slithering mass, but is not likely to form itself into a dangerous compact floe.
As soon as a second fall of snow comes to adhere to what is left of the first, it may happen that the second layer does not get properly welded to the first. The thoroughness of the attachment depends on the adhesiveness of the snow and on weather conditions. A foundation is therefore laid for the slipping of the new snow upon the surface of the old.
In the course of the winter the snow gets consolidated in one mass, but the process takes each time from two to three days, during which caution is necessary. A homogeneous layer of snow, hardened from the outside by wind pressure, or freezing over after a slight thaw, may then break up into slabs which slide down on the older snow, should one with ski, or in any other fashion, cut that snow away—at any point—from its support.
A stratum of snow on a steep open slope is like a piece of cardboard balanced on your finger. There is a limit to the inclination of the cardboard beyond which it will slip off its pivot. So it is with snow.
Newly fallen snow soon ceases to be an amorphous mealy mass. Its bottom layer models itself on the surface on which it lies and, if turned over, would show that surface en relief. The next stratum adheres to the first more or less, and finds points of support for itself, such as rocks protruding through the first stratum, trees, shrubs, fences, dykes, &c. Every ensuing layer is less shored up than the one beneath. Should there be a rise in the temperature, an increase of moisture brought on by a change in the wind, the snow becomes heavier and may start down; as a dry sponge on an inclined board, gradually absorbing water, must slide down when the inclination of the board and the quantity of water reach the critical point.
Our illustration from the cardboard balanced on a finger-tip, and from the sponge on an inclined plane, makes it clear that it is impossible to state at what definite angle the equipoise of a snow stratum must be lost or is sure to be kept. That angle depends on the finger-tip, on the weight and size of the cardboard, on the sponginess of the sponge, on the slipperiness of the plank, on your holding your breath, or mischievously blowing upon the suspended object, &c. When about to capsize, the cardboard may meet some external point of support, such as your raised hand, which, in the case of the snow stratum, would be a pre-existing prop and maintain an otherwise impossible stability.
A fall in the barometer almost always means an increase of moisture which is unfavourable to the steadiness of old snow. A dry, hot wind—such as foehn—is worse, because its heat penetrates the snow to the very bottom and sets it moving throughout its thickness.
New snow is dangerous till it has had time to set—that is, for two or three days.
Runners are generally agreed to call steep the slopes on which avalanches may occur.
Steep slopes are either concave, convex, or straight.