Snow that remains on the ground changes with time. The crystals undergo a process of change— metamorphism—that results in smaller, simpler forms and a snowpack that shrinks and settles. Because the snowpack generally becomes more stable over time, mountaineers find it useful to know the recent history of weather and snow conditions in an area.
Metamorphism begins the moment that snow falls and lasts until it melts. The equilibrium growth process gradually converts the varied original forms of the crystals into homogeneous rounded grains of ice (old snow). Both temperature and pressure affect the rate of change. When temperature within the snow is near the freezing point (32 degrees Fahrenheit; 0 degrees Celsius), change is rapid. The colder it gets, the slower the change, and it virtually stops below -40 degrees Fahrenheit (^40 degrees Celsius). Pressure from the weight of new snowfall over an older layer speeds changes within the layer. Snow that has reached old age—surviving at least one year and with all original crystals now converted into grains of ice—is called firn (or névé). Any further changes to firn snow lead to formation of glacier ice.
Another type of metamorphism takes place when water vapor is transferred from one part of the snowpack to another by vertical diffusion and is deposited in the form of ice crystals with different characteristics than those of the original snow. This kinetic growth process produces faceted crystals. When the process is carried to completion, the crystals often have a scroll or cup shape, appear to be layered, and may grow to considerable size. They form a fragile structure that loses all strength when crushed, and becomes very soft when wet. This weak and unstable snow form is known as depth hoar, popularly referred to as sugar snow. The necessary conditions for its formation are a large difference in temperature at different depths in the snow and sufficient air space so that water vapor can diffuse freely. The conditions are most common early in winter when the snowpack is shallow and unconsolidated.
As the snowpack on a slope moves very slowly downhill under the influence of gravity, the upper layers travel faster than those next to the ground. This internal deformation, called creep, proceeds most rapidly at the freezing point and diminishes with decreasing snow temperature. The entire snowpack also glides on the ground when
Fig. App. 1—2. Destructive metamorphism of a snow crystal the interface between snow and earth is at the melting point. If the ground is smooth (covered with grass, for instance), gliding is the dominant form of snow motion. The slow combined motions of creep and glide are so unhurried that they can't be noticed by the casual observer, but they cause the snowcover to exert enormous forces on obstacles in its path. The stresses produced by uneven snow creep are an important factor in avalanche formation.
Variations in the strength of snow are among the widest found in nature, with strength continually changing due to metamorphism, temperature differences, and wind. The hardness of wind-packed old snow may be fifty thousand times that of fluffy new snow. An increase in hardness is always associated with wind-drifted snow, or snow mechanically disturbed in any fashion, which undergoes a process known as age-hardening for several hours after it is disturbed.
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