Surface Forms Of Snowcover

Snow and ice undergo endless surface changes as they are worked on by wind, temperature, sun, freeze-and-thaw cycles, and rain. Following is a rundown on most of the surface permutations mountaineers typically encounter.

Powder snow: This is a popular term for light, Huffy new-fallen snow. However, powder snow is more specifically defined as new snow that has lost some of its cohesion due to the recrystallizing effects of steep temperature differences in the surface layers. These changes occur only during periods of persistent low temperatures. The changed snow is loose and powdery, commonly affords good skiing, and may form dry loose-snow avalanches.

Corn snow: After the advent of melting in early spring, a period of fair weather may be followed by formation of coarse, rounded crystals on the snow surface, often called corn snow. The crystals are formed from the daily melting and refreezing of the snow. Only when the same surface layer continues to melt and refreeze does true corn snow develop. When corn snow thaws each morning after the nighttime freeze, it's great for skiing and step-kicking.

Rotten snow: Rotten snow is a spring condition characterized by soft, wet layers that offer little support to the firmer layers above. In its worst forms, it will not support even the weight of a skier. Snow that promises good spring skiing in the morning, while there's some strength in the crust, may deteriorate to rotten snow later in the day. Rotten snow forms when lower layers of depth hoar become wet and lose what little strength they have. It's a condition that often leads to wet loose-snow or slab avalanches running clear to the ground. Continental climates, such as those of the American Rockies, often produce rotten snow, which is much less likely to occur in the more stable maritime snowcovers of the Pacific coastal ranges of the United States and Canada.

Meltwater crust: This is a snow crust formed when water melted at the surface is refrozen and bonds snow crystals into a cohesive layer. A common variety is sun crust, so called because the source of heat for melting is solar radiation. Heat to permit meltwater crusts also comes from warm air or condensation at the snow surface. In winter and early spring the thickness of a sun crust is usually determined by the thickness of the surface layer where meltwater is formed in otherwise dry snow. In later spring and summer when free water is found throughout the snowcover, the thickness depends on how cold it gets at night.

Wind crust: In contrast to meltwater crust is the crust caused by action of the wind. After the surface snow layers are disturbed by the wind, age-hardening takes place. Fragments of snow crystals broken by the wind are compacted together when they come to rest, adding to the process. The hardening is compounded when the wind provides heat, particularly through water vapor condensation. Even when there is not enough heat to cause melting, the warming of the disturbed surface layer, followed by cooling when the wind dies, provides additional metamorphic hardening.

Firnspiegel: The thin layer of clear ice some

Glacial Suncups

Fig. App. 1-3. Surface features on snow: left, suncups; center, sastrugi; right, nieve penitentes times seen on snow surfaces in spring or summer is called firnspiegel. In the right conditions of sunlight and slope angle, its reflection produces the brilliant sheen of "glacier fire." Firnspiegel forms when solar radiation penetrates the snow and causes melting just below the surface at the same time that freezing conditions prevail at the surface. Once formed, it acts like a greenhouse, bringing melting of the snow beneath while the transparent ice layer remains frozen at the surface.

Verglas: This is a layer of thin, clear ice formed from water freezing on rock. It is most commonly encountered at higher elevations in the spring or summer when a freeze follows a thaw. The water comes from rainfall or melting snow. Verglas may also be formed directly by supercooled raindrops freezing as they fall onto exposed objects ("freezing rain," also sometimes inaccurately called "silver thaw").

Drainage patterns: After melting has begun in spring, drainage patterns formed by the runoff of water appear on snowfields. However, the actual flow takes place within the snowpack, not on the surface. As snow melts at the surface, the water formed percolates downward until it encounters impervious layers, which deflect its course, or highly permeable layers, which it can easily follow. Much of the water also reaches the earth beneath. The water that flows along within the snow often causes a branching pattern of channels on the surface. This happens because the flowing water accelerates the snow settlement around its channels, which are soon outlined by depressions at the surface. The dirt that collects in these depressions absorbs solar radiation and accentuates them further by differential melting.

Suncups: Suncups are depressions in the surface of summer snowfields, and they can vary in depth from 1 inch to 2 feet or more. They always occur as an irregular pattern covering an entire snowfield and form whenever weather conditions combine to accentuate surface irregularities. There must be motion of air to cause greater heat and mass transfer at high points of the snow than at the hollows. The air must be dry enough to favor evaporation, and there must be an additional source of external heat, usually the sun.

Under these circumstances, more heat reaches the points than the hollows but a larger proportion causes evaporation rather than melting. Because evaporation of snow demands more than seven times as much heat as melting, less snow is lost from the high points in the form of vapor than is lost from the hollows in the form of meltwater. The hollows melt faster than the points evaporate, and suncups form. They are enhanced by differential melting when dirt in the hollows absorbs solar radiation. The suncups also melt faster on the south (sunny) side in the Northern Hemisphere, so the whole suncup pattern gradually migrates northward across its snowfield.

Warm, moist winds tend to destroy suncups by causing faster melt at the high points and edges. A prolonged summer storm accompanied by fog, wind, and rain will often erase a suncup pattern completely, but they start to form again as soon as dry, fair weather returns.

Nieve penitentes: When suncups grow up, they become nieve penitentes (Spanish for "penitent snow"). They are the pillars produced when sun cups intersect to leave columns of snow standing between the hollows. They are peculiar to snow-fields at high altitudes, where radiation and atmospheric conditions conducive to suncups are intense. Nieve penitentes reach their most striking development among the higher peaks of the Andes and the Himalaya, where they may get several feet high and make mountain travel very difficult. The columns often slant toward the midday sun.

Wind and erosional features: The surface of dry snow develops a variety of erosional forms from the scouring of wind, such as the small ripples and irregularities on winter snowcover. On high ridges and treeless arctic territory, under the full sweep of the wind, these features attain considerable size. Most characteristic are the wavelike forms, with sharp prows directed toward the prevailing wind, known as sastrugi. A field of sastrugi—hard, unyielding, and as much as several feet high—can make for tough going. High winds over featureless snow plains also produce dunes similar to those found in desert sand, with the crescent-shaped dune, or barchan, being most common.

Cornices: Cornices are deposits of wind-drifted snow on the lee edge of ridges or other features. They offer a particular hazard as they overhang, forming an unstable mass that may break off

from human disturbance or natural causes. Falling cornices are dangerous in themselves and also can set off avalanches.

During storms the precipitated snow furnishes material for cornice formation. Cornices also are formed or enlarged by material blown in from snowfields that lie to windward. As a general rule, cornices formed during snow storms are softer than those produced by wind drift alone.

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  • rudigar
    What is formed by melting snow fields?
    8 years ago

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