Part A Classification Of Snow

1. General

During the winter, it is imperative that you know which forms of snow are beneficial, and which are hazards to you as a climber. To clarify the danger inherent in snow and ice, it is necessary to outline the forms that snow assumes in its transition to glacier ice.

There are two types of snow: powder snow (loose snow) and compact snow (slab).

• Powder snow (loose snow). This snow is in its early stages and is further classified as new snow, or settling/settled snow.

New snow maintains individuality of flakes (crystals) or grains, which are free to move independently of each other immediately following its fall. If new snow is dry, it is feathery; if damp, it quickly consolidates into settling or settled snow. Snow is dry at subfreezing temperatures and wet at higher temperatures, where it contains water held among the ice grains.

Dry snow consists of two forms: the round and the faceted forms.

• The round form consists of well-rounded grains that are joined to give the snow slab cohesive strength. This form usually occurs when the snow cover is deep and temperatures are moderate.

• The faceted form consists of angular crystals that are poorly bonded. These usually occur near the bottom of snow layers that are not too deep; thus, they are called depth hoar. They usually develop in extremely cold weather.

When melting occurs, the dry forms disappear and other forms take their place. The snow forms into clusters of grains where the water is free to drain. Slush forms where the drainage is diverted. This form is not cohesive.

• Compact snow (slab). This is snow that has passed beyond the stage of settled snow, where the individual crystals or grains are attached to bonded to give the snow some cohesiveness.

• Glacial firn or neve is densely packed snow that has survived at least one summer melt season. Glacier ice forms from the continuing compaction of firn until it is impermeable.

2. Snow Formations.

The forms of snow change during their transition from dry new snow to glacier ice.

Temperature, humidity, and wind are important modifying factors in this transition.

Sun crust, wind crust or wind loose-packed snow, and wind slab are phenomena that should be important to you, the mountain soldier.

• Sun Crust. This is any snow that has had the top superficial layer melted by heat and subsequently refrozen often in repeated cycles. A layer of snow that is sun-crusted and weathered throughout its thickness becomes firn snow (neve). Sun crust commonly overlies loose powder snow.

• Wind Crust or Wind Loose-packed Snow. This is usually found on windward slopes and is anchored firmly to the underlying snow. Wind crust is usually safe. Freezing rain crust is also safe but once buried under new snow may form sliding layers for avalanches. Surface hoar crystals grow onto the surface on clear, cold nights. The feathery crystals also form weak layers once they are buried by subsequent snowfalls. Any type of crust, once buried under new snow is a potential slide base for avalanches.

• Wind Slab. This is formed from snow transported and deposited by winds. While the slabs are well compacted, they are loosely anchored to the underlying surface and are poorly bonded to the substratum. Having been transported by wind, the component grains are rounded and do not reflect the light. For this reason, the surface of a wind slab has a dull, chalky appearance. Wind slabs are dangerous because they often form the platform for subsequent avalanches.

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