Classification of snow avalanches

Snow avalanches can be classified by various factors, such as the start zone of an avalanche, position of a sliding plane, and the mode of movement. In general, the classification using the following three factors is utilized.

(1) Start zone of snow avalanche

• Starting from a point
• Starting from a plane

(2) Snow type of avalanche layer

• Dry snow
• Wet now

(3) Position of sliding plane

• Surface layer
• Full layer

If (1)-(3) are combined, eight types of avalanche classifications are possible. But, six types are generally known, because the existence of a full layer avalanche starting from a point has not been confirmed. Those six types are explained here using the excerpts from the "Snow Avalanches and Countermeasures against Them".

(a) Dry snow surface layer avalanche starting from a point

This type of avalanche tends to occur when the temperature is low and the snow is falling. It is often triggered by small snow masses fallen from snow cornices, tree branches, exposed rocks, etc. Dry snow sweeps down as a powder snow avalanche. The trace of the avalanche is not easily found. The movement begins from one point on the slope taking a wedge pattern. The avalanche scale is mostly small.

(b) Dry snow surface layer avalanche starting from a plane

This avalanche is likely to occur when the temperature is low and tens of centimeters of new snow has deposited on the already-accumulated considerable amount of snow. It is triggered when low temperatures continue regardless of during snowing or after snowing. Snow masses start moving simultaneously in a rather wide area on the slope, and the scale is often large. Accompanying a huge snow smoke, the avalanche can reach as far as several kilometers from the mountain foot.

(c) Dry snow full layer avalanche starting from a plane

This type of avalanche is classified into two types: those occurring mainly in Honshu and those in Hokkaido. They differ in the occurrence mechanism. In Honshu, if a large volume of new snow has rapidly deposited on the already-accumulated snow on the slope during low temperatures, the full layer of the accumulated snow starts moving in a wide range due to snow loads. The dry new snow in the surface layer generates a snow smoke and it can reach far beyond the mountain foot. If the snow below the new snow is old, it runs down the slope like a flow without generating a snow smoke. In Hokkaido, if a severe cold weather continues for a considerable time, the snow layer near the ground surface changes into a fragile layer and then collapses to become an avalanche. This type of avalanche differs from the "wet snow, full layer avalanche starting from a plane" in that the former avalanche reaches an area far away from the mountain foot.

(d) Wet snow surface layer avalanche starting from a point

This type of avalanche is caused when new snow accumulated in 20-30 cm thickness is exposed to good weather and warm temperature. Triggered by a snow ball, a wet snow layer starts a contracting movement in a wedge pattern and, if the slope is long, it turns into a collapsed flow-like avalanche. The avalanche scale is usually small. This avalanche also occurs when the accumulated snow with rough surfaces, usually seen in early spring, is exposed to sufficiently warm weather.

(e) Wet snow surface layer avalanche starting from a plane

This type of avalanche tends to be triggered when the weather becomes good and the temperature rises after snowing. The snow of this type of avalanche layer contains moisture. The snow falls down like a flow without generating a snow smoke.

(f) Wet snow full layer avalanche starting from a plane

This type of avalanche often occurs in the snowmelt season in early spring, but it also occurs in winter if the temperature becomes high. The avalanche is often triggered on the rainy day or on the warm day if a crack penetrating from the snow surface to the ground is caused around the slope top and a void is created between the bottom of snow and the ground due to melt water. The avalanche scale is often large and the hard-packed snow on the slope sometimes scrapes away the ground surface. Without generating a snow smoke, the collapsed snow falls down the slope like a flow.

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Translated from Journal of Japan Sabo Association, Sabou to Chisui, Vol.85, p.74, 1992