Soft Snow Avalanches

Connoisseurs of avalanches recognize several different types, each with its own peculiar kind of destructive power.

One type, the soft snow (new snow) avalanche occurs during or just after a heavy snowfall unaccompanied by wind to pack the snow as it falls. The avalanche occurs when the snow reaches a critical depth beyond which the binding between snow crystals at depth in the snow pack cannot sustain the snow load on the slope.

Once the soft snow avalanche starts it gains speed rapidly, and air is incorporated into the moving snow mass to help lubricate the flow. It is said that soft snow avalanches rarely exceed 360 km/hour (200 miles per hour), but obviously this speed is enough to cause a lot of damage.

Worse yet, the rapid motion of snow and air down slope creates vortices (circular motions) in the air near the front of the avalanche. It is thought that the speed of the air in these vortices can be four times greater than the speed of the moving front of the avalanche. In essence, the vortex at the front of the avalanche is a tornado laid on its side, the rotary motion in the vortex being much like that of the business end of a reel-type lawn mower. Even after the snow itself stops flowing, the air vortex can roll on downhill with a life of its own to create destruction well beyond the avalanche front.