A loose snow avalanche, or 'slough', is a common sight on steeper slopes where snow conditions appear stable.
Ideal Conditions:
Slopes of pitch in excess of 40 degrees Stable, consolidated snow pack to the ground No tendency to wind pack Cold conditions over a prolonged period Low humidity
And either:
Moderate to heavy snow fall - in sub-zero conditions; lightweight powder snow which does not pack (dry or 'champagne' powder)
Or:
Even build up of spindrift onto the lee slope without packing.
A build-up of snow over a period of time with stable temperatures and wind conditions leads to a good snow-pack; if there is then a period of consolidation without precipitation this pack becomes a good full-depth adhesive layer, both to the ground beneath and for future snow-fall on top. New snow is deposited and due to low humidity and cold snow crystals they do not bond tightly - pack - meaning that they sit lightly. This is easy to recognise when skiing as it is the Holy Grail of powder - champagne bubble light, swishing effortlessly away from the skis, leaving a blurred track as the crystals collapse in on themselves.
On the avalanche-prone slopes each turn will often cause a small slough down the fall-line as the light snow is pushed aside.
If the slope is convex the risks increase as the snow will gather momentum and push other crystals in front of the original displacement. This causes friction, which causes heat and the snow composition changes as the crystals become smaller and more dense - wet - which increases the specific mass of the original crystal. This increase in mass coupled with the concavity of the slope permit acceleration and perpetuate the cycle.
Characteristics of a large loose-snow avalanche may be a powder cloud, a hissing noise rather than the roar of any block avalanche, swift deceleration and no evidence of a crown or headwall fracture. The soft powder can be deceptive because as soon as it starts to move it changes form and becomes heavier, setting like cement around anyone caught in it.