Powder snow avalanches are by far the most complex to understand and model, as well as the most awesome to see as they thunder down the mountain at speeds up to 300 km/hr.

Reasons to Study Loose Snow Avalanches

Since there are many types of infrastructure in and around mountains, engineers and land planners need to know properties of the area such as the runout distance, flow velocity, and impact force of an avalanche originating from nearby mountains. To better predict where to build, and if an avalanche dam is needed engineers as well as physicists have developed models to predict the dynamics of an avalanche under certain conditions.

Initial Release

Loose snow avalanches usually occur as a point release avalanche.  A loose snow avalanche will usually happen in which a steep slope becomes heavily loaded with a new snowpack composed of low density and loosely packed snow. The properties of this snow do not allow it to resist the downward component of of its weight caused by gravity, thus when the slope is steep enough, and enough snow accumulates it will slide.

Trip Down the Mountain
The three basic components of a powder or loose snow slide are as follows:

1.Dense flowing avalanche

2.Powder cloud

3.Turbulent wake

    A common model used to predict the runout distance, and impact force of an avalanche such as the one above is a model based on numerical solutions to the depth averaged Navier–Stokes equations. However, models such as this do not take into account the mass exchange of the existing snow cover to the dense flow and the powder cloud which is entrained in the ambient air. This mass exchange plays a large role in determining flow velocity and runout distance.

Qualitative analysis of the dynamics of a flowing loose snow avalanche.

1. 1.As the dense flow travels down the slope it can multiply its mass by up to three times by entraining snow from the snow cover.
   

2. 2.Mass is then lost from the Dense flow in a process where snow from what is known as a Saltation layer (shown in above figure) captures air, and transforms into a state of powder snow - air suspension
   

3. 3.For a powder snow avalanche to initiate the buoyancy of this powder snow - air mixture must be greater than that of the ambient air.
   

4. 4.If sufficient energy exists then the powder cloud will form and begin to flow, being fed by the continual entrainment of snow by the dense flow.
   

5. 5.However if an excess amount of air is entrained in the powder cloud then its density will go down, causing the driving force of the avalanche to go down.
   

6. 6.It is a balance of all of these forces that determine wether a powder snow avalanche forms, and if it will accelerate or decelerate down the mountain.