In the avalanche bulletin a distinction is made between "wet snow avalanches" and "wet snow avalanches during the day", and these phrases are used to describe the two different avalanche problems.
The avalanche problem is related to a weakening of the snowpack due to the presence of liquid water. Water infiltrates the snowpack due to melt or rain.
Avalanche type and trigger
- Wet-snow slab avalanches
- Wet loose snow avalanches
- Mainly natural avalanches
When water infiltration is due to melting, the problem is often specific to certain slope aspects (solar radiation) and elevations (air temperature).
In case of rain on snow, all slope aspects are affected (below the elevation, where snow turns to rain).
Position of the weak layer in the snowpack
Anywhere in the snowpack, in case of slab avalanches often at pre-existing weak layers.
- Wet-snow slab avalanches: Weakening and failure of pre-existing weak layers in the snowpack or release at layer interfaces due to ponding water. Rain represents also an additional load on weak layers.
- Wet loose snow avalanches: Loss of cohesion between snow crystals.
- Hours to days
- Rapid loss of stability possible
- Especially critical as water infiltrates for the first time deeper down, once the snowpack has warmed up to 0 °C.
- Natural avalanches might be more likely in the course of the day, depending on aspect (unless rain is the dominating factor).
How to manage?
Identification of the problem in the field
The wet snow problem is usually easy to recognize. Onset of rain, snowballing, pin wheeling and small wet slab or loose snow avalanches are often precursors of natural wet-snow avalanche activity. Deep foot- or ski-penetration is another sign of increased wetting.
If the wet snow surface freezes overnight due to clear skies and cold temperatures then develops a strong supporting crust, favourable conditions will usually be present in the morning. After warm, overcast nights, the problem will often be in existence in the morning. Normally rain on new snow creates this problem almost immediately. Good timing and trip planning are important. Consider avalanche runout zones.