The Dynamic Mountain Snowpack
What is a snowpack?
A snowpack refers to the seasonal snowfall and the interaction of each storm. A snowpack consists of layers from various storms throughout the year. It is the interaction of these layers that can promote stability or instability. Weather, time, and gravity keep the snowpack in a state of constant change driven by weather influences as the surface and subsurface changes throughout.
Layers and Interfaces
A layer refers to a band of homogeneous snow within the snowpack. This layer is typically from a weather event such as a storm or warming. Alternatively, a layer can come from long-term changes within the snowpack over the course of weeks or months.
An interface is where two layers meet. Think of a sandwich, where each piece of bread is a layer and the jelly is what connects those layers. Layers with differing characteristics create unstable snow, which can lead to avalanches.
Understanding the layered snowpack and tracking how it changes over time helps us better understand when and where to anticipate unstable snow.
The Snowpack is Constantly Changing
Snow (and thus the snowpack) is constantly changing from the moment it hits the ground. Weather is the primary driver of change in the snowpack, though gravity and time are other important factors to consider. Below, we explore the four major weather drivers that affect the snowpack.
Temperature affects the snow from both atmospheric and metamorphic processes. Rapid warming and rapid cooling are both critical factors with regards to avalanche activity. Warming can cause deformity in slabs and cornices, thus creating instability. Warm layers can also freeze into crusts, another way that the snowpack can be destabilized. Cold temperatures are associated with weakening (faceting) of snow layers, which can cause avalanche cycles after they have been buried.
Rain is another contributing factor to instability within the snowpack. Rain creates weakness at the time of precipitation. It weakens the bonds and creates runnels within the surface layers of the snowpack. Rain can actually create a very stable snowpack when it refreezes. Though, those layers can become problematic as the season progresses as they form impermeable crusts which can lead to weakened snow grains above and below the crust.
Sun is a major driver of change within the snowpack. Solar radiation can soften and consolidate snow surfaces, which enhances the effect of warm temperatures. Depending on the night after intense solar exposure, hard crusts can form creating slick bed surfaces. The sun also heats up darker objects under the snow which breaks down bonds and can create wet loose and wet slab avalanche problems.
Wind is another element that creates change within the snowpack. Wind moves snow. It can completely remove snow from the windward aspect (where it is being pulled from) and redeposit it on a leeward aspect (where it is collecting, or loading). The snow grains will likely be very compact where they have deposited, thus forming wind slab. The amount of snow that is available for transport depends on the conditions at the surface of the snowpack. If the snow grains are loose, more will distribute to the leeward aspect.
Differences between layers are the sources of unstable snow, and thus avalanches. Of most concern are areas in the snowpack where there is strong over weak layering.
Strong snow is cohesive, meaning it sticks together. Thus, strong snow (or a slab) overriding weak snow (non cohesive) is very problematic. This type of layering creates the recipe for slab avalanches where a stronger layer fails upon a weaker layer. A slab can be formed by wind, heavy snow accumulation, settlement, and rounding processes. If a slab is on top of faceted snow or slick bed surfaces (crust) it can be hazardous.