How the Snowpack Changes
How does a snowpack change over time?
The snowpack is always in motion. Snowpack changes can affect avalanche conditions slowly and/or rapidly. We track conditions to develop pattern recognition. A weather log helps offer insight into how changing conditions can affect hazards in the mountains.
Every storm has unique characteristics, such as snowflake size, amount of moisture in the atmosphere, wind, surfaces that it lands upon, etc. These characteristics are defined as layers in the snowpack and can be the source of unstable snow. It is important that we watch how the layers interact with each other (interfaces) so we know if we have to manage or avoid a particular avalanche problem.
Metamorphism is a way to describe change within the snowpack. Metamorphic processes include rounding (strengthening), faceting (weakening), and sintering (bonding). These processes are ever-changing due to variables within the atmosphere and snowpack. Metamorphic processes occur in the snowpack itself, rather than in the atmosphere.
Metamorphism
Rounding
Rounding is a metamorphic process that promotes strong snow grains. A low temperature gradient (less than one degree over ten centimeters) will create the conditions for a grain to round. The basic principle is that the grain is losing its sharp edges (think of a snowflake falling out of the sky with its arms) and becomes more rounded, thus is able to bond with its neighboring snow grains (a process called sintering). The atmospheric and metamorphic process that promotes rounding are relatively warm temperatures and a deep snowpack.
Corn Snow
There is a particular type of change (metamorphism) that happens when the sun and/or warm temperatures melt the snow surface during the day and then the snow surface refreezes at night. This change is called melt-freeze metamorphism in which snow grains become clusters of large ice grains. Early in the day before the sun is on the slope and/or when the air temperatures are cold, the surface can be a crust. This can make for easier climbing, especially on steeper slopes. This crust can become part of the snowpack layering, and if new snow falls on top of it may form a bed surface for snow to slide on. Later in the day after the air temp and/or the sun has warmed the snow, crusts on the surface melt, becoming corn snow.
Faceting
Faceting is a metamorphic process that promotes weak snow grains. A high temperature gradient (more than one degree over ten centimeters) will create sharp edges and more pore space in the snowpack. Thus, the grains will have difficulty bonding. The longer that this process occurs, the larger and weaker the grains become. These grains typically feel “sugary” and will not stick together when trying to make a snowball with them. The atmospheric and metamorphic process that promotes faceting are cool temperature or large temperature swings that create large temperature gradients near the surface of the snow and a shallow snowpack.
Depth Hoar
Depth hoar is a very advanced facet. It occurs due to a shallow snowpack with a large temperature change between the ground (which is always right at freezing) and the atmosphere. Think of a facet getting so large that it cant go back to a round even with abundant warmth and insulation. Depth hoar usually occurs in continental climates due to long stretches without snowfall and cold temperatures in the early season. It differs from surface hoar in the fact that the process changes under the snow surface, rather than getting deposited on the snow surface.
Other Processes
Surface Hoar
Surface hoar is winter’s equivalent to dew. Think of a hot shower in a cold bathroom. All of the warm, humid air in the bathroom is looking for a place to go. Since the warm air is able to hold more moisture than the surrounding cold air and surfaces, it quickly deposits or condenses onto the mirror. It is no different when moisture in the air meets the cold surface of the snow and deposits on the surface in the form of crystals or feathers. They are often sparkly and have the appearance of snow dominos.
Surface hoar is actually an atmospheric process that creates a weak, faceted grain. It is not a metamorphic process that happens within the snowpack like other facets. Surface hoar forms during cold, clear, and relatively calm (>3mph wind) nights. It is often readily apparent near creek beds or areas with abundant moisture.
Surface hoar is quick to form (usually overnight), hard to find once it gets buried (referred to as buried surface hoar), and slow to decompose due to the size of the grains. Heavy snow, wind, and sun are all factors that can knock the large grains down. But if they remain upright, they can last for a long time as a persistent weak layer.
