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How and why does the snowpack change over time?
How and why does the snowpack change over time?
The only constant within the snowpack is change. Oftentimes snow and avalanche professions use the term "snowpack metamorphism" to describe change in the snowpack. Rounding, faceting, sintering, and melt freeze crusts are all examples of snowpack metamorphism. A trained backcountry traveler can observe evidence of these processes in the field. This section will cover the major types of snow metamorphism and their relevance with regards to avalanche conditions.
AIARE 2 Tools: Snowpack
AIARE 2 Tools: Snowpack
Observing the various types of metamorphic processes are key tools for any backcountry traveler. The weather forecast, avalanche forecast, and historical trends of terrain features offer a great starting point. Tools for the field include hand hardness tests, grain identification, and temperature readings. Once a layer of concern is located, it is important to then track it until it is no longer an issue.
How do facets form?
How do facets form?
The movement of water vapor through and within the pore spaces in between snow grains (ice) in the snowpack is the primary driver of metamorphic processes (change) in the snowpack. The temperature gradient or rate of temperature change across a vertical distance in the snowpack can be used as an indicator of the metamorphic processes taking place in the snowpack.
When air temperatures are very cold and/or the snowpack is very shallow, the temperature gradient (rate of change) across the snowpack is very high. A rule of thumb for what is considered “high” is a temperature gradient of 1 degree Celsius or more per 10 centimeters.
Large temperature gradients can indicate the formation of faceted grains. They feel like sugar and are crunchy when you chew on them. Typically, they form at or near the surface on cold, clear nights, at or near crusts, and can occur at the base of the snowpack during the early season months.
Faceting can occur at or below the surface of the snow. Once formed, they can be buried by subsequent storms which become persistent weak layers within the snowpack. The larger the faceted grain, the harder it is to morph back into a round.
Large grain facets near the bottom of the snowpack are referred to as depth hoar. They can grow so large because the early season snowpack is not very insulated thus the TG from the ground (which always remains around 0 degrees Celsius) to the surface air temperature can be very large. Also, early season storms typically have periods of high pressure in between cycles. Cold, clear nights enhance the faceting process and the surface grains are eventually buried and become problematic as the early season progresses.
Faceted grains can be easily identified in hand hardness tests as they are usually fist hardness or four finger hardness. They are problematic when anything one step further in the hand hardness scale overrides them. For instance, fist-hard (F) facets being overridden by one finger (1F) rounds would be like building a cement house on a Styrofoam foundation.
Large depth hoar snow grain
How do rounded grains form?
How do rounded grains form?
Rounded grains are exactly as they seem, round. These grains are degraded snowflakes that bond well with one another due to their small size. Rounds typically occur in the middle of the snowpack during the season as the snowpack undergoes settlement. They are strong grains that are typically associated with stable snow conditions. Though, when they override weaker grains they can be problematic.
Rounding is associated with smaller temperature gradients (less than 1 degree Celsius per 10cm) such as where snowpack depths are large and/or air temperatures are relatively mild. Rounded grains generally are generally going to be 4 Finger hardness or greater. Their hardness has a direct correlation to how well they are bonded and how little pore space is in between each individual grain.
Advanced rounding is referred to as sintering. Sintering is a process in which grains form bonds between each other to create cohesion amongst a layer. Sintering is generally associated with settlement within the snowpack and happens over longer periods of time.
Advanced rounding is referred to as sintering. Sintering is a process in which grains form bonds between each other to create cohesion amongst a layer. Sintering is generally associated with settlement within the snowpack and happens over longer periods of time.
How do melt freeze grains form?
How do melt freeze grains form?
Melt-freeze cycles form polycrystals over long periods of time, generally around springtime. The common term for polycrystals is “Corn” snow. Corn snow can be very stable when frozen. Melt freeze cycles occur when there is a large diurnal (night to day) temperature swing. During the daytime the grains start to melt and latent water moves through the snowpack. At night, the colder temperatures freeze the water and the grains together forming clumps of snow grains (corn).
When frozen, corn snow is quite strong. As temperatures begin to rise during the daytime (especially under a strong spring sun) the grains lose their bonds and can become very weak, very quickly. The result can be wet avalanche cycles. It is important to determine where potential rock outcroppings, cliffs, or tree bases connect with the snowpack as those areas are prone to even more advanced warming due to their lack of reflectivity of radiation relative to white snow.
It is important to follow temperature trends when following melt freeze cycles in order to determine when it is safe to travel on particular slopes. A multiple hour freeze of the snowpack is critical to ensure the grains have properly bonded.
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