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SnowFlurry 6 2019/20 | Degrading & building metamorphosis simply explained

What transformation processes take place in the snowpack?

by Stefanie Höpperger 12/28/2019
Snow crystals begin to change on their way from the cloud to the ground. Perhaps a layer of air is warmer and the crystal thaws slightly, or several crystals stick together to form large flakes. If it is windy, the fine branches of the crystals break off. Once they reach the ground, things don't calm down - that's when the changes really begin. For skiers, the decomposing and building transformation is crucial:

Degrading metamorphosis (transformation) also known as sintering: From fresh snow to felt-like crystals to small round grains.

When a snow crystal gently falls to the ground in its original hexagonal, still whole form, it is due to nature's physical endeavor to reduce its surface area. This happens with the decomposing transformation. How quickly the process takes place depends on the temperature and pressure. The higher the pressure (snow load) and temperature, the faster the transformation progresses.

The hexagonal snow crystal with its concave (indentations, hollows) and convex (tips, ridges) shapes begins to break down its branches and tips to become small and round.

This looks like this:

As an uneven distribution of crystals and water molecules in the new snow leads to a pressure difference, molecules begin to migrate (many molecules together form a snow crystal). Other snow crystals can branch out at the indentations of the new snow crystals, while the tips of the crystals find it difficult to join together. The water vapor pressure above convex shapes is greater than with concave shapes. Due to this pressure difference, ice sublimates (transition from solid to gas) at the tips and migrates as water vapor to the indentations, where it is deposited as ice again.

This process - also known as sintering - strengthens the bond between the crystals. Sintering describes the individual crystals moving closer together and becoming smaller as a result of the degradative transformation. The pore space becomes smaller and therefore there are fewer air pockets in the snow layer. In addition, the crystal becomes smaller and rounder and the crystals can move closer together. Sintering causes the snow cover to settle and thus the snow depth decreases. Consequence: Increase in the density and strength of the snow.

During the degrading transformation, a short-term loss of strength can occur, especially at the transition from the new snow crystal to the felt crystal. The new snow crystals are well branched, but these connections are partially lost during the transition to the felted crystal, as the new snow star breaks down its branches and pins (felt) remain. These pins can no longer branch out optimally.

If the degrading transformation takes longer, the snow also gains bonding, which in turn is one of the prerequisites for a snow slab. If there is still a weak layer (in the layers below) that can be disturbed, the risk of avalanches increases. Depending on the temperature, however, this is only a few days after the snowfall.

Both fresh new snow and felted snow can form a weak layer if it snows on top of it with the influence of wind, or if they are overlaid by drifting snow.

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Building metamorphosis: How do angular crystals, floating snow and facets form?

Building metamorphosis is the process from round grains to angular crystals to cup crystals (floating snow). The crystal starts to grow again.

The temperature gradient (temperature difference) in the snowpack is decisive for this. The greater the gradient, the faster and stronger the process of the build-up transformation takes place.

Snow is a good insulator and therefore a constant temperature of approx. 0°C prevails on the ground. The snow surface, on the other hand, can vary greatly in temperature and reach very low temperatures due to radiation (energy exchange with the atmosphere). The greater this temperature difference between the ground and the surface and the lower the snow depth, the stronger and faster the transformation progresses.

If we have a homogeneous snow cover with a very low temperature gradient, no constructive transformation takes place, but the degradative transformation.

How the constructive transformation works:

As it is warmer at ground level, the air there contains more water vapor than the colder layers above. The water vapour rises and crystallizes (deposition = conversion process from gaseous to solid) on the underside of colder crystals as ice. The grain begins to grow downwards. Small angular crystals are formed, which subsequently become more and more diverse and larger until finally the cup crystal (floating snow) is formed.

Sublimation (solid to gaseous) takes place at the same time as deposition. At the warmer top of the crystal - on which the new crystals have settled - water vapour forms again, rises to the crystals above and crystallizes again as ice on the next higher crystal. The process continues from crystal to crystal and can thus build up several centimetres.

Predominantly angular crystals form on the ground. This represents the classic old snow problem: A foundation of unconnected cup crystals (floating snow), which is overlaid by more compact, bound layers and is usually spread over a large area. The perfect combo for a snow slab!

However, if the temperature gradient is high in higher layers over a short distance of the snowpack, the build-up transformation also begins there. This happens under crusts, but also on the snow surface, although this should not be confused with surface frost. Although this is also created by a process similar to the build-up transformation, the water vapor comes from the surrounding air and settles on the surface through deposition.

Build-up transformation has a largely negative effect on the avalanche risk and usually results in an increase in this. However, there are exceptions here too. On the one hand, when the entire snowpack has undergone a build-up transformation, because then it is once again a homogeneous snowpack that has neither a bound snow slab nor a weak layer (both of which are needed for an avalanche). On the other hand, when angular crystals or surface frost form on the snow surface. Both forms are great for skiing.

However, if they are snowed in, they represent an extremely critical weak layer! Be careful!


The degrading transformation makes the snow cover compact and stable. The snow crystals become small and are well bonded to each other, there are few air inclusions and the temperature gradient is homogeneous or shows only a slight difference.

In contrast, the crystals become larger and have poor or no bonding. There are many air inclusions and the greater the temperature gradient, the more it progresses.

The Snow Stroller wishes everyone a happy new year and a healthy new year 2020!

This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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