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SnowFlurry 20 2019/20 | Melting transformation

What happens to the snow crystals in spring?

by Stefanie Höpperger 04/04/2020
The melting transformation is the transformation process of the snow crystals in the snow cover, which occurs when the snow reaches a temperature of at least 0°C due to heat input. In other words, the transformation from snow crystal to water and back to ice when the temperature drops again. The corners and edges of the snow crystals begin to melt, become rounder and move closer together. The melting transformation can occur in all winter months, but is most common in spring.

The melting transformation process

Sunlight and warm temperatures (primarily in spring) heat up the snow surface, causing it to reach temperatures in the melting range. Water ingress from rain can also promote the melting process. Initially, the melting process only takes place on the snow surface - this is referred to as a low water input into the snow cover.

The crystals become increasingly rounder as a result of the melting process. The crystals often clump together and so-called clusters are formed. These clusters can grow to several millimetres in size in a very short time.

The capillary forces between the crystals increase due to the low water input, which initially only settles in the pore angles (indentations and contact points of the crystals). This leads to solidification within the snow layer. However, this only occurs as long as the moisture and the grain diameter are low. The result is cardboard snow, which slows you down when skiing, but is great for building snowmen.

As the snow becomes increasingly moist, the pore angles fill up with meltwater until it can no longer be held in place. It then runs deeper and deeper towards the ground into the snow cover. The snow crystals are covered by a layer of water due to the large amount of meltwater, which dissolves the crystals from one another. The binding of the crystals is lost, which in turn leads to a great loss of strength in the snow cover. Also known as rotten snow.

Changes in the amount of water can occur very quickly, especially in spring. Small differences in the amount of water are decisive in determining whether the wet snow is still stable or not. It is said that the snow cover remains stable up to around three percent liquid water by volume. If this value increases, a rapid loss of stability is the result.

On clear nights, the moist/wet snow surface freezes again, creating a melting hard cap and increasing the stability of the snow cover again. Repeated freezing and re-melting causes the melt shapes (grains) to become larger and larger, resulting in "skier's firn". Let's call it that, because "firn" is actually not correct.

The absolute final stage of the melting transformation is summer-solid snow. This is primarily found on glaciers. It can be recognized by its humpy and wavy surface with large melting grains. The difference between summer snow and slush snow (rotten snow) is that summer snow only contains a small amount of meltwater and solidifies again, which means that it no longer collapses. However, I won't go into this any further, because you can read all about summer-solid snow in an older rummage.

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How does melt transformation affect the avalanche situation and skiing?

If water is only introduced into the snowpack at the snow surface, this usually has little effect on the avalanche risk. However, loose snow avalanches tend to increase here, and the subsequent night-time radiation very often results in broken snow - the nightmare of all skiers!

Under the moist snow surface, there is usually still a very compact snowpack of round or angular rounded grains. There is still no danger of breaking into the snow cover due to rotten snow or the associated loss of strength. However, when digging profiles, you can often admire the beautiful, frozen melt channels created in the snow cover by the melt water. This superficial soaking is also frequently encountered in the winter months, especially on steep, sun-exposed slopes.

With increasing warm weather and the associated progressive soaking, an even larger part of the snow cover is covered with meltwater until the entire snow cover is soaked and has transformed into melting forms. This is where we find the classic spring situation! Setting off on a ski tour early in the morning and returning home early is the order of the day! Because an increase in avalanche danger can happen very quickly and rapidly.

The nightly radiation only causes the snow on the surface to freeze. The thickness of the melted snow cover varies depending on the temperature and radiation and determines how long you can be out and about in the spring.

Underneath the melted snow cover, there is sulphur snow (rotten snow). This consists of large melted grains. The resulting cover is hard as a rock in the morning, often grippy but also sometimes slippery and slippery. The sunlight and the warm ambient air soften the surface of the snow cover. If you catch the right time here, you can enjoy perfect skier's firn.

The longer the heat lingers, the deeper the layer becomes soaked until it is finally so soft that you break through. It can happen that you sink into the snow up to your chest when you take your skis off. By then at the latest, you should be aware that you have been out too long. As soon as you break through the cover, the risk of avalanches increases rapidly.

It can still happen that a cover of harsh snow becomes so thick (the humidity and temperature during the day also play a role here, of course) that it is not possible to break through all day, even on south-facing slopes. Or the variant that is definitely unfavorable for ski tourers: no snow cover forms at all due to an overcast night. In this case, it is better not to go on a ski tour.

The first significant soaking of the snowpack in spring, due to rain or warming, should be treated with caution, as deeper-lying weak layers can be weakened again by the ingress of water, which can cause avalanches to reach greater proportions.

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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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