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SnowFlurry 4 2016/17 | The snow cover sandwich

Crusts surrounded by weak layers are not uncommon

by Lukas Ruetz 11/24/2016
Depending on the area and altitude, there are already some melting crusts within the snow cover that have been created by rain events or warm air intrusions. Weak layers often develop around these. Why is this?

Weak layer formation

Weak layers in the snow cover are caused by pronounced temperature gradients. This means that the temperature difference between the snow crystals is relatively large in a relatively small area. Snow is not always equally "warm". In practice or when creating a profile, the snow temperature ranges somewhere down to -25°C and can - nonanetically - reach a maximum temperature of 0°C.

Crust formation

Once a layer of snow reaches the melting point, it naturally does not heat up any further, but the energy (= heat) supplied beyond this point is used to convert the phase transition from solid to liquid. This creates a water-snow mixture. In practical terms: the snow becomes moist. The higher the moisture content of the snow layer, the greater the proportion of water. This continues until the mixture reaches something like saturation, which is the case at the very latest with a "liquid water content" of 15 percent by volume in the snow cover. The water begins to look for vertical and horizontal paths within the layer, i.e. simply: to flow off.

When the water-snow mixture freezes again, it is referred to as a melt crust - which is no longer moist, but dry, as the water content has frozen back into ice. Melt crusts as well as melting forms occur in the form of so-called melt lumps, which are smaller or larger in diameter. The round circle for "melting forms" in snow profiles stands for the wet state, i.e. not frozen. The spectacle symbol of the melting crust stands for melting forms whose water content has refrozen and thus encrusted and turned to ice. You can still partially recognize old grain shapes with a low original water content, which is why there is still space in the glasses for the symbol of another grain shape.

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Correlation between crust and weak layer formation

The "evil" crystal forms are thus created when snow sublimates ("evaporates") and this free water vapor "strikes" somewhere again, i.e. freezes onto an existing crystal. Unfortunately, if the temperature gradient is very pronounced, it freezes in such a way that the crystals become larger and larger and have fewer and fewer contact points between them, making the connection between them worse and worse. This is very nice to look at and soon feels like letting sugar trickle through your hands - but the stuff is not particularly sweet and can cause rather bitter situations.

For a long time, the viewpoint was held that these melt crusts (commonly known as the superficial harsh cap in spring, among other things) block the transport of water vapor in the snowpack and therefore weak layers can form more easily on them because the free water vapor cannot get past. Let's leave it at that and adopt the "modern" opinion: Weak layers form preferentially on crusts because they are better conductors of heat due to their higher density and have reached a temperature of 0°C, at least for a short time. In other words, they may have a significant temperature difference to other snow layers.

The fact is that weak layers, i.e. angular crystals, floating snow, deep frost, cup crystals - also known as "semolina" - form very frequently around crusts. This prefers to happen below crusts, but it also occurs above them. Water vapor transport within the snowpack takes place continuously: No matter how strong the temperature gradient is. In addition, the water vapor flow can take place in any direction, not just from bottom to top. This is the case, for example, with the "warm to cold" hazard pattern, where the warmer layer with the greater vapor pressure is located further up in the snowpack, while the layers further down are colder.

Weak layers form when temperature differences are very pronounced - this can be related to the entire snowpack, i.e. the temperature difference between the layer on the ground and the layer on the surface, or it can only be decisive in a very small area: For example, if it rains first and then cold fresh snow falls on a soaked old snow surface. This creates a large temperature difference over a few centimetres and a wafer-thin thin layer can form in a short space of time. The vapor flows from the moist old snow surface at 0°C to the cold, dry new snow (perhaps -10°C) directly adjacent to it and builds up angular crystals there.

Always in motion

With less pronounced temperature differences, the resulting crystal form is referred to as "point-grained" or "round-grained". Whereby one cannot speak of "end product" here either: The snow is constantly changing, no grain remains untouched: it loses mass, gains mass, gets smaller, gets bigger - the wheel never stops. Everything in the snow is constantly changing. What comes out as an intermediate product, however, depends on the temperature. It is similar with the foehn clouds in the sky: the classic foehn clouds appear to stand still. In reality, "fresh air" is continuously added to the windward side of the cloud by the wind and always condenses in the same place. On the leeward side of the cloud, the opposite happens, making the cloud appear to stand still and always be made up of the "same material". In reality, fresh material is constantly being added on one side and taken away on the other - even if the end product looks static to the observer.

Note: The snow cover is constantly changing, its state is never at rest, not for a second.

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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