SnowFlurry 7 2017/18 | Insidious weak layers due to "cold on warm"

The snowy winter of 2017/18 is nearing its peak. Weak layers close to the ground from the fall or early winter were de facto not an issue. Nevertheless, there were always times when the problem of old snow was a decisive factor in tour planning. Not through a deep persistent weak layer but through a persistent weak layer, as the Americans would call it. These weak layers are persistent weak layers of angular crystals or floating snow that last much longer than the weak layer of "powder snow" and are almost always caused by short-term temperature changes in a small area in the snow layers near the surface. And are quite toxic for us winter sports enthusiasts.

Hunting for crusts

Weak layers near the surface, which are transformed by building up, almost always form in the area of melting crusts. But beware: it is not the fusion crust that is the problem, but the weak layer that forms above or below it. This is because the crystals break apart in the weak layer and then separate the snowpack into the sliding and spilling snow slab above the weak layer and the unimportant sliding surface below the weak layer.

Weak layers and melt crusts often go hand in hand - and not just in the release mechanism. Weak layers of angular crystals also often form in the area of the crusts. And this is where the "cold on warm" hazard pattern comes into play. If the snow surface is slightly moist, whether due to radiation, warm temperatures or rain, and is then covered by much colder, loose powder snow due to an incoming cold front, a large temperature difference forms between the old snow surface and the new snow. This temperature difference between the 0°C warm old snow surface (exactly 0°C because it is slightly moist) and the -10°C cold new snow over a few millimeters, for example, is decisive for the immediate and very insidious formation of a weak layer. The more soaked the snow surface is, the stronger the weak layer formation will be. If the old snow surface is only just under 0°C and not moist, a weak layer formation will also take place, but not as strongly as with moisture content.

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Two crusts enclose a weak layer of angular crystals from mid-February. There is still no suitable snow slab above this with the unbound powder snow.

Lukas Ruetz

Within one or two days, a weak layer of angular crystals forms in this boundary area, which is also a fire hazard due to its low thickness (usually only 1 or 2 cm thick). At the same time, a melt crust forms underneath this weak layer on the former old snow surface. In this case, the melt crust is not decisive for the triggering mechanism, as it is located below the weak layer and will therefore serve as a sliding surface in the event of a potential avalanche. And the sliding surface is of no interest to us with regard to snow slab avalanches - it is always about the weak layer and the snow slab above it and how the two interact. If the melt crust is above the weak layer, it helps to trigger the avalanche: Due to its strength, it promotes the possibility of fracture propagation and thus enables the weak layer to break over a larger area.

In addition, there is the possibility that the melt crust forms even before new snowfall - namely due to falling temperatures or nocturnal radiation before the onset of precipitation. A melt crust is an excellent conductor of heat due to its high density and low air content compared to other types of snow. It cools down very well. The snow surface temperature, i.e. the temperature of the surface melt crust, is therefore far below 0°C - but at the same time the snow temperature remains relatively warm just a few centimetres below this. The temperature gradient is again very pronounced. A weak layer forms. This time below the melting crust. However, the melt crust alone is usually not suitable for a snow slab. Only when it subsequently snows on top of it - even without wind - do the ingredients for a slab avalanche fit: a thin, toxic weak layer is then located underneath a melt crust that favors fracture propagation. This in turn is covered by fresh snow, which can be great for skiing. Melt crust and fresh snow together form the ideal snow slab.

Thanks to this interaction, the avalanche warning system is often a crust hunter. If you know the altitude ranges and exposures in which a melt crust has formed, you can narrow down the problem area of the weak layer that often forms in addition. This is particularly interesting in the case of rain events: How far up did it rain and where? How far up did the snow surface get wet? The following insight is important for us: When such weak layers form, we simply have to stay away from certain exposures in certain altitude bands. For example, between 2200m and 2600m on very steep south-facing slopes. And: The rain line in high winter should always be reported to the responsible avalanche warning service.

Mostly sunny terrain affected

Normally, one learns: Weak layers that have been transformed by building up are mainly found on shady slopes. This applies primarily to winters with little snow or, in inner-Alpine areas, winters with average amounts of snow. In winters with a lot of snow, there are rarely weak layers close to the ground on shady slopes, but the "cold to warm" hazard pattern is much more frequently responsible for relevant weak layers. This is related to the constant ups and downs of temperatures, i.e. cold to warm or warm to cold.

The hazard pattern usually forms built-up converted weak layers in sunny terrain, especially in high winter. The air temperature is only high enough for superficial soaking of the snow cover in very deep locations that are less suitable for skiing. This means that direct sunlight is often the decisive factor during periods of fine weather: if it warms or moistens the snow surface, the conditions are right for the formation of the weak layer described above. Until March, this is usually only the case on steep slopes in the southern sector.

Avalanche triggering on the day of the profile recording in the vicinity of the profile slope due to this weak layer.

Lukas Ruetz

Avalanche triggering on the day of the profile recording in the vicinity of the profile slope due to this weak layer.

Lukas Ruetz

Winter 2017/18

In the current season, there have already been four more pronounced phases in Tyrol with the formation of a weak layer due to "cold to warm"

at the beginning of December on steep southern slopes around 2200m.
At the end of December in the southern sector (= south-western slopes, southern slopes and south-eastern slopes) between 2200m and 2700m. These weak layers were reactivated at the end of January during the extreme snowfall period. This means that the load caused by large amounts of fresh snow became too great on the weak layers, which had since solidified again somewhat, and slab avalanches occurred again due to these layers.
The biggest problem finally arose in mid-February: Warm, fine weather days with crust formation on steep sunny slopes at the end of January, followed by some fresh snow, then warm, fine weather days again with renewed crust formation, followed by very cold temperatures. Between the two crusts, the powder snow built up very quickly and became a significant problem for skiers on steep south-facing slopes at around 2700m after more snowfall, i.e. the formation of the snow slab. At the end of February, when the constellation of weak layer - snow slab then harmonized perfectly, numerous avalanches were triggered due to this layer.
The last phase of weak layer formation for the time being due to "cold on warm" occurred in mid-March. The LWD Tirol writes: "The distribution of the weak layer that has been transformed into a build-up depends primarily on the existence of melt crusts, and secondarily on the altitude, slope inclination and exposure. On the shaded side, it should only affect a narrowly limited altitude band between about 2100m-2300m. With increasing altitude, these weak layers can be found in other exposures and altitude bands. Initially, these are very steep slopes oriented towards the NW and NE, then these are replaced by W and E slopes, and then S slopes are added in the high Alps." In this case, the spread of the melt crusts or weak layers is due to an interplay of air temperature, diffuse and direct solar radiation. The distribution of the layers could again be narrowed down according to exposure and altitude.

Newly formed weak layers due to "cold on warm" cannot be ruled out, the primarily affected area will tend to shift upwards and into shaded slopes. The radiation becomes so strong as the sun rises higher and higher that weak layers near the surface on south-facing slopes are destroyed again in a very short time by the massive heat input.

Note: Weak layer formed on the ground and transformed by building up = persistent, longer-lasting. Weak layer formed in the atmosphere (powder, sleet) = non-persistent, only problematic for a few hours to days. Converted, persistent weak layer due to "cold on warm frequently" on steep sunny slopes!