SnowFlurry 1 2019/20 | Weak layer formation: lots of snow vs. little snow

SnowFlurry in the new season

The SnowFlurry will be enriched by a female snowflurry columnist! Steffi Höpperger, an ambitious ski mountaineer and real snow vole, will alternate the column with the long-established SchneeGestöberer in future. In addition, the SnowFlurry will focus more on the current snow cover situation in a region - usually the Stubai Alps - & discuss, explain and interpret snow profiles. Detailed articles with in-depth background knowledge, as usual from previous seasons, are interspersed for variety.

Profile 1, Grieskogelscharte, 10.11.2019, 2580m, N

The profile was recorded before the intensive November snowfalls. There was already a 42 cm thick layer of snow at the location due to earlier, smaller snowfalls. The snow is relatively soft through and through. The bottom layer consists of rounded crystals that are already slightly moist and can be clenched in the fist.

They have either become moist due to solar radiation and air temperature, i.e. before it snowed again, or the heat from the ground has moistened them. The profile was taken on a 33° steep north-eastern slope. With the current position of the sun and the temperatures at the beginning of November at this altitude, it was most likely the ground heat.

The extremely strong temperature gradient is striking: the red connecting line of the measured temperatures is extremely flat. This means that there is a massive temperature difference with a relatively low snow depth. In figures: At 42 cm, we find a difference of 0°C on the ground to -17.4°C on the snow surface. This causes the entire snow cover to build up, turning it into a weak layer of old snow.

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Profile 1 - Grieskogelscharte

This can already be seen in the crystal shapes near the surface: above a weakly pronounced, thin melt crust, the fresh snow (felt) has already developed into angular crystals. They are still very small, less than 1mm in size. Therefore, they are not yet a very prominent weak layer as soon as they are covered by fresh snow or drift snow. However, a long period of fine weather with a continuing strong temperature gradient and the resulting progression of the transformation would be very bad. The angular crystals would then become larger and larger and eventually transform into stubborn and - once snowed in - extremely dangerous cup crystals.

However, the snow cover is still low-stress because there is no harder snow slab above the only slightly pronounced weak layer. For this reason, a stability test was not carried out (see comments above left), as it is highly unlikely that a fracture would have occurred and therefore no information relevant to a potential avalanche problem could have been obtained from it.

We can only hope: No long period of good weather and lots of fresh snow as soon as possible!

Profile 2, Lampsenspitze, 20.11.2019, 2490m, N

The second profile was recorded ten days later at a similar exposure and altitude. In the meantime, there has been over a meter of fresh snow in the study area of the Northern Stubai Alps. With interim settling, there was 118 cm of snow at this location on the Lampsenspitze during the profile recording. The moderately pronounced weak layers close to the ground, as can already be seen in snow profile 1, are now developing in the other, positive direction for us. This means that they are no longer building up, but are now breaking down.

On the one hand, this is due to the pressure of the new snow (mechanical transformation), and on the other hand, due to the temperature gradient in the snow cover, which is now much less pronounced simply because of the greater snow depth. If it were as cold at the snow surface as at snow profile 1 (-17.4°C), the temperature gradient would still be much smaller with greater snow depth: the temperature change from the ground (always 0°C) to the surface is no longer distributed over 42 cm, but already over 118 cm. This makes the formation of weak layers more difficult or even stops it and everything develops back into a compact snow cover. Existing weak layers sinter (bind together better again) and the formation of new weak layers is prevented.

The snow pusher meticulously searching for weak layers. — The snow rummager meticulously searching for weak layers.

Lukas Ruetz

Steffi Höpperger

Only in the uppermost centimetres of the snow cover is the temperature gradient even more pronounced under a cloudless sky (due to the long-wave radiation from the snow surface and the associated cooling). This means that in a longer, precipitation-free phase, only weak layers are likely to develop near the surface. Due to the thick snow cover, it is unlikely that the entire snow cover or layers close to the ground will turn into floating snow in the coming season.

The situation looks worse in the areas that currently still have little snow. With each day of clear skies, the snow cover continues to build up into a weak layer, which can become dangerous with fresh snow. Hopefully, a thick base will soon form everywhere.

Note: Old, weak layers of snow mainly form in a snowpack with little snow and cloudless skies. The more snow, the better for us - in every respect!