SnowFlurry 11 2019/20 | Pimple powder as a weak layer

What is nap powder?

Nap powder is a surface snow surface that has undergone a slight build-up transformation and is mainly found after long periods of fine weather. This type of snow mainly forms on shady slopes and slopes with little sunlight, as the cold snow surface is working around the clock to build up the snow. The very cold snow surface with temperatures up to 15°C lower than the current air temperature forms due to the radiation of the snow cover and the associated cooling. As a result, there is always a strong temperature gradient under a cloudless sky, at least near the surface, which favors the build-up transformation. The pimples of the pimple powder are subsequently formed by cold downdrafts. In shady cirques and slopes, the air cools down considerably and sinks downwards due to the increase in density as a so-called katabatic downdraft, which causes a little snow to be transported on the surface and the classic structure with waves and holes to form.

The crystal shape of the nap powder is angular crystals with a grain size of around 1 - 2 mm in diameter.

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Continuously built-up converted snow cover where you break through to the ground. Taken in the continental climate of Kyrgyzstan. — Completely built-up snow cover where you break through to the ground. Taken in the continental climate of Kyrgyzstan.

Kristian Rath

Kyrgyzstan

Nap powder as a weak layer

Once the nap powder has been snowed in or blown over by drift snow, it acts as a weak layer. In other words, a weakly pronounced old snow problem - possibly in combination with a drift snow problem. Weakly pronounced because nap powder usually forms a rather thick weak layer with not too large crystals. Thick weak layers with a thickness of several dozen centimetres are usually not as prone to triggering as very thin weak layers with very large crystals.

Current situation

In the last significant snowfall at the end of January, pimple powder was locally snowed in. Particularly at high altitudes (i.e. between 2000 - 3000m) on slopes exposed from W to N to E in terrain that has had little snow cover and wind protection so far. But that's not all: due to the "desert month" of January with almost continuous sunny weather, the snow cover consisted not only superficially of crystals that had been transformed by building up, but also completely of small angular forms. A completely transformed snowpack is not a problem in terms of slab avalanches, as the snow slab is missing. You often sink through to the ground when making tracks and on the descent this type of snow can only be enjoyed with very wide skis without sinking through to the ground with every third turn. If such a snowpack is snowed in, this almost inevitably causes problems.

Excellent blasting successes, such as in Kühtai on January 30th, indicate the involvement of this old snowpack that has been transformed to build it up. If there was "only" fresh drift snow due to fresh snow and wind without a weak layer of old snow, the blasting results would not be nearly as good. This means that not as many avalanches would be triggered by the explosive charges.

Pimple powder with its characteristic surface structure. — Nap powder with its characteristic surface structure
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Conclusion

Surface pimple powder with an (even) harder snow foundation is great for skiing - similar but not quite the same as "real" powder that falls from the sky as a dendritic new snow crystal.

But as soon as the entire snowpack has been transformed to build up, you often sink through to the bottom like during a first heavy snowfall in autumn. The fun factor is massively reduced as a result.

As soon as pimple powder or a completely built-up snow cover is snowed in, they form a distinctive weak layer.