SnowFlurry 7 2018/19 | Why powder stays powdery in warm periods of fine weather

After heavy snowfalls like those last January, the snow cover settles extremely quickly. The deep or often almost bottomless powder turns into powder within hours, which becomes harder and harder towards the bottom. You sink in less and less and after two or three days at the latest, you are left with boot-high, loose powder. This then lies on a compact base. Provided the temperatures remain at a normal winter level and don't shoot up through the ceiling.

But why is 20 - 30 cm of powder left? You would think that the snow cover would continue to settle through and through. So that after a few days of settling, a compact, piste-like snow cover can be found everywhere in the terrain?

Where in the snowpack does which type of transformation predominate?

The snowpack therefore continues to settle in all layers up to the uppermost centimetres. Powder about the height of a ski boot therefore remains on the surface for a long time. This is because it does not break down but builds up moderately. It actually transforms into other powder. The "loud powder" as it is called in English, or "Noppenpulver" in German. These are no longer beautiful dendritic crystals of fresh snow, but small angular crystals. So the powder remains powder - from a skiing point of view. But it builds up throughout. It therefore remains loose. It is only a few centimetres below the snow surface that the snow cover continues to settle through degrading transformation.

This is why we can still find powder for weeks on shady slopes (in February in much larger areas and much flatter slopes than in April) even though it hasn't snowed for ages and was relatively very warm. Due to the superficial cooling caused by radiation and the associated build-up transformation near the surface, we can still find good powder well below the zero degree line, which may prevail for days - but only where the sun hardly shines or does not shine at all.

And only where the radiation manages to cool the snow surface to such an extent that build-up transformation can take place. If the air temperature is too high, the radiation will not manage to cool the surface enough for a strong temperature gradient to prevail. As a result, the snow cover near the surface becomes so warm that it does not become angular and loose but round-grained and increasingly hard until it resembles a groomed slope. Because there is then no longer a pronounced temperature difference to deeper and also quite warm snow layers and the degrading transformation instead of the building up takes over.

Note: The energy balance and the temperature gradient determine the snow quality through their influence on the snow transformation via the snow temperature. Irradiation, radiation, air temperature and humidity influence the snow temperature together and must always be considered together.