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

The air temperature alone is not decisive

by Lukas Ruetz 03/01/2019
High Dorit and Erika have been bringing warm and practically precipitation-free weather for many days. On the sunny side, you can now find spring-like conditions with firn, depending on the altitude. On the shaded side, you can still catch a few turns of good powder - at least where the wind hasn't worked - despite a zero degree limit of over 3,000m at times. Why?

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?

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The temperature gradient makes the difference!

While the snow cover on the sunny side is always warmed to 0°C during the day during a high pressure situation like the current one, the snow surface on the snowy side always remains many degrees below the air temperature. The short-wave, warming solar radiation is missing there and the snow surface remains relatively cold due to the long-wave radiation. It loses energy through radiation and therefore remains colder than the air. Even when the zero degree line rises to over 3,000m, you can still find powder on the shaded side right down to the valleys after a few very warm days. Although the air there warms up to almost double-digit plus degrees, the snow surface remains cold below 0°C - solely due to the long-wave radiation. As a result, it does not become moist and remains loose and powdery.

But shouldn't the snow cover actually decompose on the shady side and soon become compact - i.e. have a hard snow surface? Not only the lower layers of snow should settle, but also those near the surface - one would think.

Due to the radiation and the associated very low surface temperature (especially at night), the build-up transformation predominates near the surface. This means that you may find -10°C cold snow directly on the snow surface even though the air temperature fluctuates around freezing point. A few centimetres below the snow surface, the snow then becomes warmer again relatively quickly. This means that we find a large temperature difference in the uppermost centimeters of the snow cover. As a result, the accumulating transformation predominates in the top 10, 20 or 25 centimetres. Below this uppermost layer, there is usually only a slight temperature change with every centimeter that you go further down. The decomposing transformation predominates there. As a result, the snow cover only settles noticeably up to this uppermost layer, where the temperature gradient is more pronounced due to the large jump towards the surface.

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.

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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