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SnowFlurry 2 2020/21 | Why clear skies often play a more decisive role in the formation of thin layers than low temperatures

The development of weak layers of old snow continues

by Lukas Ruetz 11/28/2020
The marmot greets us daily with sunshine and a mostly cloudless sky. Especially near the surface, there is a massive temperature gradient of just a few centimetres in the snow cover - often despite plus temperatures up to high altitudes.

With every day of fine weather, the anthill of the snow cover continues to work under high pressure to form weak layers of old snow. We usually learn or hear that low temperatures are responsible for the formation of weak layers that have been transformed to build up. However, temperatures are currently only slightly below zero - if at all - even on the higher mountains. Of course, what we have heard is not wrong: the cold favors large-scale, constructive transformation in the snow cover. However, a long period of fine weather when the sun is low in the fall, early or mid-winter is much more often responsible for the weak layers than low temperatures. Because this has the same effect on the snow cover.

Influence of the temperature gradient

The temperature gradient is decisive for the type of transformation in the snow cover. In other words, the temperature change per centimeter of snow cover, or in other words: how pronounced the temperature gradient is. If the snow cover is at the same temperature everywhere, for example -5°C from top to bottom, the anthill works just as hard. However, it is not in the build-up transformation, but in the degradation transformation. The snow cover then becomes more compact and the snow crystals smaller and rounder.

The ants begin to change the pile in the form of constructive transformation as soon as the temperature changes by 0.15°C per centimeter, or 15°C per meter. The crystals then become more angular, larger and looser. The greater the temperature difference in a small space, the stronger the anabolic transformation becomes.

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Influence of the absolute temperature

But the absolute temperature is also decisive: a snow cover with a constant temperature of -15°C breaks down more slowly than a snow cover with a constant temperature of -1°C.

In physics at school, we once learned that temperature is nothing other than the speed of movement of the molecules. The warmer it is, the faster they wriggle. The warmer it is in a blanket of snow, the faster it breaks down - regardless of whether it is breaking down or building up. Because the ants can simply work faster in it. A surface temperature of -15°C in a 1m thick snowpack and a temperature of 0°C at the base of the snowpack has exactly the same temperature gradient as a snowpack with a surface temperature of -31°C and a temperature of -16°C one meter below the surface. The gradient is 15°C at a snow depth of 1 meter. Nevertheless, the build-up transformation is much stronger in the absolute warmer area because the work and transformation is faster there.

The surface temperature

Since the snowpack is always 0°C or only slightly colder at its base, the surface temperature of the snowpack plays the main role in the formation of persistent weak layers in the case of an old snow problem close to the ground.

The surface temperature is a product of

  1. radiation

  2. radiation

  3. air temperature

  4. air humidity

All four parameters play a significant role.

The air temperature is in direct exchange with the snowpack through "contact" - like when we touch a hot or cold hotplate and the heat is transferred from the plate to our hand. This is referred to as sensible heat or heat conduction.

The air humidity influences the surface temperature through the sublimation of the snow cover. The lower the humidity, the more the snow "evaporates" on the surface because there is more water in the air. Evaporation or vaporization cools the snow surface. The more snow sublimates, the more the surface is cooled. This is referred to as latent, hidden heat. Because it is only released or extracted when a phase transition takes place. From solid to gaseous removes heat. From gaseous to solid - i.e. when frost forms on the ground - a little heat is released.

When it comes to radiation, we distinguish between short-wave radiation, which includes solar radiation, and long-wave radiation, i.e. the heat radiation we know from infrared cabins. Snow does not produce any short-wave radiation on its own. It can only be heated by short-wave incident radiation. However, it cannot cool down by emitting its own short-wave radiation.

The situation is different for long-wave radiation. Snow is the perfect radiator in the long-wave range - a so-called black body. However, we humans only see short-wave rays with our eyes. Snow emits massive amounts of heat radiation throughout and thus cools down. And now we come to the crux of the matter.

The clear sky

If the sky is cloud-free, no long-wave radiation returns from the clouds to the snow cover. For the sake of simplicity, this is usually referred to as the reflection of long-wave radiation from a cloud cover. This is not quite correct, but it does not change our understanding. The clouds actually absorb the long-wave radiation from the earth's surface - in our case from the snow cover. They convert this radiation into heat and, in return, they emit "new" long-wave radiation towards the earth's surface.

In the end, the snow surface can cool down little or hardly at all with a cloud canopy. It usually remains within the air temperature range. If the air is significantly warmer than 0°C, the snow cover naturally cannot become as warm as the air. Then it simply melts away instead.

When the sky is clear, the snow surface cools down massively below the air temperature. In combination with low humidity, this can be up to 20°C below the surrounding air temperature.

At present, the snow surface is usually 7 - 15°C below the air temperature. This is the case 24 hours a day where the sun does not shine or only shines very weakly. Where the sun still brings short-wave radiation, the snow cover can warm up significantly for a few hours.

The current temperature gradient

This currently results in a massive gradient in the snow cover. Especially in unsunlit areas around the clock.

The gradient is not only massive due to the low surface temperature but also due to the low snow depth. With little snow, the temperature change per centimetre at the same surface temperature is ultimately greater than with a lot of snow.

Conclusion

In early winter, there tend to be warm, long-lasting high-pressure conditions. These are almost always responsible for the formation of an old snow problem close to the ground. Of course, a long-lasting cold spell also has a strong build-up effect on the snow cover. If it stays extremely cold for a long time with clear skies, the transformation is even more intensive. It just happens very, very rarely in this form.

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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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