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

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.

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.