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SnowFlurry 6 2021/22 | Snow-covered topsoil

Hazard pattern 8

by Stefanie Höpperger • 01/22/2022
Surface frost is certainly one of the most impressive crystal forms that we can admire: Beautiful to look at, especially when it glitters and sparkles in the sunlight. However, if it is covered by snow, it turns into a highly dangerous and treacherous weak layer. As is so often the case, beauty is so close to danger.

What is surface frost?

Surface frost is ice crystals that form on the surface of the snow. The typical appearance is platelet-shaped and transparent and the crystals have many interconnected facets. They can grow up to a few centimetres in size.


Surface frost is formed by deposition (as in the build-up transformation within the snowpack), but the water vapor comes primarily from the ambient air. Rime crystals are also caused by the water vapor that escapes from the snowpack.

Deposition is the direct transition from the gaseous to the solid state, whereby the liquid state is omitted. When dew forms in spring or fall, it is condensation (gaseous to liquid). Water vapor from the ambient air condenses on supercooled surfaces, for example meadows, and dew droplets form. If the droplets freeze, this is referred to as frozen dew, not hoar frost. When dew forms, the dew point and air temperature are above the freezing point (0°C); when hoar frost (surface frost) forms, the temperature is below 0°C.

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To create surface frost, several factors are required:

A cold snow surface, a higher air temperature than the snow surface temperature and sufficient humidity. We also need clear nights so that the snow surface can cool down. The wind has an additional influence: the stronger it blows, the lower the frost formation, because the water vapor then has too few and too short points of contact with the snow surface to be able to settle.

Whether surface frost can form also depends on the energy balance of the snow cover. The energy balance describes the energy exchange with the atmosphere and the soil flow. The exchange of energy takes place through radiation, sensible heat (e.g. foehn) and phase transitions. Long-wave thermal radiation is responsible for cooling the snow surface - which is required for the formation of surface frost. In the case of snow, the long-wave radiation can not only record an energy plus compared to the short-wave radiation (solar radiation), but also a minus. In other words, the long-wave heat radiation can both warm and cool the snow cover. For the snow cover to be able to radiate, it needs a clear view of the sky, because every obstacle, whether clouds, trees, etc., results in counter-radiation, which feeds energy back into the snow cover and thus warms it.

Long rambling, now back to the essentials: Surface frost mainly forms in periods of good weather with clear nights. This is because the snow surface can then cool down considerably and sometimes even reach considerable sub-zero temperatures. In addition, there must be enough water vapor in the air, which requires more humid air masses (high humidity). This is why surface frost tends to form in the area of high fog. The air temperature simply has to be higher than the snow surface temperature.

If all these factors are present, the process is as follows: The slightly warmer and moisture-laden ambient air passes over the cold snow surface, whereby the water vapour present settles or crystallizes on the cold surface through deposition. This process can repeat itself over days and the crystals grow facet by facet towards the sky. At such cold temperatures on the surface of the snow, the build-up transformation also works at the same time: the same process, only the water vapor comes from warmer layers in the snowpack and the crystals grow downwards, not upwards. The combination of surface ripening and the simultaneous build-up transformation means that several centimetres of loose crystals can form on the snow surface. If the snow depth is low and the processes take place over a longer period of time, the entire snowpack can be transformed, as is often the case in early winter.

As long as the rime crystals remain on the snow surface, they pose no danger. However, if they are overlaid by drifting snow or fresh snow, they serve as a perfect weak layer. This is because surface rime is usually present over a large area and consists of large, loose crystals that break easily and whose fracture propagation is usually fantastic. So if there is a suitable board on top (bound snow) and the slope is sufficiently steep, an avalanche is practically pre-programmed. It is not uncommon to hear the sound of subsidence even on flat terrain if the surface has been snowed in. This danger cannot be recognized visually in the terrain. Only if you have seen the surface frost before it overlaps, or take a look at the snow cover, can you get an indication. However, the formation of surface frost can be guessed from measuring stations.

Nigg effect

The Nigg effect is particularly treacherous. This occurs more frequently in early winter and spring, when sunny slopes are warmed up and consequently warmer and more humid air masses rise towards mountain ridges and crests and slide onto their shady side. The snow surface there is usually still quite cold and the water vapor from the warmer air is deposited on the cold snow surface. Surface frost is then formed directly below the ridges and crests only in a narrow band (a few meters wide). If new or drift snow comes to rest on top of this, you can assume that this is an absolutely treacherous trap. It is not for nothing that this phenomenon is referred to as an expert trap, because if you have not spotted the frost before it has been snowed in or overlaid, it takes a hell of a lot of experience, precise weather observation and, if necessary, a snow profile at a randomly correct location to detect it before an avalanche occurs.

Detection at measuring stations:

Some measuring stations also indicate the snow surface temperature (grey line) and the dew point (blue line) in addition to the air temperature (red line). We need these three values in order to use measurement graphs to estimate whether or not surface frost is forming.

The snow surface temperature must be below 0°C, otherwise the snow will melt. The colder it is, the easier it is for frost crystals to form.

Furthermore, the air temperature (red line) must be warmer than the snow surface temperature (gray line) and the dew point (blue line), which indicates the water content of the air, must be above the gray line. So: red line at the top, blue line in the middle and gray line at the bottom.

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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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