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They look like penitent monks in white hoods: icy steles of up to six meters in height, covering all mountain areas in the high mountains of the tropics and subtropics. For climbers, these ice-needle fields are insurmountable. But how do the uniformly shaped peaks actually develop? Almost 15 years ago, the physicist Meredith Betterton of the University of Colorado explained the formations like this: Even a flat snow surface has uplifts and depressions. When the sun's rays fall on the snow, they are reflected but encounter more resistance in the valleys, ie snow - there is greater heat in the valleys, which then melt faster and sink further.

Philippe Claudin of the ESPCI, the Technical University in Paris, was not satisfied with this statement. He and his colleagues have developed a new model for the so-called penitential rice, which explains the always characteristic shapes and the spatial distribution better than the previous theory. Their conclusion: a complex interplay of physical processes is the natural architect and master builder of these formations. When the sun shines on the snow surface, the heat is absorbed by the snow. Inside the mass of snow, the temperature rises higher than outside. Because there rises at the same time haze. This cools the surface. If dense moisture forms above the ice, less vapor can escape from the interior.

But why does the snow fall faster in the valleys? At these points more heat is absorbed, there is also a larger temperature difference between inside and outside. And there is more haze over the ice. The snow then melts faster in the hollows and breaks. Essentially, according to Claudin, the origin of penitential travel depends on how much water vapor has formed over the snow surface - and that the concentrations in close proximity to each other differ.

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© science.de - Ruth Roebuck / Karin Schlott
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