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Mathematical physics

Ice crystals pull together

04 Dec 2001

Avalanche prediction and defence relies heavily on an understanding of the structure of snow when it packs together. Now Ed Adams and colleagues at Montana State University in the US have found a new feature that develops between adjacent ice crystals, which suggests that mass may redistribute itself through snow fields more quickly than current theories predict. According to the team, the same effect may occur in a wide range of crystalline materials (E Adams et al 2001 J. Appl. Phys. 90 5782)

Slippery slope

When fallen snow is compressed by further snowfall, individual ice crystals bond with each other to form a porous mass – a process known as ‘sintering’. The bonds that join the crystalline grains determine the overall structural properties of the snow – such as strength, viscosity and creep – as well as its thermal, optical and electromagnetic characteristics. This process is complex in snow, because thermal variations cause water vapour to diffuse through the structure, which then re-freezes. This phenomenon depends on vapour pressure.

Adams and co-workers collected fresh snow and examined it at sub-zero temperatures with a scanning electron microscope after it had rested in a freezer for several months. They noticed that an elevated ridge had developed around the perimeter of nearly all the interfaces between grains of snow. This ridge – dubbed a ‘grain boundary ridge’ – is evidence for a process known as ‘grain boundary diffusion’, in which mass redistributes itself from the centre of the interface towards its edges.

The ridge could be critical to the sintering process because the vapour pressure above a surface is related to its shape. This means that ice tends to migrate – as water vapour – from sharper features to more rounded regions. According to Adams and colleagues, the abundance of these ridges should greatly increase the rate that mass redistributes itself in packed snow.

According to Adams and co-workers, the existence of the ridges suggests that the role of grain boundary diffusion has been greatly underestimated in ice, and throws into question the whole sintering process. “It is also reasonable to assume that this mass-transport mechanism exists in all crystalline materials”, they write.

A knowledge of the structure of snow is important for the mechanical removal of drifts from snow-bound communities, and for skiing conditions. Snow also plays an important role in the environment because it reflects light and heat from the Sun back into the Earth’s atmosphere.

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