May 21, 1999
Two French physicists have discovered why two distinct types of avalanche are seen on snow-covered mountains. Adrian Daerr and Stephane Douady of the Ecole Normale Supérieure in Paris observed the behaviour of glass beads on a felt slope to study avalanches. They found that disturbing a thin layer of beads causes an 'avalanche' to propagate down hill with a triangular front pattern. When a thick layer of beads was disturbed, however, the front moved both up and down the slope (Nature 399 241). Both types of avalanche have been clearly seen in real life.
Snow avalanches occur when the mass of the snow overcomes the frictional resistance of the slope. This usually takes place when snow layers near the slope are loosened by spring rains or vibrations. Daerr and Douady coated an inclined felt surface with layers of the beads, making sure that the beads remained static on the felt. (The friction between the beads and the felt is larger than the friction between the beads, making this an ideal system for modelling avalanches.) Adding more beads, or jolting a layer of them, caused the beads to roll down the slope and freeze back into a layer of glass beads. Again this was similar to the observed behaviour of snow.
Daerr and Douady repeated the experiment at a number of different inclinations and with different materials - such as crushed walnut shells and Sinai sand - and confirmed their initial hypothesis about the two different types of avalanche.