Death may come in a wide variety of colours. The nemesis of Prince Prospero in Poe's surrealistic story, for example, came in crimson: "He had come like a thief in the night, and one by one dropped the revellers in the blood-bedewed halls of their revel. And Darkness and Decay and the Red Death held illimitable dominion over all."
In medieval times the leveller was the Black Death, the bubonic plague that decimated the populations of Europe in the 14th century. And more recently, while claiming its victims in much smaller numbers, the White Death has proved as difficult to conquer as any of its predecessors: its annual harvest in the 1990s has been four or five times that of 40 years ago.
The White Death is the name given to an avalanche in parts of France and Switzerland. There are 120 deaths annually in the region from this cause, and 200 worldwide every year, with the numbers rising steadily as winter sports become more popular. As we have seen in recent times, 1999 has proved to be no exception.
An avalanche is liable to occur when a slab of strong cohesive snow rests on top of a layer that is much softer and more crumbly. The former is composed of crystals that are "spiky", so that the individual flakes become locked together: smooth crystals without any jagged edges, on the other hand, produce weak "sugar snow" from which the surface layer may shear away to start the avalanche.
Once an accumulation has become unstable, it needs only a slight mechanical or meteorological trigger to send thousands of tons of snow cascading down the mountainside at staggering speeds. A loud noise, for example, can unleash a fall, or the passage of a single skier across the surface of the snow may sometimes be sufficient stimulus.
Meteorological factors may also be important. An increase in temperature, for example, as in the case of an approaching thaw, may result in a thin film of water around each granule of ice, which acts as a lubricator and facilitates movement. For the same reason, avalanches occur most readily on slopes that catch the sun.
Rain or snow may also be a trigger. Rain percolates into the snow beneath the surface, and weakens the bonds between the different layers. And both rain and extra snow increase the shearing force, by adding weight to an accumulation near the top; this is particularly dangerous on shallow slopes, because a large amount of fresh snow can be accommodated before the critical shearing force is reached - but then the amount of snow released is very great.