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avalanche

Avalanches on 8,000-metre peaks fall into two broad categories, and the distinction matters enormously for anyone assessing risk. Snow avalanches result from an unstable layered snowpack and can, to a degree, be forecast. Ice avalanches result from serac collapse — the calving of the overhanging ice cliffs that form where a glacier flows over a break in slope — and are effectively unforecastable. A serac fails on the glacier's schedule, not the weather's.

• Wind slab: high-altitude wind strips snow from exposed ridges and deposits dense slabs on lee slopes, often over weaker underlying snow.
• Depth hoar: large temperature gradients through a shallow snowpack grow angular, poorly bonded crystals near the base that can persist for weeks.
• Warm-layer weakness: a melt-freeze crust from an unusually warm spell creates a smooth sliding surface for everything deposited above it.
• Loading rate: it is the speed of new snow accumulation, more than the total, that determines whether a slope adjusts or fails.