An avalanche occurs when snow accumulated on a slope loses cohesion and slides under gravity. Slab avalanches (the deadliest) start when a weak layer beneath a cohesive snow slab collapses. Powder avalanches form clouds advancing at 200+ km/h.
In the Pyrenees
The Spanish Pyrenees record dozens of significant avalanches each winter season. Risk is assessed using the European avalanche danger scale (1–5, from low to very high). Factors: slope angle (>28°), aspect, recent snowfall, wind (loading cornices), temperature (melting). AEMET and the Lauegi Centre issue daily danger bulletins.