Avalanche occurs when upper levels of snow can't be supported by the lower levels.
When the layers in the snowpack weaken and the upper layer loses support, the snow may collapse and begin to slide.
Temperature differences within the snowpack can cause water vapours to travel upward, forming crystals.
The crystals weaken snow layers, causing the upper layers to slide downhill. The crystals reduce friction, leading the snow to slide as fast as 90 km/h.
The avalanche takes place.