...The Avalanche Triangle: Snowpack...

If you look carefully at a cross-section, you will see layers within the snowpack that represent a history of the weather events that have produced or influenced the snow. Some of these layers are strong, some are weak. The bonds between layers can also be strong or weak. Slab avalanches occur when a weakness fails, allowing the overlying snow to slide downhill as a unit.

Strong and weak layers are caused by a number of different factors, including:

Snow crystals are not always feathery, six-sided and symmetrical like you used to draw them in Kindergarten. Snow will form other interesting shapes like hexagonal plates, columns, columns with plates at each end, bullets, needles, styrofoam pellet-like graupel and Frankenstein-ish "spatial dendrites"--classical stellar snow crystals with random arms growing out at bizarre angles from the smooth surface.

Diagram courtesy of Life-Link

The determining factors of snow crystal shape are the air temperature and saturation during formation. Crystals that are buffeted by wind will bump into super-cooled water droplets that freeze to their surface, forming rime.

The shape of the snow crystals within a snowpack layer will influence the strength of that layer: needles can pack close together and will form dense hard slabs under the influence of wind; graupel won't stick together at all but will roll downhill and collect in hollows and at the base of rock bands; stellars interlock arms with their neighbors, forming airy soft slabs; rimed crystals will tend to be sticky and pack more densely together.

Dry winter snow is the most complex substance found in nature. From the time that it forms to the time it melts away it is never the same. Snow's complexity can perhaps best be appreciated when one realizes that snow is the rare regime on earth that life has been unable to exploit. Boiling sulphur springs, deep lightless ocean clefts, inside solid rock--these and other hostile environments have provided habitat for specially adapted creatures, but until springtime brings isothermal conditions, nothing lives primarily in snow.

Since snow is constantly changing, the layers within the snowpack are either getting weaker or getting stronger. Terminology falls in and out of favor: the process of weakening is referred to as faceting, sugar snow, kinetic crystal growth, temperature gradient metamorphism, TG, or squaring.

      The process of strengthening is called sintering, necking, equitemperature metamorphosis, ET or rounding.

Weather affects the strength of the snowpack layers both at the surface and lower down. In fact, it can accurately be said that the snowpack is a physical record of the winter's past weather.