Once snow falls to the ground it begins to change in a process called metamorphism (a term borrowed from geology) where pressure and temperature, acting over time, changes the form of the crystals and the texture of the sedementary layer of snow.

Metamorphism occurs rapidly in snow and changes the structural strength, permeability, thermal conductivity and density of the snow cover. The crystalline solid is always close to the melting temperature and tends to round the grains.

The snowpit is a preferred method for visual hands-on analysis of the metamorphic changes in the snow cover. In an educational context, the students use field protocols to collect, analyze and interpet the data from the snowpit. Where snow is on the ground throughout the winter, the students address the history of winter as evidenced by the snowpack. Since the snowpack metamorphism is strongly dependent on the meteorological conditions - year to year studies are valid and extremely useful.

The rounding process builds rounded grains (rounds) which bond well to one another creating a snowpack (or layer) that is generally increasingly strong.

In weak temperature gradients(<1 degree / 10cm) sublimation typically moves ice from convex surfaces (points) to concave surfaces (hollow in 2 stages:

1.In the initial stage of rounding, the sharp ends of new crystals and the points of faceted grains sublimate and the resulting water vapour is deposited in concave areas. At high temperatures,molecules also glide along the grain surface from convexities to concavities. As well, large grains with broad curvatures grow at the expense of small grains with sharp curvatures. The result is a concentration of mass with a minimum surface area.

2.Under weak temperature gradients, water vapour moves from warm areas to cold, but the rate of movement is much slower than in strong temperature gradient environments. Slow moving vapour is deposited on the colder surfaces in a more homogenous manner and the faceted, stepped pattern associated with a strong temperature gradient does not occur.

The following conditions promote rounding:

• A weak temperature gradient generally less than 1 degree C per 10 cm (which moves water vapour slowly from warm
areas to cold)

• Dense, tightly packed snow

• Small grains (which produce denser snow)

• A high snow temperature, typically above -10 degrees C (which promotes weaker temperature gradients)

Text from "Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association

The faceting process builds angular grains (facets) which bond relatively poorly to one another and other grains creating a snowpack (or layer) that is generally increasingly weak. When the temperature gradient is strong (> 1 degree / 10cm) water vapour moves rapidly from warm grain surfaces to colder surfaces. Because the snowpack usually is warm (at or near 0 degrees C) at the ground and colder at the surface, ice sublimates from lower,warmer grains and is deposited onto colder grains higher up in the snowpack. These colder grains first develop sharp corners, then stepped facets.

If the faceting process continues, large, six - sided hollow or filled cup shaped grains called depth hoar are formed. Depth Hoar is common in Rocky Mountain climates, around large rocks and high shrubs, and where the snowpack is thin.

The following conditions promote faceting:

A strong temperature gradient, generally greater than 1 degree / 10cm (which quickly drives water vapour from warm areas to cold)

Loose, low density snow (which facilitates the free movement of water vapour between grains)

Presence of crusts (which concentrate water vapour, promoting vapour transfer in the concentrated area)

Moderate snow temperature (which maximizes the amount of vapour the snowpack can hold but does not reduce the overall temperature gradient significantly)

Text from "Advanced Avalanche Safety Course Manual" Copyright © 1998 Canadian Avalanche Association