Sliding Surfaces

Any hard or slick snow surface, such as an ice or rain crust, makes a good sliding surface for an avalanche.

Photograph of 2 people holding an ice crust cut out of a snowpack

There are three main types of crusts.

  1. Ice crusts form when relatively warm rain refreezes as it strikes a cold (below freezing) snow surface.
  2. Rain crusts form when warm rain falls on a relatively warm snow surface (at or just above freezing) and the liquid water penetrates into the upper portion of the snowpack. If temperatures drop below freezing, the water in the upper part of the pack will freeze into a very hard and well-anchored crust.
  3. Melt-freeze crusts form at or near the surface of a snowpack when temperatures get above freezing for a period of time and melt the surface snow. Once temperatures return to below freezing, the melted snow re-freezes into a firm crust. Thicker, stronger melt-freeze crusts develop when the process continues for several days. During springtime, solar radiation on a slope also accelerates the melting process during the day and forms a sun crust once temperatures cool below freezing overnight.

Problems arise once a good, firm sliding surface is buried by new snowfall. This is especially true when a weak layer forms on top of a good sliding surface prior to the next snowstorm.

Here’s another Glory Bowl question. Two crusts formed in October and November. What type were they? (Choose the best answer.)

The correct answer is A.

The periods of warmer temperatures in mid-October and early November briefly melted the snow surface. This was quickly followed by colder temperatures, which refroze it.