Sean Zimmerman-Wall just wrote this piece for Powder Magazine.  Sean is a pro ski patroller at Snowbird, an avalanche educator, and a mountain guide.

This piece is about faceted snow which is the cause of many avalanches especially in the American Mountain West.

Backcountry Essentials: Faceted Snow

by Sean Zimmerman/Powder Magazine

“Early-season snowfall and a thin snowpack are some of our biggest enemies in the avalanche game. That’s because these conditions are prime for faceting, which creates persistent weak layers. So far this season, with October promises of an epic season still unfulfilled by January in many places, a faceted weak layer has been the culprit of a majority of the fatalities in North America and Europe.

The process of faceting is quite simple to understand and it happens in seemingly benign conditions. During cold, clear, and calm spells, the snow undergoes a process known as metamorphism. Those beautiful stellar snowflakes that fell from the sky immediately start to change the moment they hit the ground.

For faceting to occur, a temperature gradient must exist, or there must be a difference in temperature between the ground and the air. An early season snowpack is often thinner and therefore the temperature gradient is steeper. High vapor pressure moves from the ground up through the snowpack to the surface where a lower vapor pressure exists. That process weakens the snow crystals. It can take a few days or it can occur over the course of a week or more. In places like Colorado where the snowpack is usually thin and the temperatures are quite cold, this weak layer is a problem throughout the year.” – Sean Zimmerman/Powder Magazine