Why Thin Snow Is Weak Snow

Compared with last winter's snowpack, we're dealing with a far more insidious beast this year.  We have already had one avalanche fatality and natural and human triggered avalanches are still being reported in the backcountry.  Brett Kobernik summarized the situation quite well in today's avalanche advisory (my emphasis added below).

Our main concern remains triggering an avalanche that breaks into old faceted snow. As I see it, we are in a holding pattern right now where I have not seen any significant improvement with the weak layer over the last seven days.

Faceted snow is a type of snow characterized by angular crystals.  Facets can be small, but when they reach large size with steps and striations on their surfaces, they are called depth hoar (see below).

Source: zacstracks.com

Faceted snow is weak, lacks cohesion, and forms where there is a large temperature gradient in the snowpack.  A temperature gradient is a measure of how quickly the temperature changes with distance.  For faceted crystals to form, the temperature gradient needs to be at least 10ºC per meter.  Or, equivalently, at least 1ºC per 10 cm.

The temperature gradient is important because it plays a role in the movement of water molecules in the snowpack.  Water molecules in a snowpack are not locked in the ice phase.  They can sublimate (i.e., change from ice to vapor phase) or condense (i.e., change from vapor to ice phase).  Water vapor can also move through pore spaces in the snowpack.  What happens in a snowpack with a large temperature gradient is that there is a net movement of water molecules from the warmer snow crystals to the colder snow crystals.  The warmer ice crystals lose mass to net sublimation while the colder ice crystals grow through net condensation.  In addition, because of the close proximity of the warm snow crystals, the cold snow crystals grow in a water-vapor-rich environment, which, through the wonders of ice physics, leads to faceted crystals.

Snow crystals in snowpacks with a weak temperature gradient also experience sublimation and condensation.  However, the net transfer of water molecules between snow crystals is much slower, which, through the wonders of ice physics, leads to rounded rather than faceted snow crystals.  These rounded crystals tend to be strong.

In a snowpack, the temperature near the ground is usually near freezing (0ºC), so the temperature gradient is strongly determined by the temperature at the top of the snowpack.  If the snowpack is deep, the temperature gradient in the snowpack will generally not be very large even in cold weather because the difference in temperature between the bottom and top of the snowpack is spread over a large distance (an important exception is right near the snow surface where a strong, localized temperature gradient can develop in cold, clear conditions even in the deepest snowpacks).  However, in a thin snowpack, the temperature gradient can be quite large, especially in cold weather.

What happened in the Wasatch this year is our mid-October snow sat for a few weeks on high-altitude northerly aspects, was fairly thin in depth, and developed faceted snow crystals due to the presence of a large temperature gradient.  Mother Nature has now dropped a load on that weak snow, and we are dealing with a cranky snowpack.  The weak layer is not strengthening because the snowpack remains thin and the temperature gradient is too large for the facets slowly transform into stronger rounded grains.

If we can get several storms to make the snowpack deeper and reduce the temperature gradient, then the snowpack will likely strengthen gradually.  I emphasize gradually because this process doesn't happen over night.  In addition, there will be some places where the snowpack remains thin and may continue to be weak.  I call this a Russian Roulette snowpack because the snowpack can become strong in some areas, but remain weak and "loaded" in others.

Important Clarification: The Wasatch Weather Weenies is an educational blog, not an avalanche advisory service.  We like to take advantage of what is happening in our backyard and mountains, but for professional, up-to-date avalanche advisories for the Wasatch backcountry, please consult the Utah Avalanche Center web site.