Persistent Weak Layers

Photo: Ben Pritchett

How do I keep track of persistent weak layers?

Persistent weak layers (PWL’s) are of special concern to avalanche forecasters. Weak layers that tend to resist strengthening can cause multiple cycles of elevated avalanche hazard throughout a season. PWL’s are made up of the large, angular, flat-sided grains: facets, depth hoar, and surface hoar. As a slab builds over a PWL, potential avalanches get increasingly large. PWL’s are anisotropic snow grain types, which means that they behave differently in different directions. These snow grains are typically strong in compression, capable of holding up a lot of slab weight, but they are weak in shear. Self-propagating shear fracture is the primary mechanism by which avalanches fail downslope. 

It is difficult to predict how different PWL’s will react to new loads across terrain. Risk managers should treat these layers with respect and caution, tracking them, testing them, and assessing them throughout a season. It is common that PWL’s that become dormant by mid-season, start causing avalanches again in the spring as the snowpack thins and weakens. Sometimes they remain an issue until the snow melts away.

What conditions promote PWL formation?

Facets form in places and at times where there are high temperature gradients in the snowpack. Any snowy range on Earth has faceting occur at times. High temperature gradients often exist in locations with cold air temperatures, under clear skies, or in shallow snowpacks (in the early season or in locations where little snow accumulates). Facets form where there are frequent, large diurnal snow temperature swings. Facets grow faster in warmer snow temperatures than colder ones, as all thermodynamic processes occur more effectively at warmer temperatures, and pore spaces can hold more water vapor in them at higher snow temperatures. Lastly, facets are commonly found above or below crusts, due to reasons such as: conditions as the crust formed, increased pore space size and/or available water vapor at crust interfaces, and micro temperature gradients due to physical properties of the crust compared to adjacent snow layers. 

Depth Hoar is often found at the bottom of the snowpack in the early season snow. Early season snow is generally low density, with relatively large pore spaces. It falls at the coldest time of year and forms a shallow early-season snowpack. All of this leads to a high temperature gradient and conditions that promote facet growth. With a long enough duration in these conditions, the facets advance in their growth to become considered depth hoar. Depth hoar also forms in cavities and hollow spots on the mountain slopes, like around rocks, bushes, stumps, or creek beds where snow does not pack in densely when it falls. Depth Hoar can be so persistent that it is known to avalanche and reload to cause multiple avalanche cycles on the same layer, with a different slab, in the same path. The large size and angular shape of depth hoar can be very slow to round. Avalanches on depth hoar typically fail on or near the ground.

Surface Hoar forms best in cool climates, on clear evenings. Clear nights help cool the snowpack surface through radiative cooling. High relative humidity in the air helps make more water vapor available for deposition. This means the climate can’t be too cold or too dry. Surface hoar forms especially in areas near an open water source such as lakes and streams. Sheltered drainages and clearings can pool cold air and concentrate humidity. Winds mix the air and don’t allow time for it to cool and deposit moisture onto the snow surface. On the other hand, a very gentle breeze can help replenish air moisture and increase surface hoar growth.

Near surface facets. Adapted from The International Classification for Seasonal Snow on the Ground. IHP-VII Technical Documents in Hydrology N°83, IACS Contribution N°1, UNESCO-IHP, Paris. By: Fierz, C., Armstrong, R.L., Durand, Y., Etchevers, P., Greene, E., McClung, D.M.,Nishimura, K., Satyawali, P.K. and Sokratov, S.A. 2009.

Depth Hoar. Adapted from The International Classification for Seasonal Snow on the Ground. IHP-VII Technical Documents in Hydrology N°83, IACS Contribution N°1, UNESCO-IHP, Paris. By: Fierz, C., Armstrong, R.L., Durand, Y., Etchevers, P., Greene, E., McClung, D.M.,Nishimura, K., Satyawali, P.K. and Sokratov, S.A. 2009.

Buried Suface Hoar. Adapted from Textrue and Strength changes of buried surface hoar layers with implications for dry snow-slab avalanche release. By:Jamieson and Schweizer 2000

Ice Formation.  Growth of facets on either side of an established crust can occur. Adapted from The International Classification for Seasonal Snow on the Ground. IHP-VII Technical Documents in Hydrology N°83, IACS Contribution N°1, UNESCO-IHP, Paris. By: Fierz, C., Armstrong, R.L., Durand, Y., Etchevers, P., Greene, E., McClung, D.M.,Nishimura, K., Satyawali, P.K. and Sokratov, S.A. 2009.

Implications of PWL’s for the Observer

These PWL’s, may not become a critical factor until they are buried by an overlying slab. PWL’s that get too much wind or sun before getting buried may not be an issue. Surface hoar is a particularly persistent and weak grain type with low fracture toughness, but it is so delicate that it can decompose or be destroyed before it gets buried. If it lays down or gets pushed into softer snow beneath it may make it stronger. Depth hoar without an overlying slab, may just be deep, weak snow over ground cover. PWL’s are given names on the day they are buried (e.g. the January 8th surface hoar layer). This is how observers and forecasters track and discuss them throughout the season. Once a layer is buried and shows evidence that it can cause avalanches, it can become a topic of conversation for a long while.

PWL’s are the leading cause of avalanches that produce fatalities, both by recreational travelers and professionals. PWL’s can produce large and destructive avalanches that are difficult to escape or survive because of their ability to propagate fractures long distances, even into places where the layer is very deeply buried. When PWL’s are the problem, expect to encounter uncertainty and risk. Learn to manage it with team communications and careful observations over an extended time period. Conservative terrain use is the best decision-making approach in these times.