Persistent and non-persistent weak Layers American Institute for Avalanche Research & Education Level II Avalanche Course Persistent and non-persistent weak Layers Overview Introduces the concept of persistent weak layers, common examples, and illustrates potential bonding problems.   Learning Outcomes Know the difference between persistent and non-persistent weak layers Be aware of some potential bonding problems associated with persistent weak layers.

Non-persistent weak layers American Institute for Avalanche Research & Education Level II Avalanche Course Weak Layers For a slab avalanche to occur, a weak (failure) layer that prevents the slab from bonding to the bed surface is required. It is in this layer that shear (and perhaps compression) failure occur and propagate. Types of Weak Layers Non-persistent weak layers Persistent weak layers

Non-persistent weak Layers American Institute for Avalanche Research & Education Level II Avalanche Course Non-persistent weak Layers Non-persistent weak layers are layers that strengthen quickly and bond to the slab and bed surface readily. Non-persistent weak layers commonly consist of new snow grains, decomposing and fragmented grains, rounds, and perhaps wet grains. Although a wet layer may persist if temperatures are very warm and the grains do not freeze.

American Institute for Avalanche Research & Education Level II Avalanche Course Non-persistent weak Layers New snow grains (rounds or wet) metamorphose readily into stronger forms which bond well to each other and surrounding layers. Generally speaking, non-persistent weak layers will show observable strength gain and improvement in bonding in a matter of hours or days.

American Institute for Avalanche Research & Education Level II Avalanche Course Persistent weak Layers Persistent weak layers are ones which strengthen slowly and do not bond readily to the bed surface or slab. Persistent weak layers commonly consist of graupel, faceted grains, depth hoar, and surface hoar.

American Institute for Avalanche Research & Education Level II Avalanche Course Persistent weak Layers Faceted grains do not metamorphose readily into stronger forms and do not bond well to each other or other layers. Persistent weak layers may get weaker over time and even when strengthening, persistent weak layers take many days, weeks, and sometimes months to show significant increases in strength or improvement in bonding.

American Institute for Avalanche Research & Education Level II Avalanche Course Persistent and Non-persistent weak Layers Non-persistent weak layers are often associated with maritime climates and warms storms that have high precipitation intensities and significant accumulations of precipitation in a short period of time. Persistent weak layers are often associated with continental climates and extended periods of cold, dry weather where there are few precipitation events and accumulations are light.

American Institute for Avalanche Research & Education Level II Avalanche Course Weak Layer Characteristics Thick or thin Variable in density Variable in hardness Easy to see or hard to see Various grain types or combinations of different grains Varying in quality of bond to the slab and bed surface Regardless of whether weak layers are non-persistent or persistent, their characteristics are diverse .

Non-persistent weak layers It is in the weak layer that shear (perhaps compression) failure occur and propagate. If a non-persistent weak layer is related to new or recent snow (new snow or DF grains) - - note the depth of the fracture lines and watch the trend of avalanche activity. Weak Layer Characteristics Thick or thin Variable in density Variable in hardness Easy to see or hard to see Various grain types or combinations of different grains Varying in quality of bond to the slab and bed surface

Non-persistent weak layers If avalanches are running in the layer of storm snow that was deposited and activity tapers off in the first 24 – 36 hours after the storm ends, instability is almost certainly related to a non-persistent weak layer. Stability will likely improve dramatically by the time 48 hours has elapsed. Weak Layer Characteristics Thick or thin Variable in density Variable in hardness Easy to see or hard to see Various grain types or combinations of different grains Varying in quality of bond to the slab and bed surface

American Institute for Avalanche Research & Education Level II Avalanche Course If a non-persistent weak layer is related to wet grains, watch and measure air and snow temperatures. As temperatures fall below freezing, the problem will likely resolve itself quickly as the wet grains freeze. Weak Layer Characteristics Thick or thin Variable in density Variable in hardness Easy to see or hard to see Various grain types or combinations of different grains Varying in quality of bond to the slab and bed surface

Weak Layers Persistent weak layers, on the other hand, are much harder to assess and forecast. Avalanches associated with these layers may occur sporadically when the layer first forms. strength fluctuations and persistent weak layers often go into extended dormant periods before becoming sensitive to triggering again.

Weak Layers Sometimes no avalanches occur until some time after the layer has formed and some combination of seemingly minor events triggers failure. Persistent weak layers are difficult to assess as they may go through cycles where strength decreases, increases, then decreases again.

American Institute for Avalanche Research & Education Level II Avalanche Course Persistent weak layers require ongoing, long-term monitoring using a variety of observation, testing, and recording methods to ensure one does not lose track of their location and characteristics.

Factors Affecting Bonds Grain Form Grain Size Riming Density Hardness American Institute for Avalanche Research & Education Level II Avalanche Course Here’s a few things to look for when trying to assess how serious a problem the bond between a weak layer and the slab or bed surface might be. Factors Affecting Bonds Grain Form Grain Size Riming Density Hardness Differences

American Institute for Avalanche Research & Education Level II Avalanche Course Factors Affecting Bonds Grain Form: Grains with faceted forms and features often bond poorly. This includes facets, depth hoar, surface hoar, new snow plates, etc. Grain Size: In many cases, larger grains bond more poorly than smaller ones. Riming: Often, grains that are heavily rimed bond more poorly than those which are not rimed.

Density: Generally, lower density layers have poorer bonds. American Institute for Avalanche Research & Education Level II Avalanche Course Factors Affecting Bonds Density: Generally, lower density layers have poorer bonds. Hardness: Often, softer layers have poorer bonds than harder ones. Difference in Characteristics: The greater the difference in characteristics between a weak layer and the layers above and below, the poorer the bond generally is.

Factors Affecting Bonds of Surface Hoar Shape Coverage Bed surface American Institute for Avalanche Research & Education Level II Avalanche Course Factors Affecting Bonds of Surface Hoar Shape Coverage Bed surface Next snowfall

Factors Affecting Bonds of Surface Hoar American Institute for Avalanche Research & Education Level II Avalanche Course Factors Affecting Bonds of Surface Hoar Surface hoar grains tend to bond poorly due to: 1) Shape: V-shaped feathery or cup-shaped grains tend to bond more poorly than smaller and needle shaped grains. 2) Coverage: layers that create an “umbrella” effect (isolated pore spaces between and underneath the grains) often bond poorly to the slab above.

Factors Affecting Bonds of Surface Hoar American Institute for Avalanche Research & Education Level II Avalanche Course Factors Affecting Bonds of Surface Hoar Surface hoar grains tend to bond poorly due to: 3) Bed surface: surface hoar that forms on a firm bed surface (e.g. a hard crust) will have a smaller point of contact and will bond more poorly to the bed surface. 4) Next snowfall: Large new snow crystals, especially when they fall as snowflakes, are less prone to filling in pore spaces between surface hoar crystals and are more likely to bond poorly to the slab above.

Factors Affecting Bonds of Wet Grains Timing Grain Form Grain Size American Institute for Avalanche Research & Education Level II Avalanche Course Factors Affecting Bonds of Wet Grains Timing Grain Form Grain Size Moisture Content

Factors Affecting Bonds of Wet Grains 1) Timing: wet grains are often weakest from mid-afternoon to early evening when the most heat is entering and held in the snowpack. They are often strongest in the early morning before sunrise when the snowpack has lost most of the heat it gained the previous day. 2) Grain form: the more “icy” looking and spherical, (the more well developed the wet grain is from melt-freeze metamorphism) the weaker the bond is likely to be.

4) Moisture content: the more free water Factors Affecting Bonds of Wet Grains 3) Grain size: Larger grains tend to bond more poorly than smaller ones. 4) Moisture content: the more free water

American Institute for Avalanche Research & Education Level II Avalanche Course Factors Affecting Persistence Cold Snow Temperatures Strong Temperature Gradients Shallow Snowpacks Incline and Aspect Early Season

1) Cold Snow Temperatures: inhibit settling and bonding. Factors Affecting Persistence With the exception of wet grains, weak layers tend to persist longer due to: 1) Cold Snow Temperatures: inhibit settling and bonding. 2) Strong Temperature Gradients: promote faceting. 3) Shallow Snowpacks: provide less overlying weight and pressure which means less settlement. Promote stronger temperature gradients.

Factors Affecting Persistence With the exception of wet grains, weak layers tend to persist longer due to: 4) Incline and Aspect: grains on steep, shaded terrain tend to persist longer because snow temperatures tend to be colder and temperature gradients stronger. 5) Early Season: shallower snowpack and colder temperatures = stronger temperature gradients.

Factors Affecting Persistence of Wet Grains Aspect Incline Season American Institute for Avalanche Research & Education Level II Avalanche Course Factors Affecting Persistence of Wet Grains Aspect Incline Season Air Temperatures

Factors Affecting Persistence of Wet Grains Aspect: wet grains will probably persist longer on sunny aspects where air and snow temps are warmer due to solar radiation. Incline: wet grains will likely persist longer on steep terrain (sunward slopes) where solar radiation is enhanced. On shaded slopes, wet grains will persist longer on flatter terrain.

Warm Air Temperatures: inhibit freezing. Factors Affecting Persistence of Wet Grains Season: wet grains are more likely to be more persistent later in the season (late winter, spring, early summer) when above-freezing temperatures are more common. Warm Air Temperatures: inhibit freezing. Depth of Burial: sometimes wet grains that are deeply buried take longer to freeze because they are insulated from cold air temps.

American Institute for Avalanche Research & Education Level II Avalanche Course These discussion covers some of the common scenarios and potentials. There are far too many variables and combinations of variables to even begin discussing all the possibilities. The idea is to think and ask yourselves “what if…”.   We will continue our discussion in the field and in stability analysis when it is applicable and relevant.