What conditions promote PWL formation?

Persistent weak layers are often associated with mountain climates where extended periods of cold, dry weather are common, and where there are fewer precipitation events and accumulations are moderate or light.

Facets occur during clear, and cold nights. As the snowpack loses warmth in the form of radiation, the temperature gradient becomes drastic near the surface of the snow. This increases the transfer of water vapor and the growth of angular crystals.

Surface hoar forms similarly to facets but is actually an atmospheric deposit, not a metamorphic process within the snowpack. Surface hoar forms on cold, clear nights with a calm wind (less than 3mph). The wind can recirculate just enough moisture to keep the feathers/ crystals growing as the moisture condenses upon the snowpack. Surface hoar is winter’s equivalent to dew. It forms on the surface of the snow due to an extreme temperature gradient. Relatively warm, moist air masses flow above the surface of the snow and are forced to deposit moisture onto the cooling snowpack (cooling from radiational loss to the clear sky). Think about taking a hot shower in a cold bathroom without a vent. The warm, moist air will condense upon the cold mirror. This is basically what is happening at the surface of the snow.

Depth hoar occurs near the base of the snowpack and is synonymous with basal faceting. Depth hoar is generally large in size and has striations from growth. The grains typically form a cup shape and can also growth a small tail in their most advanced stages. Depth hoar occurs from a large temperature gradient within an early season snowpack. Shallow snow packs are not insulated from cold atmospheric temperatures thus the snow has a constant vapor exchange from the warm ( o degree Celsius) earth below. The vapor exchange can drive massive growth within the grains forming depth hoar that can sometimes never morph back to rounds.

What conditions promote persistence?

Large temperature gradients within the snowpack, long stretches of cold and clear nights, shallow snowpacks, and large diurnal temperature swings during winter time are all drivers of persistence. It is important to note these atmospheric conditions and track them as the season progresses.

Where might I commonly find PWLs in the snowpack?

Persistent grain types forming above or below stiff layers and crusts can be durable and last for weeks, months, or even throughout the season in a mountain snowpack. PWLs can be found everywhere in the snowpack. Depth hoar is generally associated with the base of the snowpack. Facets can form in the mid pack, specifically under crusts that cause an impermeable layer above and below them. Surface hoar is found at the surface of the snow, and then can become buried. Surface hoar can remain problematic for weeks after it is buried if it remains upright. Think of dominoes standing up as you lay a heavy mattress (slab) above them. They can only hold for so long before they lose compressive strength and fail.

Treatment

Persistent weak layers are very hard to assess and forecast. Avalanches associated with these layers may occur sporadically when the layer first forms. Sometimes no avalanches occur until sometime 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, and then decreases again.

In conjunction with these strength fluctuations, persistent weak layers often go into extended dormant periods before becoming sensitive to triggering. 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 locations and characteristics (observation and testing methods are discussed later when we talk about observing and recording instability factors). Typically, persistent weak layers also call for a more conservative approach to terrain selection and hazard forecasting.