Typical Weak Layers

Document Actions

Typical Weak Layers

Faceted Crystals and Temperature Gradients
Depth hoar and surface hoar are types of snow grains snow scientists call faceted grains. These types of grains have flat sides and don’t bond together very well. The have little cohesion and are common causes of avalanches. Faceted snow grains form in any temperature gradient above 1 degree Celcius per Centimeter. This means that if the temperature changes 1 degree Celcius over 10 cm depth in the snowpack, faceted grains are capable of growing in this region. Under a hand lens, faceted snow grains have sharp edges and may look similar to little cubes.

Surface Hoar
During a clear, cold night the surface of the snowpack loses heat to the atmosphere and becomes very cold. When this happens moisture in the air can freeze to the top of the snow surface, growing feather like crystals called surface hoar. Surface hoar can be identified by its shimmery, glittering appearance on top of the snow. Once buried by a storm, these faceted crystals form an extremely weak unstable layer within the snowpack. The may persist in the snowpack from days to weeks, depending on the ambient air temperature. Colder conditions mean that weak layers will persist longer.

Crusts
Crusts form when the top few centimeters of snow begins to melt from rain or exposure to sun, then refreeze, creating a thin layer of hard, dense snow at the surface. Crusts often form a slippery surface that is hard for new snow to bond to. This makes for a good bed surface for avalanches. Also, crusts tend to have thin layers of faceted, weak, cohesionless snow above or below them which contribute to instability.

Graupel
Graupel is a type of round, hard, ball bearing like snow crystal that is heavily rimed. A snow crystal becomes rimed when super cooled water droplets in a cloud stick to the crystal as it falls from the sky. Hail is produced by the same phenomenon. Think of graupel like ball bearings made of ice. They tend to roll into shallow areas below cliffs or steep slopes, creating pockets of instability. Beware of avalanches starting in these areas after a storm that has precipitated graupel.

Depth Hoar
Depth hoar is often referred to as “sugar snow” because of it’s sugary texture. It grows in shallow snowpacks in layers near the ground in response to the temperature gradient that exists between the warm ground and cold air above the snow. Depth hoar is common in shallow continental snowpacks like those found in Colorado.

Depth hoar crystals look cube like under a hand lens. They also have a very granular, sugary texture. The picture at left is of depth hoar under a microscope. Faceted grains of depth hoar tend to be larger than other grains of snow.

Picture of a well developed rain crust. Notice the shiny, slick surface. This layer may function as a bed surface for avalanches once buried.

Logan Avalanche Forecast Center

Density Inversions
Remember that slab avalanches can occur whenever there is a layer of stronger (more cohesive) over weaker snow. Sometimes, a slab avalanche can occur within the new snow due to subtle differences in density. This typically occurs when a warm front drops cold, light, less dense snow, then drops higher density, warmer snow on top. Luckily, most winter storms are associated with cold fronts, so this situation doesn’t happen that often.

Surface hoar and other types of surface facets can be distinguished by its shiny, sparkly appearance on the snow surface. Up close, surface hoar crystals look delicate and feathery.

www.avalanche.org Doug Wewer

Logan Avalanche Forecast Center

Beltsville Agricultural Research Center Electron Microscopy Unit Snow Page

avalanche.org

from Avalanche Atlas unesco 1981

Back to Course Schedule

Copyright 2008, by the Contributing Authors. Cite/attribute Resource. factcouraud. (2007, May 08). Typical Weak Layers. Retrieved November 23, 2009, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/avalanche-and-snow-dynamics/AvClass/SnowpackandStabilityTests/Typical%20Weak%20Layers.html. This work is licensed under a Creative Commons License.

Course Contents Avalanche and Snow Dynamics Home Avalanche Basics Decision Making Rescue Start Here Snow Pack and Stability Terrain Weather Why Avalanches are Important	Personal tools Log in You are here: Home → Wildland Resources → Avalanche and Snow Dynamics → AvClass → SnowpackandStabilityTests → Typical Weak Layers
	Info Typical Weak Layers Document Actions Typical Weak Layers Depth hoar crystals look cube like under a hand lens. They also have a very granular, sugary texture. The picture at left is of depth hoar under a microscope. Faceted grains of depth hoar tend to be larger than other grains of snow. Picture of a well developed rain crust. Notice the shiny, slick surface. This layer may function as a bed surface for avalanches once buried. Logan Avalanche Forecast Center Surface hoar and other types of surface facets can be distinguished by its shiny, sparkly appearance on the snow surface. Up close, surface hoar crystals look delicate and feathery. www.avalanche.org Doug Wewer Logan Avalanche Forecast Center Beltsville Agricultural Research Center Electron Microscopy Unit Snow Page avalanche.org from Avalanche Atlas unesco 1981 Back to Course Schedule Next Page Copyright 2008, by the Contributing Authors. Cite/attribute Resource. factcouraud. (2007, May 08). Typical Weak Layers. Retrieved November 23, 2009, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/avalanche-and-snow-dynamics/AvClass/SnowpackandStabilityTests/Typical%20Weak%20Layers.html. This work is licensed under a Creative Commons License.	Reuse Course Download this course

Sections

Personal tools

Typical Weak Layers

Document Actions