Lecture 13: Snow avalanche

Question 1

snow pack

Answer 1

• composed of several layers with different characteristics
• thickness, density, hardness, cohesion, strength
• formed under different weather conditions
• properties can change over time

Question 1

cohesion

Answer 1

• property of the particles of a material to stick together
• low-cohesion snow: fresh powder snow, 95% volume is void space
• cohesive snow: wet snow, liquid water between particles of ice
• compacted snow: snow particles densely packed and tightly bounded

Question 2

snow crystals

Answer 2

• new snow characterized by 6 sided crystals (diameter 0.5mm)
• aging crystals: more round; more densely packed; more cohesive

Question 3

weak layers

Answer 3

• within the snowpack, weak layers are sandwiched between stronger layers
• weak layers form: when wind is increasing during a storm; when hoar crystals develop
• denser layer of broken crystals on top of less dense layer of unbroken crystals, deposited under increasing wind conditions

Question 4

Hoar

Answer 4

• relatively large crystals formed by the deposition of water vapor - when hoar crystals grow, the density of the snowpack decreases
• depth hoar: vapor from underlying soil deposits on snow crystals at the base of the snowpack; density of the lower layer of the snowpack decreases
• surface hoar: surface hoar forms on cold clear nights; vapour from the atmosphere deposits on the surface of the snowpack

Question 5

three main parts of snow avalanches

Answer 5

• start zone: most volatile part of the slope where unstable snow begins to slide
• track: path followed by the avalanche moving downhill
• runout zone: zone where snow and debris stop and are deposited

Question 6

point release avalanche

Answer 6

• most often develop in low cohesion snow ***
• inverted v shape

Question 7

slab avalanche

Answer 7

• involves several layers of cohesive snow ***
• avalanche starts when fracture spreads to a weak layer at the base
• more dangerous
• larger and greater force

Question 8

slope angle of the start zone

Answer 8

• > 50 degrees - frequent point-release avalanches
• 25-50 degrees - infrequent, large dry slab avalanches
• 25 degrees - slab avalanches only

Question 9

cornice

Answer 9

snow structure formed by wind loading snow on the downwind side of an obstacle, often a mountain ridge line

Question 10

what triggers avalanches

Answer 10

• weather conditions: common after snow storms,
• human activities: recreational activities and purposful