Take the 2-minute tour ×
The Great Outdoors Stack Exchange is a question and answer site for people who love outdoor activities, excursions, and outdoorsmanship. It's 100% free, no registration required.

Avalanches are extremely dangerous when mountaineering, winter backpacking/camping, or backcountry skiing/snowboarding.

What causes them and is there any way I can predict when an avalanche is about to occur?

share|improve this question
add comment

1 Answer

up vote 11 down vote accepted

This is an extremely deep topic which has entire forum sites dedicated to it. However I will attempt to summarize the key points for the average enthusiast.

First - Don't ever travel into uncontrolled avalanche terrain if you have not received proper training from an expert in avalanche conditions, triggers, mitigation, and rescue. It's stupid, seriously you can die, and many people each year do.

Second - If you receive training the first thing that you will be taught is to Always wear a transceiver, and know how to use it. It is very possible that your life or that of someone you care about will be saved by a transceiver. Your better off forgetting your ski's or snowboard than your transceiver.

Third - Always carry a shovel, avalanche probe pole, and extra clothing when in avalanche terrain.

Now that's out of the way

Avalanche Overview

Information is summarized from the Canadian Avalanche Center

Snowpack Layering

  • The snowpack is made of many layers of snow which are formed by climate factors from above the snow, and within the snow.
  • These layers create a stratification of the snow crystals where some adhere more to one another than others.
  • This creates a situation where fractures can occur, and some layers of the snow shift as a unit.
  • The snow "unit" causes more snow to fracture in similar ways.
  • This sometimes creates a chain reaction we call an "Avalanche".

Types of Avalanches

  1. Loose Dry
    • Usually limited to surface layers, and pitches at or above 40 degrees
    • Sometimes called slough, although they can still be very dangerous.
  2. Loose Wet
    • Typically form in spring or summer, due to solar radiation (direct sun).
    • Usually require slopes of 30 - 40 degrees to initiate.
  3. Wet Slab
    • Generally created by extended periods above freezing.
    • Pinwheeling, snowballing, and loose wet avalanches are predictors
  4. Storm snow
    • Caused by a cohesive layer of new snow overloading the sub-surface snow layer bond.
    • Generally avalanches peak during intense snowfall and decrease 24-36 hours following a storm cycle.
  5. Wind Slab
    • Caused by wind deposited snow building up and overloading the sub-surface layer's bond.
    • Indicators include "ribbing" and "cornicing" on leeward slopes.
  6. Persistent Slab
    • Caused by a weak bond within the snowpack created weeks, and sometimes months prior to the trigger.
    • Prediction requires digging a snow test pit, reviewing weather logs over weeks and months, and speaking to local experts.
    • Very dangerous avalanches result from "persistent slab" conditions.
  7. Persistent Deep Slab
    • Caused by a thick, hard, and deep slab of snow loosing it's bond with sub-surface layers.
    • Most commonly caused by early season snowpack being exposed to long periods dry weather.
    • Most common above tree line.
    • Prediction requires digging a snow test pit, reviewing weather logs over weeks and months, and speaking to local experts.
  8. Cornice Falls
    • Avalanches caused by large quantities of wind blown overhanging snow fracturing and falling onto a slope.
    • Most common on leeward slopes where a ridgeline breaks the wind.

Signs of Unstable Snow

  1. Whoompfing - A sound the snow makes (just say the word) when the sub-layer fractures from the surface layer.
    • Sometimes creates shooting surface cracks.
    • Check your underwear in steep terrain, it's scary.
  2. Recent Avalanche Activity - Observe the nearby slopes and ridges for activity of sloughing , wet slides, fracturing, or cornacing.
    • Be aware of the environment around you. Always place your head on a swivel.
  3. Hollow sounds - While traveling across the snow surface.
    • Stay off steep ridgelines, look for terrain elsewhere.
  4. Difficult travel due to breaking through snow layers.
    • If the snow cannot support you while skinning (traveling to and from steep slopes using grippy ski bottoms), it is not stable.
    • Consider lower angles or not skiing that area.

Most Common Triggers

  1. Environmental

    • Loading from new snow.
    • Temperature or humidity Changes.
  2. Human

    • Ski and snowboard travel
    • Snowmobiling
    • Explosives

Reducing Avalanche Risk

  1. Do not stop or group in exposed areas
  2. Do not travel directly above someone in a party.
  3. Do not underestimate the danger of a slab avalanche, or how easy they can be triggered.
  4. Recognize terrain traps.
  5. Do Not travel alone, or apart from your party.
  6. Do not become distracted and stay in an exposed place for too long.

Good Habits

  1. Use releasable bindings
  2. Follow low angle slopes
  3. Don't box yourself into a region when the slide will be, always have an escape plan.
  4. Avoid travel on cornices.
  5. Always monitor for environmental signs of avalanche.
  6. Watch for decreasing mental judgement, or influence from "Powder Fever".

General Avalanche Prediction

Note this is by NO means comprehensive, and is only the tip of the iceberg.

  1. Check with the local avalanche prediction center
  2. Follow historical and current weather, and know how they impact the snowpack.
  3. Look for environmental signs of danger.
    • Cracking
    • Ribbing
    • Wind loading
    • Cornicing
    • Tree flagging (where the branches have been sheered off)
    • Nearby avalanche activity
    • Dig snow pits, and test for snow stability.(Outside scope of article)
    • Make local pole tests to check for variation.

Please read - Additional materials and resources.

share|improve this answer
    
+1 - Great answer! –  berry120 Jan 26 '12 at 0:17
    
+1 fantastic answer. –  Hartley Brody Jan 26 '12 at 2:18
    
Good answer. I would emphasize avoiding 30-40 degree (or 25-45 degree) slopes, especially with new or wind loaded snow. Also, look for and avoid being above or in avalanche chutes -- areas with missing trees that have had avalanches in past years. If you hear and feel a "whump" of the snow giving way underneath you, even in a level area, the snow is UNSTABLE and you should definitely avoid steep slopes (25-45 degree). –  xpda Jan 26 '12 at 3:10
    
+1 - Massive topic, impossible to cover everything in a single answer. –  Qwerky Jan 26 '12 at 12:31
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.