Friday, December 9, 2011

Avalanche!!!! The Weather Variable 

The first significant contributing factor is weather.  80% of avalanches occur during or shortly after a storm.  For this reason, the information gathering process must begin BEFORE you leave on your outing.

Before you leave home, gather as much information as possible!  Utilize your local avalanche forecast center's web site and/or recorded avalanche hot line and listen to weather reports on the TV and radio.  The local Ski Patrol may also have information regarding the latest avalanche forecast.

Invest in a commercially sold cross-country trail map if one exists for the area you plan to visit.  Sometimes, dangerous avalanche zones are noted, giving you advance warning of problem areas.

What should you look for when on the trail?  Beware of changing weather patterns, especially unusual changes in wind, snowfall and temperatures.

The first thing to look for is storms.  Remember that 80% of avalanches occur during or shortly after a storm, often because of the fact that the existing snowpack cannot support the weight of the new snow, especially if stressed by the added weight of a skier or snowmobiler.

You must also be alert to the presence of winds.  Winds of over 15 M.P.H. cause avalanche hazard to increase greatly.  Under these conditions, the wind lifts snow from windward slopes and redeposits it onto leeward slopes.  This produces greater accumulations of heavier, denser snow on these leeward slopes, which stresses the existing snowpack.  Snow plumes off the tops of ridges are a good indication that wind is moving the snow.  Cornices on leeward slopes indicate accumulations of wind-deposited snow.

New Snow 
Snow falling at a rate of one inch per hour or greater increases the avalanche danger as a result of the increased weight.  If a foot or more of fresh snow is deposited at one time, then avalanche danger is often extreme.  Even four inches of fresh snow is dangerous, in conditions of high wind.

Snow remains unstable (or may become less stable) in cold temperatures, due to the temperature difference between the surface of the snow and the surface of the ground.  Once temperatures climb into the range of
20-32 degrees, the snow cover will rapidly stabilize, due to settling.  Temperatures above freezing produce very dangerous conditions, because melting snow introduces water into the snowpack.

weakens the existing snow crystals and acts as a lubricant in the snowpack.  In other words, temperatures significantly above freezing increase the danger

In our next few posts we will continue to discuss the three main variables that help develop a potentially unstable snowcover: weather, terrain and snowpack. By understanding these variables, backcountry users will have a better chance of predicting avalanche danger.

For more information on avalanche safety check out the Mountain Rescue Associations public education program @ as well as our Backcountry Skiing & Riding Safety Video  

Courage - Commitment - Compassion
     Mountain Rescue Association