FOX44 - Burlington / Plattsburgh News, Weather & Sports2.20.11 All About Avalanches!

2.20.11 All About Avalanches!

We had a story tonight on a deadly avalanche that happened over the weekend in Washington state. (click for the full story from USAToday ).  

So we decided to highlight this and talk about it on our 10pm newscast on Fox44... I got to put on my "station scientist" hat on and talk about the science of avalanches! 

The clip above is what we aired on our 10 pm newscast on Fox44 on Monday night (the explaination is after the news package...be sure to scroll near the end for what I had to say).

The main question posed to me was, why do avalanches happen more often in the western US?

This is an entirely fair question, its true. Generally speaking, the western US tends to see a higher avalanche risk.... So why? First, I should start by saying avalanches absolutely can and do happen, on the east coast (Mount Washington has its own Avalanche Center to monitor the avalanche risk, click to read about it).

A quick and dirty look at avalanches consists of 3 things

1)      A steep mountain slope. Typically, a slope greater than 30 degrees will be ideal for avalanches. The mountains in the western US elevate avalanche danger as they are widely taller and steeper. Logic says, gravity "works better" with steep mountain slopes.

2)      Unstable snowpack

3)      A trigger (often times, a human...a backcountry skier, a snowshoer, etc...)

So lets dig just a little deeper...

The steep mountain slope concept  is pretty self explanatory.

Also, the trigger...typically people, are what sets off an avalanche. While it is not always human activity that sets off an unstable snowpack to create an avalanche, it is the most common reason, hence why avalanches are often so deadly.

So let's dig in a bit more to the snowpack....this is what makes an avalanche so unpredictable.

A snowpack is simply layer upon layer of snow, this comes with each new snowfall or layer of precipitation. In the instance of this avalanche in Washington, the mountains there just came off of a fresh heavy snowfall. The most treacherous type of unstable snowpack occurs when a slab sets on top of a weak layer. A slab is a layer of snow that has at least some cohesion.  Slabs can be quite hard or they can be rather soft; the slab layer just has to be more cohesive or harder than the layer below it. Slab layers tend to occur with a heavier, wetter snow (which is more cohesive).

The most dangerous situation occurs when the upper layer is very cohesive and the lower layer is much less cohesive. In those situations, the upper or slab layer is literally a block that you can cut out and pick up and the lower or weak layer is like sugar. 

 Sometimes the weak layer can just barely hold up the block.

Another type of unstable and potentially dangerous snowpack occurs when above freezing temperatures melt portions of the snow pack and saturate certain layers.  This condition can cause wet avalanches.

This is only scratching the surface of avalanches. There are plenty of great resources to learn more... here are some that I recommend.

http://www.avalanche.org/

http://www.fsavalanche.org/Default.aspx?ContentId=6&LinkId=12&ParentLinkId=9

A good learning tool are the Comet modules from NCAR (you can sign up for a free acount and learn LOTS!) Here's the module on avalanche safety... http://www.meted.ucar.edu/afwa/avalanche/index.htm