Avalanches

With the Ski season around the corner, I thought it'd be good to have a look at avalanches and what causes them.

An avalanche is a rapid flow of snow/ice down a slope. Now this can vary from just a small ripple of snow on a ski slope, to vast volumes of snow and ice rushing down the side of a mountain, with varying consequences. Avalanches are caused by a number of factors:

 1. The recent weather. If a snowstorm has been occurring recently then an avalanche is more probable considering recent snowfall puts extra stress on the already existing snow pack, this is especially dangerous if the snow does not bond with the layers of snow already there. The 24 hours after a major snowstorm is the most critical time as this is when the crust is most likely to give way to the pressure.

2. Temperature change. Small changes in temperature for short periods of time usually do not have an effect on whether an avalanche will occur or not, however, if the change in temperature is substantial or sustained then this can seriously weaken the upper layers of snow, causing an avalanche.  

3. Wind direction. Wind on a mountain usually blows up the windward side and down the leeward side, however, as the wind blows, it picks up snow (on the windward side by scouring the snow off the mountain side) and deposits it on the other side of the mountain (leeward side), this increases the volume and weight of the snow pack on the leeward side which can often trigger an avalanche.  

4. Snow pack conditions. This is perhaps the most important factor when looking at avalanche probability ad slope stability as determining whether the snow pack is bound well or loosely will give an idea of how probable an avalanche is.

5. Vegetation. Lone trees and rocks can often weaken the snow pack significantly triggering avalanches. However, not all vegetation is bad; having lots of trees and rocks can break up the snow pack and also help to stop the avalanche if it begins.



Here is an example of the layers in snow pack. Courtesy of http://www.fsavalanche.org/encyclopedia/layer_snow.htm