The sheer terror of watching a mountain of snow hurtling down a slope and sweeping away everything in its path can only be imagined by most, but the number of fatalities that occur every year makes it a force to be respected for its potential to devastate. The causes are varied.
The Dry Slab Avalanche
This is known to be the most deadly avalanche of all. Fractures cause a solid slab of snow to dislodge and begin an en masse slow slide on the snow beneath at initial speeds of around 20 mph but can within seconds then accelerate to speeds of up to 80 mph. This type of avalanche can sometimes disguise the weaknesses lying beneath for quite lengthy periods until sudden added weight, perhaps that of a lone skier, finally rips away any stable facade.
These avalanches occur when temperatures suddenly increase due to sun or rain and cause the snow to become slushier and decrease in strength. It then begins to slide at a much slower pace than the dry avalanche. It is also not as susceptible to sudden weight changes which is why it accounts for far fewer fatalities.
Ice Fall Avalanches
When ice begins to cause overhangs on glaciers the weight can eventually cause it to break away. The subsequent fall onto snow below can then cause fractures, which will precipitate a movement of snow.
Cornice Fall Avalanches
These avalanches are similar in many ways to the ice fall variety. Snow collects behind obstacles such as ridgelines and eventually the sheer weight or amount of snow will cause it to break up into hundreds of pieces. This again has a destabilising effect on the snow structure causing it to slide downwards.
Glide avalanches are due to a slow process of melting water lubricating the ground allowing the snow on top to begin gliding downhill. Not as dangerous as the other varieties they are nevertheless not to be taken lightly.
These happen on the gentle slopes and very seldom on slopes steeper than 25 degrees. These avalanches occur when water pools and quickly weakens a snow structure. Because this usually only happens at high latitudes these slides seldom cause fatalities, but can destroy structures unwisely built in certain locations.
It is wise to remember that packed snow is dependent on conditions allowing it to remain so. Weather conditions should always be ascertained before skiing and attention paid to any alerts.