AVALANCHES
There are 3 main types of avalanches.
Loose snow avalanches:
They occur at the start of winter with pulverulent snow. It happens when the weather is very cold, after heavy snowfalls of light snow on steep slopes; just the weight of the snow adding on the first layer can make the avalanche happens. It could be just a small avalanche, with a speed that doesn't exceed 60 to 80km/h, it is a very cold powder snow wave that can kill people by frost or drowning.
The speed of the avalanche will increase on the way down, going at 100 to 400 km/h. It makes a tidal snow wave with a lot of air mixed in it. Very dangerous, this type of avalanches can create great disasters; in Himalayas some of them were so big that they went up 600m on the opposite side of the valley!
A person not physically caught in the avalanche, would be hit by the shock wave that is present in front of the snow cloud and die from injuries like somebody exposed to an explosion.
If the weather stay very cold, the phenomenon can take few weeks and the danger be still high.
Slab avalanches:
They happen when a strong, cohesive layer of snow stand above a weak layer which doesn't provide good anchoring and play as a break plan and sliding surface. The ground configuration play a strong part and fractures will occur on convex parts of the ground. The overload of a new snowfall, a skier, or anything that add some weight locally, breaks the layer in one point and instantly, it is all the layer that breaks into pieces in a rumbling noise due to the expulsion of the air that was stuck between the two snow layers. In the Alps, after a long period of cold and sunny weather, this type of avalanche can happen due to hoar frost formation in a deep coat of fresh snow.
The speed of these avalanches is between 20 to 50km/h and they are very heavy (density>200kg/m3).
Isothermal avalanches:
They are heavy and powerful but they don't kill much as they are quiet predictable. They occur in spring or in winter after a rise in temperature when snow is full of melting water and lose all cohesion.
Balls of snow from an egg to a water melon size go down the slope (30 to 100 km/h), crushing everything on its way. It leaves a trench and finishs in a big cone of dejection. As hard as cement, they block things and people caught during the fall.
Some avalanches of this type are just a heavy mix of snow and water, going down slowly (10 to 20 km/h) but dangerous due to their high density; they occur in the afternoon on well known areas.
In the French Alps, the French Weather Forecast "Météo France", indicates a level of risks concerning avalanches for all mountains areas.
Their website is: france.meteofrance.com/france/montagne
They use the European avalanches risks scale describe below.
It gives a very good indications but of course doesn't replace a good knowledge of the local conditions and of the area.
If you go snowshoeing or ski touring, always take advices from local people like mountain guides, people of PGHM (French National Mountain rescue).
The European phone number for rescue is: 112.
