An avalanche may be dry or wet, according to whether free water is present in the snow. It may be of loose snow, when the avalanche starts at a single point or a slab avalanche which occurs when an area of more cohesive snow separates form the surrounding snow and slide out. So to sum it up simply, avalanches occur because of gravity. The most dangerous slopes are those between 30 degrees and 60 degrees. Snow is unlikely to detach on a lesser slope, while much snow will not accumulate on stronger slopes, sliding off as it settles.
Snow crystals formed in the clouds can have a variety of shapes, depending on the temperature.
Once the snow has fallen, it undergoes a series of physical variations, caused by the wind and by the exposure of the slope to the sun and the air temperature, creating a varied sequence of layers which are almost indistinguishable from the outside, due above all to the density and shape of the crystals. All these elements, studied in a fairly recent discipline known as Nivology, also affect cohesion and translate into a level of mechanical resistance of the snow mantle to the force of gravity, which is, the essence of avalanches. Three main factors determine whether avalanches are likely to occur - the weather, the snow pack and the terrain. Weather - This is the most important factor in determining whether avalanches are likely, and the evaluation of the snow pack is entirely dependent on this. Both wind and temperature are the key weather variables that effect avalanches.