An avalanche is a mass of snow falling down a mountain or an incline.

The makings of an avalanche begin with the accumulation of snow. As snow falls, it sticks together to form layers. Numerous snowfalls and variations in weather form different layers of snow. These combined layers are called snow pack. The strength of snow pack depends on the shape of the ice crystals. Ice crystals become more rounded with frequent melting and refreezing. These rounded crystals do not stick together well and form a weak layer within the snow pack. Avalanches occur at weak layers.

A trigger causes the weak layer to break. Just one small trigger is needed to create an avalanche. The weak layer breaks as the pull of gravity exceeds the strength of the layer bond. The most common natural trigger is rapid, heavy snowfall but can also include small things like a chunk of snow falling from trees or large things like earthquakes. The trigger of most avalanches that harm people are the victims themselves.

There are many factors that play a role in determining whether an avalanche will occur. Most avalanches occur on slopes between 35 and 40 degrees. The pull of gravity is usually not strong enough to create an avalanche on slopes less than 35 degrees. On slopes greater than 40 degrees, the snow accumulations do not get deep enough to create an avalanche. Vegetation on the slope can prevent or slow avalanches. Weather also affects the likelihood of avalanches. Avalanches often occur during or immediately after heavy snowfall. The new snow puts extra stress on the snow pack. This extra stress can be a trigger that causes an avalanche. Another possibility is that the new snow might not bond strongly enough with the previous surface layer and could slide down the slope.