Avalanches occur when snow builds up to unstable levels on a steep surface, causing it to slide and fall. This can be due to a lack of vegetation supporting the snow, multiple layers of snow or ice, an increase in temperature or too heavy a snowfall over a short period.
When snow falls on a mountain, each part of the slope collects some of the snowfall, allowing the excess to slide or fall down to lower levels safely. Avalanches can occur when large amounts of snow build up in unstable areas of the mountainside either due to heavy snowfall or temperature conditions. For instance, cold temperatures may allow large snowdrifts to build up and freeze together, only to present an avalanche danger when temperatures rise and the mass begins to melt and fracture.
When a slope holds an unstable mass of snow, it is prone to avalanches. Loud sounds and vibrations can trigger avalanches although this propensity is often exaggerated in movies and cartoons. Most avalanches are caused by people climbing or skiing into these unstable snow zones, with their movement and weight providing the tipping point to bring the snow field down around them. Gravity can also be enough to trigger an avalanche as can increased load due to precipitation.