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What is an Avalanche?

How Do Avalanches Form?

The weather is the ultimate driver of avalanche conditions and determines how the snowpack develops. The snowpack is not a uniform mass of snow; it consists of layers deposited by storms. Different weather creates different types of layers. As they accumulate, a complex snowpack develops with stronger and weaker layers.

Snow crystals within the snowpack change over time, and these changes are also affected by weather. Some weather patterns tend to make crystals bond better to each other, strengthening the layers within the snowpack. Other weather patterns do the opposite, making layers less cohesive and resulting in a weaker snowpack.

Slab avalanches require a weak layer in the snowpack that is overlaid by a slab of stronger snow above.

Loose snow avalanches are associated with poor bonds between grains within a layer.

Cornice avalanches can be caused by a weak layer or when even relatively strong bonds between grains are broken due to the stress of overhanging, unsupported snow.

If you dig into the snowpack, you can often see the different layers and sometimes you can even spot the weak layers dividing the stronger ones. How these layers are arranged determine how likely avalanches are to occur.
If you dig into the snowpack, you can often see the different layers and sometimes you can even spot the weak layers dividing the stronger ones. How these layers are arranged determine how likely avalanches are to occur.
Mark Grist
Dig Deeper: Weak Layers +

Weak layers can take many forms:

Storm Snow

These are the classic, six-pointed snowflakes you drew in school. If these crystals are about 3mm or larger, they can form a weak layer. This type of weak layer will last anywhere from a few hours to a few days before they start bonding to the surrounding snow.
These are the classic, six-pointed snowflakes you drew in school. If these crystals are about 3mm or larger, they can form a weak layer. This type of weak layer will last anywhere from a few hours to a few days before they start bonding to the surrounding snow.
Jim Bay

Surface Hoar

This is a form of frozen dew that creates feathery, vertical snow crystals that grow on the surface on cold, clear nights. Surface hoard tends to maintain its vertical structure even when buried, creating an airy layer in the snowpack. This type of weak layer can take weeks and even months to break down.
Surface hoar is essentially frozen dew that creates feathery crystals on the snow surface after a few cold, clear nights. Once buried, the vertical structure of these crystals often creates a weak layer in the snowpack. This type of weak layer can take weeks, and sometimes even months, to heal.
Wes Gano

Facets

These crystals form in response to differences in temperature throughout the snowpack. They are sharp-sided grains that don’t bond well with each other or with surrounding snow. A layer of facets looks and feels like sugar, with virtually no cohesion between the grains.
These crystals form in response to differences in temperature throughout the snowpack. They are sharp-sided grains that don’t bond well with each other or with surrounding snow. A layer of facets looks and feels like sugar, with virtually no cohesion between the grains.
Leslie Crawley

Depth Hoar

These are large, cup-shaped crystals that most commonly form at the base of shallow snowpacks in cold climates. It can create a deep persistent weak layer that lasts for an entire season.
These are large, cup-shaped crystals that most commonly form at the base of shallow snowpacks in cold climates. Depth hoar can create a deep persistent weak layer that lasts for an entire season.
Jennifer Coulter

Crusts

There are many types of crusts, which are formed when the surface snow is melted by sun, rain, or temperature changes, and then refreezes. While they are not weak on their own, avalanches are more likely when a crust lies next to a weak layer because they make for such a great sliding surface.
There are many types of crusts, which are formed when the surface snow is melted by sun, rain, or temperature changes, and then refreezes. While they are not weak on their own, avalanches are more likely when a crust lies next to a weak layer because they make for such a great sliding surface.
Raven Eye Photography