Avalanche

What Is an Avalanche?

An avalanche is a rapid, powerful flow of snow down a mountainside. It can reach speeds of up to 100 mph, carrying snow, ice, rocks, and debris in its path. Avalanches range from small slides that cause minimal damage to massive ones capable of flattening forests and burying entire villages. In North America alone, avalanches cause around 40 fatalities every year.

Types of Avalanches

1. Loose Snow Avalanches (Sluffs):These start from a single point and fan out as they move downhill, creating an inverted "V" shape. They occur when the surface layer of snow loses its grip and slides down. While usually small and predictable, they can be dangerous if they drag someone into hazards like cliffs, trees, or narrow gullies.

2. Slab Avalanches: Slab avalanches happen when a thick layer of snow (the slab) breaks away from weaker snow layers underneath. This type is particularly dangerous because it involves large amounts of snow moving at high speeds. Slab avalanches are responsible for most avalanche-related deaths.

3. Glide Avalanches: These occur when an entire slab of snow slowly slides over a smooth surface like bedrock or grassy slopes. They can happen in both cold and warm conditions. Glide cracks (deep crevices in the snowpack) often form before the avalanche occurs, but predicting exactly when the snow will give way is challenging—it could be hours, days, or even months after the crack appears.

   
   

What Causes Avalanches?

• Natural Triggers: Avalanches can be caused by heavy snowfall, rapid temperature changes, strong winds depositing snow unevenly, or melting snow that destabilizes the snowpack.

• Human Triggers: People are a common cause of avalanches, especially in backcountry areas. Skiers, snowboarders, snowmobilers, and hikers can trigger avalanches simply by adding extra weight to unstable snow layers.

Source: Avalanche.org

Avalanche Annualized Frequency

An Avalanche annualized frequency value represents the average number of recorded Avalanche hazard occurrences (events) per year over the period of record (60 years).

Source Data

National Avalanche Center's Avalanche Forecast Zone Map

Arizona State University's Center for Emergency Management and Homeland Security's Spatial Hazard Events and Losses Database of the United States (SHELDUS)

See the Official FEMA MAP

Source: FEMA.gov