From SEG Wiki
Jump to: navigation, search

An avalanche is a natural disaster that occurs when snow rapidly flows down a mountain. During an avalanche a combination of snow and ice (snowpack) [1] is formed. The avalanche begins when the snowpack is unstable and breaks off along a mountain slope. When the snow breaks apart from the mountain, it begins to accelerate downhill and picks up other material along the way. A fully developed snow avalanche can travel as fast as 320 kilometer per hour.[2]

Seen above is an avalanche that occurred in Juneau,Alaska

Anatomy and formation of an avalanche

An avalanche occurs when the slope of a mountain is between thirty and forty-five degrees. An avalanche generally occurs in three different climates. Maritime (coastal) avalanche climate, Continental avalanche climate, and Intermountain (transitional) avalanche climate are the three most common climatic zones where the avalanche occur.

Avalanche climate zones

Maritime avalanche climate has the deepest snowfall, (which produces more of a snowpack). It is estimated that the snowpack can be over nine feet deep. There is also more precipitation over the course of the winter season, however, there is a less chance of an avalanche occurring because the snowpack is controlled and stable.[3]

Continental avalanche climate, is a type of climate that has very cold temperatures throughout the winter and drought. There are less storms and precipitation. The snowpack on the mountain is unstable, meaning that there is a stronger chance of an avalanche occurring.

Intermountain (transitional) avalanche climate has fewer rainstorms. The snowpack depth ranges from 5 to 9 feet.

Avalanche path

Avalanches travel in three main sections: the starting zone, track, and the runout zone.[4]

Diagram of the structure of an avalanche

Starting Zone is where the avalanche begins.

Avalanche track or (track) is the pathway than an avalanche follows as it travels downhill.

Runout Zone is where the snow and debris comes to an end.[5]

Types of Avalanches

Avalanches travel through many different forms; however, there are three specific types of avalanches that are most likely to occur. Slab avalanches, power snow avalanches and wet avalanches usually occur on mountain slopes.

Slab avalanche

Slab avalanche are the most common and most deadly form of an avalanche. They are formed by snow that has been moved by wind in different locations on top of a mountain.. Slab avalanches appear as a snow slab, which a layer of snow that is on top of a weak layer of snow. The length of a slab can be roughly the size of half a football field. The depth of a slab can be thirty to sixty centimeters thick. Slabs can be divided between soft and hard. A soft slab is formed with fresh snow. Over time, the wind contributes into making aloft slabs into hard slabs. Hard slabs are formed when crystalline structures from the fresh snow harden. When wind speeds 80 mph, the slab begins to shatter, causing an avalanche to occur. The harder and denser the snowpack is on top of the slope, the deadlier the avalanche outcome.[6]

Powder snow avalanche

Dry avalanches or power snow avalanches are typically seen as a powder cloud. Although these avalanche appear to be falling gracefully form mountain slope, they are actually extremely dangerous. This form of avalanche is developed from 99% of air and only 1% of snow, which is how it gets it power dry and soft appearance. Powder snow avalanches are considered to be the largest. As it travels down the mountain slope, dry avalanches can reach a speed of 300 km/h.

Wet snow avalanche

Wet avalanche are triggered by warm air temperatures, sun, and rain. They occurred in this type of climate due to the warmth from the rain water. These factors infect the snowpack and decreases the strength of the snow. A wet avalanche travels at 10 to 20 mph down the slope. Wet snow avalanches can be triggered from either loose snow releases.[7] or slab releases.[8]


There is annual average of roughly ten thousand avalanches occurring in The United States. The risk for a potential avalanche to occur is more severe in Europe than in North America due to the heavily populated in the Alps than in the Rocky Mountains.[9].

Human activity

Humans have activated many avalanches in recent years. Seventy percent of snow avalanches worldwide are caused by human activity. There are many athletes from snowboarding, skiing and many other winter sports addition, there are other article factor that contribute to avalanches which include skiers and snowmobiles.[10] Vibrations from snowmobiles and other vehicles place a lot of pressure on top of mountains. The combination of both gravity and vibrations cause the avalanche to occur very quickly.


Many avalanches form as an effect of a large amount of snowfall. Twenty four hours after a snowstorm occurs, wind speeds increase transfers a great amount of snow from one side of a mountain to the other side, causing an unstable mountain slope. The second largest cause of natural avalanches is the melting ice and snow due to radiation in the atmosphere. Also there are many other natural disasters that have an influence on the frequency of avalanches which include earthquakes, rockslides, and icefall.[11]

See also

Other closely related articles in this wiki include:


  1. US EPA, O. (n.d.). Less Snowpack. Retrieved November 8, 2015, from http://www3.epa.gov/climatechange/kids/impacts/signs/snowpack.html
  2. Avalanche. (n.d.). Retrieved November 8, 2015, from [1]
  3. Avalanche Weather Forecasting: Types of Avalanches. (n.d.). Retrieved October 18, 2015, from [2]
  4. Avalanche Weather Forecasting: Types of Avalanches. (n.d.). Retrieved October 18, 2015, from [3]
  5. National Snow and Ice Data Center :: Advancing knowledge of Earth's frozen regions Snow Avalanches | National Snow and Ice Data Center. (n.d.). Retrieved September 24, 2015, from [ http://nsidc.org/cryosphere/snow/science/avalanches.html]
  6. National Avalanche Center Hard Slab Avalanche. (n.d.). Retrieved October 28, 2015, from [4]
  7. Hard Slab Avalanche. (n.d.). Retrieved October 28, 2015, from [5]
  8. [6]
  9. Smith, K. (2013). Environmental Hazards: Assessing Risk and Reducing Disaster. Routledge.
  10. Avalanche. (n.d.). Retrieved November 8, 2015, from Avalanche Problems. (n.d.). Retrieved October 17, 2015, from [7]
  11. [Types, Causes and Effects of Avalanches - Conserve Energy Future. (n.d.). Retrieved from http://www.conserve-energy-future.com/types-causes-effects-of-avalanches.php]

External links