Editing Avalanche (section)

== Formation ==
[[File:Loose snow and slab avalanches near mt shuksan.jpg|thumb|Loose snow avalanches (far left) and slab avalanches (near center) near [[Mount Shuksan]] in the [[North Cascades]] mountains. Fracture propagation is relatively limited.]]
[[File:Snowboarder triggered soft slab near mount baker.jpg|thumb|15 cm deep, soft slab avalanche triggered by a snowboarder near Heliotrope Ridge, [[Mount Baker]] in March 2010. Multiple crown fracture lines are visible in the top-middle of the image. Note the granular characteristic of the debris in the foreground that results from the slab breaking up during descent.]]

Most avalanches occur spontaneously during storms under increased load due to snowfall and/or [[erosion]]. Metamorphic changes in the snowpack, such as melting due to solar radiation, is the second-largest cause of natural avalanches. Other natural causes include rain, earthquakes, rockfall, and icefall. Artificial triggers of avalanches include skiers, snowmobiles, and controlled explosive work. Contrary to popular belief, avalanches are not triggered by loud sound; the pressure from sound is orders of magnitude too small to trigger an avalanche.<ref>{{cite conference|last1=Reuter|first1=B.|last2=Schweizer|first2=J.|year=2009|title=Avalanche triggering by sound: Myth and truth|url=http://gblanc.fr/IMG/pdf/reuter2009.pdf|conference=ISSW 09 – International Snow Science Workshop, Proceedings|pages=330–333|quote=Based  on order of  magnitude  estimates  of the pressure amplitude of various sources that cause elastic or pressure (sound) waves it can be ruled out that shouting or loud noise can trigger snow slab avalanches. The amplitudes are at least about two orders of magnitude smaller than  known  efficient  triggers.  Triggering  by sound really is a myth.}}</ref>

Avalanche initiation can start at a point with only a small amount of snow moving initially; this is typical of wet snow avalanches or avalanches in dry unconsolidated snow. However, if the snow has sintered into a stiff slab overlying a weak layer, then fractures can propagate very rapidly, so that a large volume of snow, possibly thousands of cubic metres, can start moving almost simultaneously.{{fact|date=January 2024}}

A snowpack will fail when the load exceeds the strength. The load is straightforward; it is the weight of the snow. However, the strength of the snowpack is much more difficult to determine and is extremely heterogeneous. It varies in detail with properties of the snow grains, size, density, morphology, temperature, water content; and the properties of the bonds between the grains.<ref name="McClung 2006">McClung, David and Shaerer, Peter: ''The Avalanche Handbook, The Mountaineers'': 2006. {{ISBN|978-0-89886-809-8}}</ref> These properties may all metamorphose in time according to the local humidity, water vapour flux, temperature and heat flux. The top of the snowpack is also extensively influenced by incoming radiation and the local air flow. One of the aims of avalanche research is to develop and validate computer models that can describe the evolution of the seasonal snowpack over time.<ref>{{Cite journal|url=https://www.mendeley.com/catalogue/3f58ea80-6488-31f0-867c-a94843e225e3/|archiveurl=https://archive.today/20130128103159/http://www.mendeley.com/research/physical-snowpack-model-swiss-avalanche-warning-part-i-numerical-model/|archivedate=2013-01-28|url-status=dead|title=A physical SNOWPACK model for the Swiss avalanche warning Part I: Numerical model|first1=Perry|last1=Bartelt|first2=Michael|last2=Lehning|date=24 May 2002|journal=Cold Regions Science and Technology|volume=35|issue=3|pages=123–145|via=www.mendeley.com|doi=10.1016/S0165-232X(02)00074-5|bibcode=2002CRST...35..123B }}</ref> A complicating factor is the complex interaction of terrain and weather, which causes significant spatial and temporal variability of the depths, crystal forms, and layering of the seasonal snowpack.<ref name=":1">{{Cite web |title=Avalanches: Their Dangers and How to Reduce Your Risks |url=https://www.wunderground.com/cat6/avalanches-their-dangers-and-how-to-reduce-your-risks |access-date=2024-04-15 |website=www.wunderground.com |language=en}}</ref>

=== Slab avalanches ===
Slab avalanches are formed frequently in snow that has been deposited, or deposited by wind. They have the characteristic appearance of a block (slab) of snow cut out from its surroundings by fractures. Elements of slab avalanches include a crown fracture at the top of the start zone, flank fractures on the sides of the start zones, and a fracture at the bottom called the stauchwall. The crown and flank fractures are vertical walls in the snow delineating the snow that was entrained in the avalanche from the snow that remained on the slope. Slabs can vary in thickness from a few centimetres to three metres. Slab avalanches account for around 90% of avalanche-related fatalities.{{fact|date=January 2024}}

=== Powder snow avalanches ===
{{Main|Powder snow avalanche}}
The largest avalanches form turbulent suspension currents known as [[powder snow avalanche]]s or mixed avalanches,<ref>Simpson JE. 1997. Gravity currents in the environment and the laboratory. Cambridge University Press</ref> a kind of [[gravity current]]. These consist of a powder cloud, which overlies a dense avalanche. They can form from any type of snow or initiation mechanism, but usually occur with fresh dry powder. They can exceed speeds of {{convert|300|kph|abbr=on}}, and masses of 1,000,000 tons; their flows can travel long distances along flat valley bottoms and even uphill for short distances.<ref>{{Cite web |title=Avalanche |url=https://education.nationalgeographic.org/resource/avalanche |access-date=2024-04-10 |website=education.nationalgeographic.org |language=en}}</ref>

=== Wet snow avalanches ===
[[File:Lawinenkegel auf dem Simplonpass (2019).jpg|thumb|Avalanche on [[Simplon Pass]] (2019)]]
In contrast to powder snow avalanches, wet snow avalanches are a low velocity suspension of snow and water, with the flow confined to the track surface (McClung, 1999, p.&nbsp;108).<ref name="McClung 2006" /> The low speed of travel is due to the friction between the sliding surface of the track and the water saturated flow. Despite the low speed of travel (≈10–40&nbsp;km/h), wet snow avalanches are capable of generating powerful destructive forces, due to the large mass and density. The body of the flow of a wet snow avalanche can plough through soft snow, and can scour boulders, earth, trees, and other vegetation; leaving exposed and often scored ground in the avalanche track. Wet snow avalanches can be initiated from either loose snow releases, or slab releases, and only occur in snowpacks that are water saturated and isothermally equilibrated to the melting point of water. The isothermal characteristic of wet snow avalanches has led to the secondary term of isothermal slides found in the literature (for example in Daffern, 1999, p.&nbsp;93).<ref name="Daffern 1999">Daffern, Tony: Avalanche Safety for Skiers, Climbers and Snowboarders, Rocky Mountain Books: 1999. {{ISBN|0-921102-72-0}}</ref> At temperate latitudes wet snow avalanches are frequently associated with climatic avalanche cycles at the end of the winter season, when there is significant daytime warming.{{fact|date=January 2024}}

=== Ice avalanche ===
An ice avalanche occurs when a large piece of ice, such as from a serac or calving glacier, falls onto ice (such as the [[Khumbu Icefall]]), triggering a movement of broken ice chunks. The resulting movement is more analogous to a rockfall or a landslide than a snow avalanche.<ref name="McClung 2006" /> They are typically very difficult to predict and almost impossible to mitigate.{{fact|date=January 2024}}

=== Avalanche pathway ===
As an avalanche moves down a slope it follows a certain pathway that is dependent on the slope's degree of steepness and the volume of snow/ice involved in the [[Mass wasting|mass movement]]. The origin of an avalanche is called the Starting Point and typically occurs on a 30–45 degree slope. The body of the pathway is called the Track of the avalanche and usually occurs on a 20–30 degree slope. When the avalanche loses its momentum and eventually stops it reaches the Runout Zone. This usually occurs when the slope has reached a steepness that is less than 20 degrees.<ref>{{Cite book|title=Natural Disasters|last=Abbott|first=Patrick|publisher=McGraw-Hill Education|year=2016|isbn=978-0-07-802298-2|location=New York}}</ref> These degrees are not consistently true due to the fact that each avalanche is unique depending on the stability of the [[snowpack]] that it was derived from as well as the environmental or human influences that triggered the mass movement.{{fact|date=January 2024}}