Editing Avalanche (section)

== Terrain, snowpack, weather ==
[[File:North Ridge of Mount Rohr.jpg|thumb|In steep avalanche-prone terrain, traveling on [[ridge]]s is generally safer than traversing the slopes.]]
[[File:Mount Windsor Cornice2.jpg|thumb|A [[cornice (climbing)|cornice]] of snow about to fall. Cracks in the snow are visible in area (1). Area (3) fell soon after this picture was taken, leaving area (2) as the new edge.]]
Doug Fesler and Jill Fredston developed a conceptual model of the three primary elements of avalanches: terrain, weather, and snowpack. Terrain describes the places where avalanches occur, weather describes the meteorological conditions that create the snowpack, and snowpack describes the structural characteristics of snow that make avalanche formation possible.<ref name="McClung 2006" /><ref>Fesler, Doug and Fredston, Jill: ''Snow Sense'', Alaska Mountain Safety Center, Inc. 2011. {{ISBN|978-0-615-49935-2}}</ref>

=== Terrain ===
Avalanche formation requires a slope shallow enough for snow to accumulate but steep enough for the snow to accelerate once set in motion by the combination of mechanical failure (of the snowpack) and gravity. The angle of the slope that can hold snow, called the [[angle of repose]], depends on a variety of factors, such as crystal form and moisture content. Some forms of drier and colder snow will only stick to shallower slopes, while wet and warm snow can bond to very steep surfaces. In coastal mountains, such as the [[Cordillera del Paine]] region of [[Patagonia]], deep snowpacks collect on vertical and even overhanging rock faces. The slope angle that can allow moving snow to accelerate depends on a variety of factors such as the snow's shear strength (which is itself dependent upon crystal form) and the configuration of layers and inter-layer interfaces.{{fact|date=January 2024}}

The snowpack on slopes with sunny exposures is strongly influenced by [[Sunlight|sunshine]]. Diurnal cycles of thawing and refreezing can stabilize the snowpack by promoting settlement. Strong freeze-thaw cycles result in the formation of surface crusts during the night and of unstable surface snow during the day. Slopes in the lee of a ridge or of another wind obstacle accumulate more snow and are more likely to include pockets of deep snow, [[wind slabs]], and [[cornice (climbing)|cornices]], all of which, when disturbed, may result in avalanche formation. Conversely, the snowpack on a windward slope is often much shallower than on a lee slope.<ref>{{Cite web |title=Avalanche Safety Guidelines |url=https://www.ehss.vt.edu/uploaded_docs/201006231613220.Avalanche_Safety_Guideline.pdf |access-date=April 10, 2024 |website=www.ehss.vt.edu}}</ref>

[[File:Avalanche path 7271.JPG|thumb|left|Avalanche path with {{convert|800|m|ft}} vertical fall in the [[Glacier Peak Wilderness]], [[Washington (state)|Washington state]]. Avalanche paths in alpine terrain may be poorly defined because of limited vegetation. Below tree line, avalanche paths are often delineated by vegetative trim lines created by past avalanches. The start zone is visible near the top of the image, the track is in the middle of the image and clearly denoted by vegetative trimlines, and the runout zone is shown at the bottom of the image. One possible timeline is as follows: an avalanche forms in the start zone near the ridge, and then descends the track, until coming to rest in the runout zone.]]
Avalanches and avalanche paths share common elements: a start zone where the avalanche originates, a track along which the avalanche flows, and a runout zone where the avalanche comes to rest. The debris deposit is the accumulated mass of the avalanched snow once it has come to rest in the run-out zone. For the image at left, many small avalanches form in this avalanche path every year, but most of these avalanches do not run the full vertical or horizontal length of the path. The frequency with which avalanches form in a given area is known as the [[return period]].<ref>{{Cite web |title=Return period calculated for study snow avalanche paths using the existing method |url=https://www.researchgate.net/figure/Return-period-calculated-for-study-snow-avalanche-paths-using-the-existing-method_fig4_327258239 |access-date=April 10, 2024 |website=www.researchgate.net}}</ref>

The start zone of an avalanche must be steep enough to allow snow to accelerate once set in motion, additionally [[Convex function|convex]] slopes are less stable than [[concave function|concave]] slopes because of the disparity between the [[tensile strength]] of snow layers and their [[compressive strength]]. The composition and structure of the ground surface beneath the snowpack influences the stability of the snowpack, either being a source of strength or weakness. Avalanches are unlikely to form in very thick forests, but boulders and sparsely distributed vegetation can create weak areas deep within the snowpack through the formation of strong temperature gradients. Full-depth avalanches (avalanches that sweep a slope virtually clean of snow cover) are more common on slopes with smooth ground, such as grass or rock slabs.<ref>{{Cite web |title=Glossary |url=https://avalanche.ca/glossary |access-date=2024-04-10 |website=avalanche.ca |language=en}}</ref>

Generally speaking, avalanches follow drainages down-slope, frequently sharing drainage features with summertime watersheds. At and below [[tree line]], avalanche paths through drainages are well defined by vegetation boundaries called [[trim line]]s, which occur where avalanches have removed trees and prevented regrowth of large vegetation. Engineered drainages, such as the [[avalanche dam on Mount Stephen in Kicking Horse Pass]], have been constructed to protect people and property by redirecting the flow of avalanches. Deep debris deposits from avalanches will collect in catchments at the terminus of a run out, such as gullies and river beds.

Slopes flatter than 25&nbsp;degrees or steeper than 60&nbsp;degrees typically have a lower incidence of avalanches. Human-triggered avalanches have the greatest incidence when the snow's [[angle of repose]] is between 35 and 45&nbsp;degrees; the critical angle,<ref name=":1" /> the angle at which human-triggered avalanches are most frequent, is 38&nbsp;degrees. When the incidence of human triggered avalanches is normalized by the rates of recreational use, however, hazard increases uniformly with slope angle, and no significant difference in hazard for a given exposure direction can be found.<ref name="Pascal Hageli et al">{{Cite web |last=Hageli |first=Pascal |display-authors=etal |title=AVISUALANCHE – SELECTED PUBLICATIONS |url=http://www.avisualanche.ca/publications.html |website=www.avisualanche.ca}}</ref> The rule of thumb is: ''A slope that is flat enough to hold snow but steep enough to ski has the potential to generate an avalanche, regardless of the angle.''{{fact|date=January 2024}}

=== Snowpack structure and characteristics ===
[[File:D Hoarfrost 3.jpg|thumb|After surface [[hoarfrost]] becomes buried by later snowfall, the buried hoar layer can be a weak layer upon which upper layers can slide.]]
The snowpack is composed of ground-parallel layers that accumulate over the winter. Each layer contains ice grains that are representative of the distinct meteorological conditions during which the snow formed and was deposited. Once deposited, a snow layer continues to evolve under the influence of the meteorological conditions that prevail after deposition.{{fact|date=January 2024}}

For an avalanche to occur, it is necessary that a snowpack have a weak layer (or instability) below a slab of cohesive snow. In practice the formal mechanical and structural factors related to snowpack instability are not directly observable outside of laboratories, thus the more easily observed properties of the snow layers (e.g. penetration resistance, grain size, grain type, temperature) are used as index measurements of the mechanical properties of the snow (e.g. [[tensile strength]], [[friction]] coefficients, [[shear strength]], and [[Ductility|ductile strength]]). This results in two principal sources of uncertainty in determining snowpack stability based on snow structure: First, both the factors influencing snow stability and the specific characteristics of the snowpack vary widely within small areas and time scales, resulting in significant difficulty extrapolating point observations of snow layers across different scales of space and time. Second, the relationship between readily observable snowpack characteristics and the snowpack's critical mechanical properties has not been completely developed.{{fact|date=January 2024}}

While the deterministic relationship between snowpack characteristics and snowpack stability is still a matter of ongoing scientific study, there is a growing empirical understanding of the snow composition and deposition characteristics that influence the likelihood of an avalanche. Observation and experience has shown that newly fallen snow requires time to bond with the snow layers beneath it, especially if the new snow falls during very cold and dry conditions. If ambient air temperatures are cold enough, shallow snow above or around boulders, plants, and other discontinuities in the slope, weakens from rapid crystal growth that occurs in the presence of a critical temperature gradient. Large, angular snow crystals are indicators of weak snow, because such crystals have fewer bonds per unit volume than small, rounded crystals that pack tightly together. Consolidated snow is less likely to slough than loose powdery layers or wet isothermal snow; however, consolidated snow is a necessary condition for the occurrence of [[slab avalanche]]s, and persistent instabilities within the snowpack can hide below well-consolidated surface layers. Uncertainty associated with the empirical understanding of the factors influencing snow stability leads most professional avalanche workers to recommend conservative use of avalanche terrain relative to current snowpack instability.{{fact|date=January 2024}}

=== Weather ===
{{More citations needed section|date=June 2021}}
[[File:Avalanche testing snow pit.JPG|thumb|left|After digging a snow pit, it is possible to evaluate the snowpack for unstable layers. In this picture, snow from a weak layer has been easily scraped away by hand, leaving a horizontal line in the wall of the pit.]]
Avalanches only occur in a standing snowpack. Typically winter seasons at high latitudes, high altitudes, or both have weather that is sufficiently unsettled and cold enough for precipitated snow to accumulate into a seasonal snowpack. [[Continentality]], through its potentiating influence on the meteorological extremes experienced by snowpacks, is an important factor in the evolution of instabilities, and consequential occurrence of avalanches faster stabilization of the snowpack after storm cycles.<ref>Whiteman, Charles David: ''Mountain Meteorology: Fundamentals and Applications'', Oxford University Press: 2001. {{ISBN|0-19-513271-8}}</ref> The evolution of the snowpack is critically sensitive to small variations within the narrow range of meteorological conditions that allow for the accumulation of snow into a snowpack. Among the critical factors controlling snowpack evolution are: heating by the sun, [[radiational cooling]], vertical [[temperature gradient]]s in standing snow, snowfall amounts, and snow types. Generally, mild winter weather will promote the settlement and stabilization of the snowpack; conversely, very cold, windy, or hot weather will weaken the snowpack.<ref>{{Cite web |last=US EPA |first=OAR |date=2016-07-01 |title=Climate Change Indicators: Snowpack |url=https://www.epa.gov/climate-indicators/climate-change-indicators-snowpack |access-date=2024-04-15 |website=www.epa.gov |language=en}}</ref>

At temperatures close to the freezing point of water, or during times of moderate solar radiation, a gentle freeze-thaw cycle will take place. The melting and refreezing of water in the snow strengthens the snowpack during the freezing phase and weakens it during the thawing phase. A rapid rise in temperature, to a point significantly above the freezing point of water, may cause avalanche formation at any time of year.<ref>{{Cite web |last=O'Neill |first=Donny |date=2021-04-12 |title=How Climate Change Impacts Avalanche Conditions |url=https://protectourwinters.org/how-does-climate-change-impact-avalanches/ |access-date=2024-04-10 |website=Protect Our Winters |language=en-US}}</ref>

Persistent cold temperatures can either prevent new snow from stabilizing or destabilize the existing snowpack. Cold air temperatures on the snow surface produce a temperature gradient in the snow, because the ground temperature at the base of the snowpack is usually around 0&nbsp;°C, and the ambient air temperature can be much colder. When a temperature gradient greater than 10&nbsp;°C change per vertical meter of snow is sustained for more than a day, angular crystals called [[depth hoar]] or facets begin forming in the snowpack because of rapid moisture transport along the temperature gradient. These angular crystals, which bond poorly to one another and the surrounding snow, often become a persistent weakness in the snowpack. When a slab lying on top of a persistent weakness is loaded by a force greater than the strength of the slab and persistent weak layer, the persistent weak layer can fail and generate an avalanche.{{fact|date=January 2024}}

Any wind stronger than a light breeze can contribute to a rapid accumulation of snow on sheltered slopes downwind. Wind slabs form quickly and, if present, weaker snow below the slab may not have time to adjust to the new load. Even on a clear day, wind can quickly load a slope with snow by blowing snow from one place to another. Top-loading occurs when wind deposits snow from the top of a slope; cross-loading occurs when wind deposits snow parallel to the slope. When a wind blows over the top of a mountain, the leeward, or downwind, side of the mountain experiences top-loading, from the top to the bottom of that lee slope. When the wind blows across a ridge that leads up the mountain, the leeward side of the ridge is subject to cross-loading. Cross-loaded wind-slabs are usually difficult to identify visually.{{fact|date=January 2024}}

Snowstorms and rainstorms are important contributors to avalanche danger. Heavy snowfall will cause instability in the existing snowpack, both because of the additional weight and because the new snow has insufficient time to bond to underlying snow layers. Rain has a similar effect. In the short term, rain causes instability because, like a heavy snowfall, it imposes an additional load on the snowpack and once rainwater seeps down through the snow, acts as a lubricant, reducing the natural friction between snow layers that holds the snowpack together. Most avalanches happen during or soon after a storm.{{fact|date=January 2024}}

Daytime exposure to sunlight will rapidly destabilize the upper layers of the snowpack if the sunlight is strong enough to melt the snow, thereby reducing its hardness. During clear nights, the snowpack can re-freeze when ambient air temperatures fall below freezing, through the process of long-wave radiative cooling, or both. Radiative heat loss occurs when the night air is significantly cooler than the snowpack, and the heat stored in the snow is re-radiated into the atmosphere.<ref>{{Cite web |title=Physical Properties of Snow |url=https://www.inscc.utah.edu/~campbell/snowdynamics/reading/Pomeroy.pdf |access-date=April 9, 2024 |website=University of Utah}}</ref>