In Europe, the avalanche risk is widely rated on the following scale, which was adopted in April 1993 to replace the earlier non-standard national schemes. Descriptions were last updated in May 2003 to enhance uniformity.

In France, most avalanche deaths occur at risk levels 3 and 4. In Switzerland most occur at levels 2 and 3. It is thought that this may be due to national differences of interpretation when assessing the risks.

Risk Level	Snow Stability	Flag	Avalanche Risk
1 - Low	Snow is generally very stable.		Avalanches are unlikely except when heavy loads are applied on a very few extreme steep slopes. Any spontaneous avalanches will be minor (sluffs). In general, safe conditions.
2 - Limited	On some steep slopes the snow is only moderately stable. Elsewhere it is very stable.		Avalanches may be triggered when heavy loads are applied, especially on a few generally identified steep slopes. Large spontaneous avalanches are not expected.
3 - Medium	On many steep slopes the snow is only moderately or weakly stable.		Avalanches may be triggered on many slopes even if only light loads are applied. On some slopes, medium or even fairly large spontaneous avalanches may occur.
4 - High	On most steep slopes the snow is not very stable.		Avalanches are likely to be triggered on many slopes even if only light loads are applied. In some places, many medium or sometimes large spontaneous avalanches are likely.
5 - Very High	The snow is generally unstable.		Even on gentle slopes, many large spontaneous avalanches are likely to occur.

[1] Stability:

• Generally described in more detail in the avalanche bulletin (regarding the altitude, aspect, type of terrain etc.)

[2] additional load:

• heavy: two or more skiers or boarders without spacing between them, a single hiker or climber, a grooming machine, avalanche blasting.
• light: a single skier or snowboarder smoothly linking turns and without falling, a group of skiers or snowboarders with a minimum 10 m gap between each person, a single person on snowshoes.

Gradient:

• gentle slopes: with an incline below about 30°.
• steep slopes: with an incline over 30°.
• very steep slopes: with an incline over 35°.
• extremely steep slopes: extreme in terms of the incline (over 40°), the terrain profile, proximity of the ridge, smoothness of underlying ground.

Professional and volunteer rescue teams respond when a victim needs more help than their companions can provide. Traditionally, organized rescue responded after companion rescue efforts failed. However, today, thanks to mobile telephones, helicopters and snow-machines, the distinction between organized and companion rescue sometimes blur together as organized rescue can respond quickly to assist companions. In a some cases in recent years, organized rescue has even replaced companion rescue and saved lives when organized rescue teams reached the debris before the victim's companions.

There are four primary goals of any rescue operation and in organized rescue the goals can be initiated simultaneously.

• Immediate search: get rescuers to the site; find and uncover buried victims.
• Medical: care for victims and companions
• Transport/evacuation: transport rescuers in quickly and safely; get victims out and to advanced medical care; return rescuers safely
• Support/Logistics: care for rescuers in the field (food, shelter, rest and replacement)

Immediate search

The first teams travel fast and light to locate and uncover buried victims. These teams carry basic rescue equipment, including rescue dogs and RECCO detectors, and emergency-care gear. These rescuers are generally not equipped for prolonged operations.

Medical

While the immediate search teams carry some basic medical equipment, a special team that can provide advanced life-support follows quickly. This team usually includes paramedic, trauma nurse, or physician, and may also transport in a rescue toboggan and other equipment needed to revive, stabilize, protect and transport their patient.

Transport/evacuation

Upon the first alert of an avalanche incident the rescue leader will appoint a team to arrange transportation for both rescuers and patients.

Support/logistics

Rescue leaders will assess the complexity of the search and rescue operation to determine and anticipate the needs for support. Every incident is different depending upon the number of victims, avalanche danger, weather conditions, terrain, access, availability of rescuers, etc. Support includes getting appropriate resources of people and equipment, transporting the resources, caring for and replacing rescuers.

In the United States all agencies are mandated to manage search and rescue operations, including avalanche, under the Incident Command System (ICS).

Survival time is short, if a victim is buried. The search for victims must start immediately; many people have died because the surviving companions or witnesses failed to do even the simplest search.

Witnesses to an avalanche that engulfs people are frequently limited to those in the party involved in the avalanche. Those not caught should try to note the locations where the avalanched person or persons were last seen. In fact, anyone planning to enter an avalanche area should discuss this step as part of their preparation. Once the avalanche has stopped and the danger of secondary slides has passed, witnesses should mark these points with objects for reference. Then, survivors should take a headcount to determine who may be lost. If the area is safe to enter, the searchers should visually scan along a downslope trajectory from the marked points last seen. Victims who are partially or shallowly buried can often be located quickly by visually scanning the avalanche debris and pulling out clothing or equipment that may be attached to someone buried.

Because survival rates plummet as time passes, do not send a searcher for help until you feel you can do no more. However, do use your mobile phone or radio to call for help as soon as you suspect a burial. Generally, the telephone connection will be better from the top of a slope than from the bottom. Go to and mark the Last Seen Area, switch transceivers to receive mode, and check them. Select likely burial areas and search them, listening for beeps (or voices), expanding to other areas of the avalanche, always looking and listening for other clues (movement, equipment, body parts). Probe randomly in probable burial areas. Mark any points where signal was received or equipment found. Continue scanning and probing near marked clues and other likely burial areas. After 30 to 60 minutes, consider sending a searcher to get more help, because at this point, the remaining victims have probably not survived.

Line probes are arranged in most likely burial areas and marked as searched. Continue searching and probing the area until it is no longer feasible or reasonable to continue. Avoid contaminating the scent of the avalanche area with urine, food, spit, blood, etc., in case search dogs arrive.

Buried victims are most likely to be found--

• Below the marked point last seen
• Along the line of flow of the avalanche
• Around trees and rocks or other obstacles
• Near the bottom runout of the debris
• Along edges of the avalanche track
• In low spots where the snow may collect (gullies, crevasses, creeks, ditches along roads, etc.)

Although less likely, check other areas if initial searches are not fruitful.

Once buried victims are found and their heads and chests are freed, perform first aid (airway, breathing, circulation/pulse, arterial bleeding, spinal injuries, fractures, shock, hypothermia, internal injuries, etc.), according to local law and custom.

Victims caught in an avalanche are advised to try to escape to side of the avalanche. If not possible, grab onto a tree, brush or rock (each second one hangs on lets snow pass by that cannot bury one). If knocked off one's feet, then jettison their equipment (if possible) and FIGHT for your life. Rolling like a log may help one escape to the side. Conventional wisdom says to make swimming motions to stay on the surface. Anecdotal stories tell successes; however, analysis of avalanche motion and physics dispute swimming. Avalanches stop quickly and if under the snow it is critical to get a hand in front of the face to create an airspace before the snow stops. If one is near the surface they may try to thrust an arm, leg or object above the surface. If it is possible, try to break free once the snow stops. If unable to move do not struggle except to enlarge the air space.

Chances of a buried victim being found alive and rescued are increased when everyone in a group is carrying and using standard avalanche equipment, and have trained in how to use it. A beacon, shovel and probe is considered the minimum equipment to carry for companion rescue. Organized rescue involves ski patrols and mountain rescue teams who are often equipped with other technologies to search for buried victims. Rescue equipment can make a difference, and in 2010 the French National Association for the Study of Snow and Avalanches (ANENA) recommended that all off-piste skiers should carry beacons, probes, shovels, and Recco reflectors.

Avalanche cords

The use of avalanche cords goes back just over 100 years to a Bavarian mountaineer named Eugen Oertel. In the United States the concept was recommended as early as 1908 in the Colorado newspaper - the Ouray Herald (November 13) - when the editor repeated the suggestion that miners in the San Juans Mounains adopt "snowslide ribbons" to safeguard their travels to and from the mines. The principle is simple. An approximately 15 meter long red cord (similar to parachute cord) is attached to the person in question's belt. While skiing, snowboarding, or walking the cord is dragged along behind the person. The assumption is that if the person gets buried in an avalanche, the light cord stays on top of the snow. Due to the color the cord would be easily visible for companions. Commercial avalanche cords have metal markings every one to three meters indicating the direction and length to the victim.

Avalanche cords were popular before beacons became available, and while cords were thought to be effective markers there was never any proof of their effectiveness. In the 1970s Melchoir Schild of the Swiss Federal Institute for Snow and Avalanche Research (SLF) reviewed 30 years of Swiss avalanche accidents and rescues from 1944/45 to 1973/74. Of the 2042 avalanche victims he found only seven cases where avalanche cords were used (not including the 2 mentioned above). In five cases part of the cord was visible on the surface but so too were part of the victims. In the sixth case the victim was completely buried, but part of the cord was visible. Sadly, this victim died of trauma. In the seventh case the completely buried avalanche cord was located by an avalanche rescue dog, however, the cord had become detached from the victim. Her body was found much later. In Knox Williams' and Betsy Armstrong's 1986 book The Avalanche Book they cite an early 1970s study where avalanche cords were tested on sandbag dummies. The dummies were placed onto steep slopes where explosives were used to trigger avalanches. Trials showed a portion of the cord remained on the surface only 40% of the time. The other 60% of the time the cord was completely buried along with the dummy. Typically the cord had spooled around the dummy. 1975, at a symposium of avalanche rescue experts hosted by the International Foundation Vanni Eigenmann, Schild concluded, “On the basis of these results the avalanche cord can no longer be considered reliable.”

In the United States there have been two accidents with five buried victims, all wearing avalanche cords. In one accident, an avalanche cord remained on the surface. In the second accident five ski mountaineers with cords deployed triggered an avalanche. One skier was partly buried, but his four friends and cords were completely buried. Moderate snow and the loss of the survivor’s eyeglasses only worsened the situation. The search was called off a few days later. The four were eventually found many months later after their bodies with attached cords melted out of the snow. On one victim the cord was wrapped tightly around the body.

The avalanche cord should not be used or accepted as being an adequate sole safety measure.

Beacons

Beacons — known as "beepers", peeps (pieps), ARVAs (Appareil de Recherche de Victimes en Avalanche, in French), LVS (Lawinen-Verschütteten-Suchgerät, Swiss German), avalanche transceivers, or various other trade names, are important for every member of the party. They emit a "beep" via 457 kHz radio signal in normal use, but may be switched to receive mode to locate a buried victim up to 80 meters away. Analog receivers provide audible beeps that rescuers interpret to estimate distance to a victim. To use the receiver effectively requires regular practice. Some older models of beepers operated on a different frequency (2.275 kHz ) and a group leader should ensure these are no longer in use.

Since about 2000 nearly all avalanche rescue transceivers use digital displays to give visual indications of direction and distance to victims. Most users find these beacons easier to use, but to be effective still requires considerable practice by the user. Beacons are the primary rescue tool for companion rescue and are considered active devices because the user must learn to use and care for their device.

Probes

Portable (collapsible) probes can be extended to probe into the snow to locate the exact location of a victim at several yards / metres in depth. When multiple victims are buried, probes should be used to decide the order of rescue, with the shallowest being dug out first since they have the greatest chance of survival.

Probing can be a very time-consuming process if a thorough search is undertaken for a victim without a beacon. In the U.S., 86% of the 140 victims found (since 1950) by probing were already dead. Survival/rescue more than 2 m deep is rare (about 4%). Probes should be used immediately after a visual search for surface clues, in coordination with the beacon search.

Shovels

Even when the snowpack consists of loose powder, avalanche debris is hard and dense. The energy of the avalanche causes the snow to melt, and the debris refreezes immediately after it stops.

Shovels are essential for digging through the snow to the victim, as the snow is often too dense to dig with hands or skis. A large strong scoop and sturdy handle are important. Plastic shovels often break, whereas metal ones are less prone to failure.

As excavation of the avalanche victim is extremely time-consuming and many buried victims suffocate before they can be reached, shovelling technique is an essential element of rescue.

Shovels are also useful for digging snow pits as part of evaluating the snow pack for hidden hazards, such as weak layers supporting large loads.

RECCO Rescue System

The Recco system is used by organized rescue services around the world. The Recco system is a two-part system where the rescue team uses a small hand-held detector. The detector receives a directional signal that is reflected back from a small, passive, transponder called a reflector that is included into outerwear, boots, helmets, and body protection. Recco reflectors are not a substitute for avalanche beacons. The Recco signal does not interfere with beacons. In fact, the current Recco detector also has an avalanche beacon receiver (457 kHz) so one rescuer can search for a Recco signal and a beacon signal at the same time.

Avalung

Recently, a device called an Avalung has been introduced for use in avalanche terrain. The device consists of a mouth piece, a flap valve, an exhaust pipe, and an air collector. Several models of Avalung either mount on one's chest or integrate in a proprietary backpack.

During an avalanche burial, victims not killed by trauma usually suffer from asphyxiation as the snow around them melts from the heat of the victim's breath and then refreezes, disallowing oxygen flow to the victim and allowing toxic levels of CO₂ to accumulate. The Avalung ameliorates this situation by drawing breath over a large surface area in front and pushing the warm exhaled carbon dioxide behind. This buys additional time for rescuers to dig the victim out.

Avalanche airbags

Avalanche airbags help a person avoid burial by making the user an even larger object relative to the moving snow, which forces the person toward the surface. Avalanche airbags work on the principle of inverse segregation. Avalanches, like mixed nuts and breakfast cereal are considered granular materials and behave fluid-like (but are not liquids) where smaller particles settle to the bottom of the flow and larger particles rise to the top. Provided the airbag is properly deployed, the chances of a complete burial are significantly reduced.

Other devices

More backcountry adventurers are also carrying Satellite Electronic Notification Devices (SEND) to quickly alert rescuers to a problem. These devices include the SPOT Messenger, Emergency Position-Indicating Radio Beacon (EPIRB) or Personal Locating Beacons (PLBs) containing the Global Positioning System (GPS). This device can quickly notify search and rescue of an emergency and the general location (within 100 yards), but only if the person with the EPIRB has survived the avalanche and can activate the device. Survivors should also try to use a mobile phone to notify emergency personnel. Unlike the other devices mentioned above, the mobile phone (or satellite phone) provides two-way communications with rescuers.

On-site rescuers (usually companions) are in the best position to save a buried victim. However, organized rescue teams can sometimes respond very quickly to assist in the search for a buried victim. The sooner organized rescue can be notified the sooner they can respond, and this difference can mean the difference in living or dying for a critically injured patient. The International Commission for Alpine Rescue recommends, “early notification is essential, e.g., by mobile phone, satellite phone, or radio, wherever possible”

Other rescue devices are proposed, developed and used, such as avalanche balls, vests and airbags, based on statistics indicating that decreasing the depth of burial increases the chances of survival.

Although inefficient, some rescue equipment can be improvised by unprepared parties: ski poles can become short probes, skis or snowboards can be used as shovels. A first aid kit and equipment is useful for assisting survivors who may have cuts, broken bones, or other injuries, in addition to hypothermia.

Safety in avalanche terrain

• Terrain management - Terrain management involves reducing the exposure of an individual to the risks of traveling in avalanche terrain by carefully selecting what areas of slopes to travel on. Features to be cognizant of include not under cutting slopes (removing the physical support of the snow pack), not traveling over convex rolls (areas where the snow pack is under tension), staying away from weaknesses like exposed rock, and avoiding areas of slopes that expose one to terrain traps (gulleys that can be filled in, cliffs over which one can be swept, or heavy timber into which one can be carried).

• Group management - Group management is the practice of reducing the risk of having a member of a group, or a whole group involved in an avalanche. Minimize the number of people on the slope, and maintain separation. Ideally one person should pass over the slope into an area protected from the avalanche hazard before the next one leaves protective cover. Route selection should also consider what dangers lie above and below the route, and the consequences of an unexpected avalanche (i.e., unlikely to occur, but deadly if it does). Stop or camp only in safe locations. Wear warm gear to delay hypothermia if buried. Plan escape routes. In determining the size of the group balance the hazard of not having enough people to effectively carry out a rescue with the risk of having too many members of the group to safely manage the risks. It is generally recommended not to travel alone, because there will be no-one to witness your burial and start the rescue. Additionally, avalanche risk increases with use; that is, the more a slope is disturbed by skiers, the more likely it is that an avalanche will occur. Most important of all practice good communication with in a group including clearly communicating the decisions about safe locations, escape routes, and slope choices, and having a clear understanding of every members skills in snow travel, avalanche rescue, and route finding.

• Risk Factor Awareness - Risk factor awareness in avalanche safety requires gathering and accounting for a wide range of information such as the meteorological history of the area, the current weather and snow conditions, and equally important the social and physical indicators of the group.

• Leadership - Leadership in avalanche terrain requires well defined decision making protocols that use the observed risk factors. These decision making frameworks are taught in a variety of courses provided by national avalanche resource centers in Europe and North America. Fundamental to leadership in avalanche terrain is honestly assessing and estimating the information that was ignored or overlooked. Recent research has shown that there are strong psychological and group dynamic determinants that lead to avalanche involvement.

Preventative measures are employed in areas where avalanches pose a significant threat to people, such as ski resorts and mountain towns, roads and railways. There are several ways to prevent avalanches and lessen their power and destruction; active preventative measures reduce the likelihood and size of avalanches by disrupting the structure of the snow pack; passive measures reinforce and stabilize the snow pack in situ. The simplest active measure is by repeatedly traveling on a snow pack as snow accumulates; this can be by means of boot-packing, ski-cutting, or machine grooming. Explosives are used extensively to prevent avalanches, by triggering smaller avalanches that break down instabilities in the snow pack, and removing over burden that can result in larger avalanches. Explosive charges are delivered by a number of methods including hand tossed charges, helicopter dropped bombs, Gazex concussion lines, and ballistic projectiles launched by air cannons and artillery. Passive preventive systems such as Snow fences and light walls can be used to direct the placement of snow. Snow builds up around the fence, especially the side that faces the prevailing winds. Downwind of the fence, snow buildup is lessened. This is caused by the loss of snow at the fence that would have been deposited and the pickup of the snow that is already there by the wind, which was depleted of snow at the fence. When there is a sufficient density of trees, they can greatly reduce the strength of avalanches. They hold snow in place and when there is an avalanche, the impact of the snow against the trees slows it down. Trees can either be planted or they can be conserved, such as in the building of a ski resort, to reduce the strength of avalanches.

To mitigate the effect of avalanches, artificial barriers can be very effective in reducing avalanche damage. There are several types. One kind of barrier (snow net) uses a net strung between poles that are anchored by guy wires in addition to their foundations. These barriers are similar to those used for rockslides. Another type of barrier is a rigid fence like structure (snow fence) and may be constructed of steel, wood or pre-stressed concrete. They usually have gaps between the beams and are built perpendicular to the slope, with reinforcing beams on the downhill side. Rigid barriers are often considered unsightly, especially when many rows must be built. They are also expensive and vulnerable to damage from falling rocks in the warmer months. In addition to industrially manufactured barriers, landscaped barriers, called avalanche dams stop or deflect avalanches with their weight and strength. These barriers are made out of concrete, rocks or earth. They are usually placed right above the structure, road or railway that they are trying to protect, although they can also be used to channel avalanches into other barriers. Occasionally, earth mounds are placed in the avalanche's path to slow it down. Finally, along transportation corridors, large shelters, called snow sheds, can be built directly in the slide path of an avalanche to protect traffic from avalanches.

When an avalanche occurs, as the snow slides down the slope any slab present begins to fragment into increasingly smaller tumbling fragments. If the fragments become small enough the avalanche takes on the characteristics of a fluid. When sufficiently fine particles are present they can become airborne and, given a sufficient quantity of airborne snow, this portion of the avalanche can become separated from the bulk of the avalanche and travel a greater distance as a powder snow avalanche. Scientific studies using radar, following the 1999 Galtür avalanche disaster, confirmed suspicions that a saltation layer forms between the surface and the airborne components of an avalanche, which can also separate from the bulk of the avalanche.

Driving a (non-airborne) avalanche is the component of the avalanche's weight parallel to the slope; as the avalanche progresses any unstable snow in its path will tend to become incorporated, so increasing the overall weight. This force will increase as the steepness of the slope increases, and diminish as the slope flattens. Resisting this are a number of components that are thought to interact with each other: the friction between the avalanche and the surface beneath; friction between the air and snow within the fluid; fluid-dynamic drag at the leading edge of the avalanche; shear resistance between the avalanche and the air through which it is passing, and shear resistance between the fragments within the avalanche itself. An avalanche will continue to accelerate until the resistance exceeds the forward force.

Avalanche Modelling

Attempts to model avalanche behaviour date from the early 20th century, notably the work of Professor Lagotala in preparation for the 1924 Winter Olympics in Chamonix. His method was developed by A. Voellmy and popularised following the publication in 1955 of his Ueber die Zerstoerungskraft von Lawinen (On the Destructive Force of Avalanches).

Voellmy used a simple empirical formula, treating an avalanche as a sliding block of snow moving with a drag force that was proportional to the square of the speed of its flow:

He and others subsequently derived other formulae that take other factors into account, with the Voellmy-Salm-Gubler and the Perla-Cheng-McClung models becoming most widely used as simple tools to model flowing (as opposed to powder snow) avalanches.

Since the 1990s many more sophisticated models have been developed. In Europe much of the recent work was carried out as part of the SATSIE (Avalanche Studies and Model Validation in Europe) research project supported by the European Commission which produced the leading-edge MN2L model, now in use with the Service Réstitution Terrains en Montagne (Mountain Rescue Service) in France, and D2FRAM (Dynamical Two-Flow-Regime Avalanche Model), which was still undergoing validation as of 2007.

Avalanches are always caused by an external stress on the snow pack; they are not random or spontaneous events. Natural triggers of avalanches include additional precipitation, radiative and convective heating, rock fall, ice fall, and other sudden impacts; however, even a snow pack held at a constant temperature, pressure, and humidity will evolve over time and develop stresses, often from the downslope creep of the snow pack. Human triggers of avalanches include skiers, snowmobiles, and controlled explosive work. The triggering stress load can be either localized to the failure point, or remote. Localized triggers of avalanches are typified by point releases from solar heated rocks. Remotely triggered avalanches occur when a tensile stress wave is transmitted through the slab to the start zone, once the stress wave reaches the start zone a fracture initiates and propagates the failure. Of exceptional note is that avalanches can not only entrain additional snow within the failing slab, but can also, given the sufficient accumulation of overburden due to a smaller avalanche, step down and trigger deeper slab instabilities that would be more resilient against smaller stresses. The triggering of avalanches is an example of critical phenomenon.

Avalanches can only occur in a standing snow pack. Typically winter seasons and high altitudes have weather that is sufficiently unsettled and cold enough for precipitated snow to accumulate into a snow pack. The evolution of the snow pack is critically sensitive to small variations within the narrow range of meteorological conditions that allow for the accumulation of snow into a snow pack. Among the critical factors controlling snow pack evolution are: heating by the sun, radiational cooling, vertical temperature gradients in standing snow, snowfall amounts, and snow types. Generally, mild winter weather will promote the settlement and stabilization of the snow pack; and conversely very cold, windy, or hot weather will weaken the snow pack.

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 snow pack 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 a slope to avalanche, especially in the spring.

Persistent cold temperatures can either prevent the snow from stabilizing or destabilize a snow pack. Cold air temperatures on the snow surface produce a temperature gradient in the snow, because the ground temperature at the base of the snow pack is close to freezing; unless the snow pack is standing on glaciated terrain, in which case the temperature at the base of the snow pack can be significantly below freezing. When a temperature gradient greater than 10^oC change per vertical meter of snow is sustained for more than a day depth hoar will form in the snow pack, through the thermal transport of moisture away from the depth hoar along the temperature gradient, from bottom to top. This layer of depth hoar becomes a persistent weakness in the snow pack, characterized by faceted grains forming either above or below crusts and slabs. When a slab lying on top of this persistent weakness is loaded by a force above the tensile and ductile strength of the slab and the shear strength of the persistent weak layer, the persistent weak layer will fail and generate an avalanche.

Any wind stronger than a light breeze can contribute to a rapid accumulation of snow on sheltered slopes downwind. Wind pressure at a favorable angle can stabilize other slopes. A "wind slab" is a particularly fragile and brittle structure which is heavily loaded and poorly bonded to its underlayment. Even on a clear day, wind can quickly shift the snow load on a slope. This can occur in two ways: by top-loading and by cross-loading. Top-loading occurs when wind deposits snow perpendicular to the fall-line on a slope; cross-loading occurs when wind deposits snow parallel to the fall-line. 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.

Snowstorms and rainstorms are important contributors to avalanche danger. Heavy snowfall will cause instability in the existing snow pack, 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 snow pack; and, once rainwater seeps down through the snow, it acts as a lubricant, reducing the natural friction between snow layers that holds the snow pack together. Most avalanches happen during or soon after a storm.

Daytime exposure to sunlight will rapidly destabilize the upper layers of a snow pack. Sunlight reduces the sintering, or necking, between snow grains. During clear nights, the snow pack can strengthen, or tighten, through the process of long-wave radiative cooling. When the night air is significantly cooler than the snow pack, the heat stored in the snow is re-radiated into the atmosphere.

The snow pack is composed of deposition layers of snow that are accumulated over time. The deposition layers are stratified parallel to the ground surface on which the snow falls. Each deposition layer indicates a distinct meteorological condition during which the snow was accumulated. Once deposited a snow layer will continue to evolve and develop under the influence of the meteorological conditions that prevail after deposition.

For an avalanche to occur, it is necessary that a snow pack have a weak layer (or instability) below a slab of cohesive snow. In practice the mechanical and structural determinants of snow pack stability 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 proxy measurements of the mechanical properties of the snow (e.g. tensile strength, friction coefficients, shear strength, and ductile strength). This results in two principal sources of uncertainty in determining snow pack stability based on snow structure: First, both the factors influencing snow stability and the specific characteristics of the snow pack vary widely within small areas and time scales, resulting in an inability to extrapolate point observations of snow layers. Second, the understanding of the relationship between the readily observable snow pack characteristics and the snow pack's critical mechanical properties has not been completely developed.

While the deterministic relationship between snow pack characteristics and snow pack 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. Shallower snow, that can lie above or around boulders, plants, and other discontinuities in the slope, will weaken from the presence of a stronger temperature gradient. Larger and more angular snow crystals are an indicator of weaker bonds within the snow pack, because the sintering process that forms bonds within the snow pack will also cause the snow crystals to become smaller and rounder. Consolidated snow is less likely to slough than either loose powdery layers or wet isothermal snow; however, consolidated snow is a necessary condition for the occurrence of slab avalanches, and will mask persistent instabilities within a snow pack. The empirical understanding of the factors influencing snow stability only places broad predictive bounds on the stability of the snow, consequently a conservative use of avalanche terrain, well within the recommended guidelines of the local avalanche forecasts and bulletins, is always recommended.

Terrain affects avalanche occurrence and development through three factors: First, terrain affects the evolution of the snow pack by determining the meteorological exposure of the snow pack. Second, terrain affects the stability of the snow pack, through the geometry and ground composition of the slope. Third, the down slope features of the terrain affects the path and consequences of a flowing avalanche.

For a slope to generate an avalanche it must be simultaneously capable of retaining snow, and allowing snow to accelerate once set in motion. The angle of the slope that can hold snow depends on the ductile and shear strength of the snow, which is determined by the temperature and moisture content of the snow. Drier and colder snow, with lower ductile and shear strength, will only bond to lower angle slopes; while wet and warm snow, with higher ductile and shear strength, can bound to very steep surfaces. In particular, in coastal mountains, such as the Cordillera del Paine region of Patagonia, deep snow packs collect on vertical, and overhanging, rock faces. The angle of slope that can allow moving snow to accelerate depends on the shear strength of the snow. Snow that has been water saturated to the point of slush can accelerate on shallow angled terrain; while a cohesive snow pack will not accelerate on very steep slopes, such as the typical snow pack in the Chugach Mountains of Alaska.

The snow pack on slopes with sunny exposures are strongly influenced by sunshine. Daily cycles of mild thawing and refreezing can stabilize the snow pack by promoting settlement, strong freeze thaw cycles will result in the formation of surface crusts during the night, and the formation of unstable isothermal snow during the day. Slopes in the lee of a ridge or other wind obstacle accumulate more snow and are more likely to include pockets of abnormally deep snow, wind slabs, and cornices, all of which, when disturbed, may trigger an avalanche. Conversely a windward slope will be bare of snow.

The start zone of an avalanche must be steep enough to allow snow to accelerate once set in motion, additionally convex slopes are less stable than 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 snow pack influences the stability of the snow pack, either being a source of strength or weakness. Thickly forested areas are unlikely to be avalanche paths, however boulders and sparsely distributed vegetation can create weak areas deep within the snow pack, 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 cover, such as grass or rock slabs.

Avalanches follow drainages down slope, frequently sharing drainage features with summertime watersheds. At and below tree line these drainages are well defined by vegetation boundaries where the avalanches have prevented the growth 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 degrees or steeper than 60 degrees typically have a lower incidence of avalanche involvement, likewise slopes with windward and sunny exposure have a lower incidence of avalanche involvement . Human triggered avalanches have the greatest incidence when the snow's angle of repose is between 35 and 45 degrees; the critical angle, the angle at which the human incidence of avalanches is greatest, is 38 degrees. But when the incidence of human triggered avalanches are normalized by the rates of recreational use hazard increases uniformly with slope angle, and no significant difference in hazard for a given exposure direction can be found. 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.

All avalanches share common elements: a trigger which causes the avalanche, a start zone from which the avalanche originates, a slide path along which the avalanche flows, a run out where the avalanche comes to rest, and a debris deposit which is the accumulated mass of the avalanched snow once it has come to rest. As well avalanches have a failure layer that propagates the failure and the bed surface along which the snow initially slides, in most avalanches the failure layer and the bed surface are the same. Additionally slab avalanches have a crown fracture at the top of the start zone, flank fractures on the sides of the start zones, and a shallow staunch fracture at the bottom of the start zone. 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.

The nature of the failure of the snow pack is used to morphologically classify the avalanche. Slab avalanches are generated when an additional load causes a brittle failure of a slab that is bridging a weak snow layer; this failure is propagated through fracture formation in the bridging slab. Loose snow, point release, and isothermal avalanches are generated when a stress causes a shear failure in a weak interface, either within the snow pack, or at the base. When the failure occurs at the base they are known as full depth avalanches. Spin drift avalanches occur when wind lifted snow is funneled into a steep drainage from above the drainage.

Loose snow avalanches occur in freshly fallen snow that has a lower density and are most common on steeper terrain. In fresh, loose snow the release is usually at a point and the avalanche then gradually widens down the slope as more snow is entrained, usually forming a teardrop appearance. This is in contrast to a slab avalanche.

Slab avalanches account for around 90% of avalanche-related fatalities, and occur when there is a strong, cohesive layer of snow known as a slab. These are usually formed when falling snow is deposited by the wind on a lee slope, or when loose ground snow is transported elsewhere. When there is a failure in a weak layer, a fracture very rapidly propagates so that a large area, that can be hundreds of meters in extent and several meters thick, starts moving almost instantaneously.

A third starting type is a wet snow avalanche or isothermal avalanche, which occurs when the snow pack becomes saturated by water. These tend to also start and spread out from a point. When the percentage of water is very high they are known as slush flows and they can move on very shallow slopes.

Among the largest and most powerful of avalanches, powder snow avalanches can exceed speeds of 300 km/h, and masses of 10,000,000 tonnes; their flows can travel long distances along flat valley bottoms and even up hill for short distances. A powder snow avalanche is a powder cloud that forms when an avalanche accelerates over an abrupt change in slope, such as a cliff band, causing the snow to mix with air. This turbulent suspension of snow particles then flows as a gravity current.

An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the descending snow. Powerful avalanches have the capability to entrain ice, rocks, trees, and other material on the slope. Avalanches are primarily composed of flowing snow, and are distinct from mudslides, rock slides, and serac collapses on an icefall. In contrast to other natural events which can cause disasters, avalanches are not rare or random events and are endemic to any mountain range that accumulates a standing snow pack. In mountainous terrain avalanches are among the most serious objective hazards to life and property, with their destructive capability resulting from their potential to carry an enormous mass of snow rapidly over large distances.

Avalanches are classified by their morphological characteristics, and are rated by either their destructive potential, or the mass of the downward flowing snow. Some of the morphological characteristics used to classify avalanches include the type of snow involved, the nature of the failure, the sliding surface, the propagation mechanism of the failure, the trigger of the avalanche, the slope angle, direction, and elevation. Avalanche size, mass, and destructive potential are rated on a logarithmic scale, typically of 5 categories, with the precise definition of the categories depending on the observation system or forecast region.