There are several classifications of avalanches, which are based on different characteristics: type of snow (loose or dense), water content in the snow, nature of movement, sliding surface, morphology of the path.
However general classification avalanche information should reflect their most essential features and serve the practical purposes of organizing avalanche protection. These requirements are best met by two approaches to dividing avalanches into main types. The first is genetic - based on taking into account the causes of avalanches, which were discussed above; its value lies in the possibility of developing a forecast for the onset of avalanche danger. The second approach is based on taking into account the topography of the snow collection basin and the path of the avalanche. This principle of dividing avalanche devices allows one to calculate the volumes and ranges of avalanches, i.e., it is necessary when mapping avalanche-prone areas. In this tutorial we will look at the first approach to classifying avalanches.
Genetic classification avalanches, most fully developed by the Soviet researcher V.N. Akkuratov, includes the following classes and types of avalanches.
I. Class of dry (cold) avalanches.
Such avalanches usually consist of dry snow; disappear mainly in winter; The escape routes are not strictly limited - they can descend along a flat slope and partially through the air. They have maximum speed and can form an air wave. The following types of avalanches belong to the dry class:
1. Avalanches from freshly fallen snow. Such avalanches occur due to overloading of slopes during prolonged snowfalls. For avalanches, 0.3-0.5 m of fresh snow is enough. In snowy areas of temperate climates, this type of avalanche is the main one.
2. Avalanches of blizzard snow. The reason for their occurrence is the high growth rate of the gravity component on the slope. This is the most typical type of avalanche for areas with a moderately cold climate and stormy wind conditions.
3. Avalanches associated with recrystallization of snow and the formation of layers of deep frost (the adhesion forces in which are weakened). Usually rare but powerful avalanches.
4. Avalanches of temperature reduction of snow cover. These avalanches occur as a result of a sharp drop in air temperature. Also a rare type of avalanche.
II. Class of wet (warm) avalanches.
Such avalanches are formed from wet or wet snow; they disappear mainly in the spring; the escape paths are usually constant; movement is carried out along the lower horizons of snow or on the ground; the movement speed is lower than that of dry avalanches; the impact is mainly due to the pressure of heavy (water-saturated) snow masses.
1. Avalanches resulting from radiation thaws. These are low-power avalanches of southern (sunny) slopes.
2. Avalanches associated with thaws and spring snowmelt usually consist of wet, less often wet snow. The sliding surface is usually the interface between snow layers, i.e. avalanches belong to the category of reservoir avalanches.
3. Ground avalanches are formed in the spring from wet snow completely saturated with water, as a result of prolonged thaws and rains or during rapid snow melting during hair dryers. They always go along certain paths, therefore, as a rule, they have names. They transport significant amounts of debris. The inhabitants of the Alps call the roar of these avalanches “avalanche thunder.” The most destructive in the class of wet avalanches.
Avalanches are one of the most widespread and dangerous natural phenomena mountainous countries. Mentions of avalanches are found in the writings of ancient writers who lived more than 2000 years ago. The ancient Greek historian Polybius (201 -120 BC) writes about losses from avalanches when Hannibal's troops crossed the Alps (218 BC). The ancient Roman geographer Strabo (63 BC - 20 AD) wrote about the avalanche danger that awaits a traveler in the Alps and the Caucasus.
In January 1951, the entire Alpine mountain range, about 700 km long and up to 150 km wide, was in the zone of avalanche disasters. Snowfall, accompanied by blizzards, continued in many areas for seven days and ended with a sharp warming. The amount of snow that fell in some places exceeded the annual precipitation norm by 2-3 times and reached 2-3 m. The slopes were overloaded with snow, and massive avalanches began. The entire transport network of the Alps was disrupted - highways and railways were in some places destroyed or littered and temporarily closed. Avalanches occurred in places where many generations of residents had not known them. Hotel buildings and protected forests were destroyed. The season was called "Winter of Terror".
In February 1999, an avalanche weighing 170 thousand tons completely destroyed the village of Galtur in Austria, causing the death of 30 people, and in early March 2012, a series of avalanches in Afghanistan destroyed residential buildings, causing the death of at least 100 people.
In Russia, snow avalanches are common in the mountainous regions of the Caucasus, the Urals, Eastern and Western Siberia, Far East, on Sakhalin.
Nowadays, many countries have accumulated significant experience in avalanche protection.
A set of anti-avalanche measures consists of two main categories - preventive and engineering.
Preventive actions boil down to warning about avalanche danger and its elimination by artificial dumping. To prevent avalanche danger, maps of avalanche zones and avalanche time forecasts are compiled.
Preventive measures also include warning the population about the onset of avalanche periods.
Artificial avalanches are carried out by mortars or by detonating the avalanche catchment area with explosives. Avalanche collections are also fired for control purposes, to check the stability of the snow on the slope.
Engineering activities They are usually used to protect populated areas and permanent structures from avalanches. For this purpose, tunnels, galleries, and canopies are built. Typically, these structures are used to cover certain areas on railways and highways passing through the mountains.
For many years, structures have been erected that change the path of an avalanche, reducing the speed and range of the release - avalanche cutters, wedges, guide walls, wallpaper dams, etc.
They partially extinguish the avalanche energy or divert it away from the protected object. Engineering methods such as terracing and building slopes with snow-retaining shields are also often practiced. They prevent snow from sliding out of avalanche catchment areas. It's expensive, but effective method avalanche control. The protection and restoration of forests on mountain slopes is still considered one of the most important measures in avalanche-prone areas. In the Alps, a forest destroyed by an avalanche is immediately restored. Forest planting is usually combined with the construction of slopes with snow-retaining structures.
Dense forest provides natural protection against avalanches. It prevents the redistribution of snow by the wind and divides the snow cover into separate areas. In Switzerland, a law prohibiting logging on mountain slopes has existed since the 14th century. Destruction of forests on mountain slopes always stimulates avalanche activity.
Mudflows
Mudflow is a rapid mud or mud-stone flow, consisting of a mixture of water and rock fragments, that suddenly appears in the basins of small mountain rivers. Mudflows pose a threat settlements, railways and roads and other structures located on their way.
The immediate causes of mudflows are rainfall, intense snow melting, outburst of reservoirs, and, less commonly, earthquakes and volcanic eruptions.
Avalanches. Every year, many people die under them, either because they ignore the danger or because they know little about avalanches.
Many of us don't take the threat from avalanches seriously until someone is killed or injured in one. The sad fact is that people caught in an avalanche usually provoke it themselves. Skiers cut slopes, climbers walk in avalanche times. Moreover, the victims are often professionals in their field, but they neglect the avalanche danger. This article provides basic knowledge about avalanches.
Avalanches.
Potential threats
An avalanche can move at a speed of 200 kilometers per hour. Such a force can smear you against trees and rocks, grind you into rocks, make a mess of your insides and impale you on your own skis or snowboard. About one third of all avalanche victims die due to injury.
If you were not injured by an avalanche, you will be struggling with a mass of snow as dense as concrete, which squeezes your body. An avalanche, which begins as snow dust, heats up from friction with the slope as it moves down, melts a little and then freezes tightly around your body. All this mass is enough to squeeze all the air out of your lungs.
If you manage to create an air pocket around you before the snow settles, you have a good chance of survival. If you and your friends have an avalanche transmitter and know how to use it, then your chances of survival are even greater. However, this is where the race against time begins. Most people are unable to survive an avalanche for more than 30 minutes (Black Diamond AvaLung backpacks can extend that time to up to one hour), so it makes sense to purchase and learn how to use avalanche transmitters. A must-have item for winter freeride enthusiasts. About 70% of avalanche victims die from asphyxiation.
The best protection against avalanches is, of course, knowledge of avalanche conditions and slopes, and avoidance of dangerous situations.
Loose avalanches.
Such avalanches form when there is little or no grip on the snow cover. As a rule, such avalanches begin from one point either on the surface of the slope or close to it. Such avalanches gain greater snow mass and momentum while moving down the slope, often forming a triangular-shaped path behind them. The causes of such avalanches can be blocks of snow falling onto the slope from the cliffs above or melting snow cover.
Such avalanches occur on dry and wet snow and occur both in winter and summer. Winter loose avalanches usually occur during or after a snowfall. In warmer seasons, wet, loose avalanches are caused by snow or melt water. These avalanches are dangerous both in winter and summer.
Reservoir avalanches.
These avalanches pose much more danger. Sheet avalanches form when one layer of snow slides off the bottom layer and rushes down the slope. Most freeriders end up in such avalanches.
They are caused by snowfalls and strong winds, which deposit layers of snow that change over time. Some layers are tracked and held together, while others, on the contrary, are weakened. Weak layers are often grainy or very light snow (powder) so that other layers cannot grip them.
An avalanche occurs when the top layer, called the “plank,” is not sufficiently bonded to the underlying layer and is set in motion by some external agent, usually a skier or climber. Unlike loose avalanches, which start from a single point, sheet avalanches increase in depth and width, usually along a separation line at the top of the slope.
Avalanche release on Cheget:
Factors contributing to avalanches.
Terrain.
Slope steepness: Pay attention to the steepness of the slope when you are skiing or climbing. Avalanches often occur on slopes steeper than 30-45 degrees.
Slope side: In winter, southern slopes are much more stable than northern slopes, as the Sun melts and compacts the snow. Unstable layers of “deep rime,” dry, icy snow that does not adhere to adjacent layers, are most often located on northern slopes. Therefore, be vigilant when you see a tempting northern slope with excellent powder, because they are more dangerous than the southern slopes, due to the fact that they do not get enough solar heat, which will compact the snow over the winter. At the same time, in spring and summer, southern slopes melt more, which leads to dangerous wet avalanches. Warmer weather at this time of year hardens the snow on northern slopes, making them safer.
Terrain hazards: Snow cover is most often unstable on convex slopes, rocky outcroppings, boulders or trees where the snow cover is interrupted, leeward slopes or under eaves. It is best to avoid bowls, circuses and pits where snow can accumulate after an avalanche (avalanche discharges). Steep, narrow couloirs (or gullies) tend to accumulate a lot of snow and pose a huge danger to hikers and skiers who get caught in them. Often, it is impossible to get out of such places due to the steep side slopes, so in the event of an avalanche there is nowhere to run.
Weather
Precipitation: snow is least stable after snowfalls or rains. A large amount of snow falling in a short period of time is a sign of avalanche danger. Heavy snowfall, especially wet or dense snow falling on powder, creates unstable layers in the snowpack. Rain seeps through and heats the lower layers of the snowpack and also reduces friction between the layers, making them less stable. After heavy snowfall, you should wait at least two days before going to avalanche areas.
Wind: Another indicator of snow cover instability is wind. Often strong winds carry surface snow from one slope to another part of the ridge, where the snow falls down, forming an avalanche. Pay attention to the intensity and direction of the wind throughout the day.
Temperature: A large number of problems with snow cover are caused by temperature fluctuations. The formation of snow crystals can vary due to temperature differences between the surface and overlying layers, different layers in the center of the cover, and even between the air temperature and the upper snow layer. A particularly dangerous snow crystal, due to its inability to bond with other crystals, is “frost.”
Deep frost ("sugar snow"), due to its similarity to granulated sugar, can be located at any depth or several depths of deep snow cover. Often a sharp increase in temperature leads to wet avalanches, especially in the spring, so be careful when it gets warm in the mountains.
Snow cover
Snowfalls occur one after another throughout the winter. Temperature changes cause metamorphosis of snow crystals. If the composition of the snow remains the same, then the snow cover is uniform and stable. Snow becomes dangerous and unstable when layers of different types of snow form within the snowpack. To every freerider it is imperative to check snow layers for stability, especially on slopes of 30-45 degrees.
How to test a slope for avalanche danger:
Human factor
While terrain, weather and snow cover play a big role in triggering avalanches, it is important to remember that ego, emotions and herd mentality can seriously cloud your judgment and lead you to make rash decisions. In fact, in a recent survey of Canadian avalanche experts, respondents cited 'human error' and 'poor terrain selection' as the main causes of avalanche accidents. Most avalanches are caused by people!
Typical mistakes when making decisions:
- Familiar places: It is most likely that you will take risks in a place that is familiar to you. Conditions, however, can change from minute to minute, so treat any terrain as if you were seeing it for the first time.
- OK: encouragement from a group can put a lot of pressure on you. “Everything will be fine, relax!” Even if you feel that something is wrong, in order to please the group you may take unnecessary risks.
- Get to the place at any cost: if you want to get to your destination too much, you may act contrary to your common sense and ignore signs of danger, concentrating only on your goals. Foreign climbers call this phenomenon “summit fever.”
- "We have an expert with us": You imply that there is someone else in your group with more experience than you. Do you think so based on the fact that this person was in this place before you or he went through some special training. It's better to ask than to guess.
- Existing trails: you can feel safe because you see a well-trodden path ahead of you. In our mountains, I was once walking along a seemingly excellent path, but I felt that the slope under the path was very unreliable. Just because someone has been here before you doesn't mean it's safe to walk here.
- "Virgin Fever": You can turn a blind eye to the signs of avalanche danger when there is fresh, deep and untouched snow in front of you. Don't give in to temptation!
- “Others have passed!”: It is very easy to succumb to the “herd instinct” and go on a dangerous slope when other people have already passed in front of you. Always assess the situation as if you are alone. Tell me if you feel something is wrong.
Avalanches form when there is sufficient snow accumulation and on treeless slopes with a steepness of 15 to 50°. At a slope of more than 50°, the snow simply falls off, and conditions for the formation of a snow mass do not arise. Optimal situations for avalanches occur on snow-covered slopes with a steepness of 30 to 40°. There, avalanches occur when the layer of freshly fallen snow reaches 30 cm, and old (stay) snow requires a cover of 70 cm thick. It is believed that a smooth grassy slope with a steepness of more than 20° is avalanche dangerous if the snow height on it exceeds 30 cm. With increasing slope steepness the likelihood of avalanches increases. Shrub vegetation is not an obstacle to the gathering. Best condition for the snow mass to begin to move and gain a certain speed, the length of the open slope is from 100 to 500 m. Much depends on the intensity of the snowfall. If 0.5 m of snow falls in 2-3 days, then this usually does not cause concern, but if the same amount falls in 10-12 hours, then snowfall is quite possible. In most cases, the snowfall intensity of 2-3 cm/h is close to critical.
Wind also plays a significant role. Yes, when strong wind An increase of 10 - 15 cm is enough for an avalanche to occur. The average critical wind speed is approximately 7-8 m/s.
One of the most important factors influencing the formation of avalanches is temperature. In winter, when the weather is relatively warm, when the temperature is close to zero, the instability of the snow cover increases greatly, but quickly passes (either avalanches occur or the snow settles). As temperatures drop, periods of avalanche danger become longer. In spring, with warming, the likelihood of wet avalanches increases. The lethality varies. An avalanche of 10 m3 already poses a danger to humans and light equipment. Large - able to destroy capital engineering structures, form difficult or insurmountable blockages on transport routes.
Speed is one of the main characteristics of a moving avalanche. In some cases it can reach 100 m/s. The ejection range is important for assessing the possibility of hitting objects located in avalanche zones. Distinguish maximum range emission and the most probable, or long-term average.
The most probable ejection range is determined directly on the ground. It is assessed if it is necessary to place structures in the avalanche zone for a long period. It coincides with the boundary of the avalanche fan. The frequency of avalanches is an important temporal characteristic of avalanche activity. A distinction is made between average long-term and intra-annual recurrence rates. The first is defined as the frequency of avalanches on average over a long-term period. Intra-annual frequency is the frequency of avalanches during the winter and spring periods. In some areas, avalanches can occur 15-20 times a year.
The density of avalanche snow is one of the most important physical parameters, which determines the impact force of the snow mass, the labor costs for clearing it, or the ability to move on it. For dry snow avalanches it is 200 - 400 kg/m 3 for wet snow - 300 - 800 kg/m 3.
An important parameter, especially when organizing and conducting emergency rescue operations, the height of the avalanche flow is used, most often reaching 10 - 15 m.
The potential period of avalanche formation is the time interval between the first and last avalanches. This characteristic must be taken into account when planning the mode of activity of people on dangerous territory. avalanche snow destructive natural
It is also necessary to know the number and area of avalanche foci, the start and end dates of the avalanche period. These parameters are different in each region. In Russia, such natural disasters most often occur in the Kola Peninsula, the Urals, the North Caucasus, in the south of Western and Eastern Siberia, Far East. Avalanches on Sakhalin have their own characteristics. There they cover all altitude zones - from sea level to mountain peaks. Descending from a height of 100 - 800 m, they cause frequent interruptions in train traffic on the Yuzhno-Sakhalinsk Railway. In the vast majority of mountainous regions, avalanches occur annually, and sometimes several times a year. How are they classified?
Avalanche - a sudden movement of a mass of snow, ice, rocks down the slopes of mountains, posing a threat to human life and health. The conditions for the formation of avalanches are a snow-covered mountain slope with a steepness of 15 - 30 degrees, heavy snowfall with an increase rate of 3 - 5 m/h. The most avalanche-dangerous periods of the year are winter-spring - up to 95% of avalanches are recorded at this time. The movement of an avalanche begins in conditions when the component of gravity of the snow cover in the direction of the slope exceeds the adhesion force of snow crystals to each other
Causes of avalanches:
· heavy snowfall or accumulation of large amounts of snow on the slopes when it is carried by the wind;
· low adhesion force between the underlying surface and recently fallen snow;
· thaw and rain with the subsequent formation of a slippery layer of water between the underlying surface and freshly fallen snow;
· sharp change in air temperature;
· mechanical, acoustic, wind effects on snow cover.
The speed of avalanches is 20 - 100 m/s.
The damaging factor of avalanches is their enormous destructive power. Avalanches sweep away everything in their path, in the mountains they damage and destroy buildings, communications, power lines, roads, equipment, injure and kill people. The main cause of death in avalanches is suffocation (asphyxia). During the movement of an avalanche, it is almost impossible to breathe in it, the snow clogs the airways, and snow dust penetrates the lungs. In addition, a person can freeze, receive mechanical injuries to the head and internal organs, fractures of the limbs or spine. This occurs as a result of impacts on the ground, rocks, trees, stones.
Avalanche protection includes:
· study, observation, forecasting, informing the population about the possible threat of avalanches;
· training people to act safely in avalanche zones;
· artificially causing avalanches;
· use of avalanche plantings;
· creation of engineering structures in avalanche-prone areas, including canopies, tunnels, and corridors.
If there is a threat of avalanches, ski slopes, mountain roads and railways are closed, people are prohibited from going to the mountains, and the work of rescue teams is intensified.
Landslide - sliding displacements of rock masses down a slope under the influence of gravity.
Up to 90% of landslides occur in areas located at altitudes from 1000 to 1700 m. This natural disaster most often occurs in spring and summer on slopes whose steepness is at least 19 degrees. Landslides also occur on the banks of large rivers.
Based on the speed of movement, landslides are divided into:
· exceptionally fast (0.3 m/min);
· fast (1.5 m/day);
· to moderate (1.5 m/month);
· very slow (1.5 m/g);
· to exceptionally slow ones (0.06 m/g).
Landslide - displacement of rock masses along a slope under the influence of its own gravity.
Landslide classification:
|
Area, ha | ||
|
Grandiose | ||
|
Very large | ||
|
Very small | ||
Causes of landslides:
· increase in slope steepness as a result of erosion of the base by water;
· weakening of the strength of rocks when they are weathered or waterlogged;
· seismic tremors;
· violation of mining technology;
· deforestation and destruction of other vegetation on the slopes;
· incorrect agricultural technology for using slopes for farmland.
The power of a landslide is characterized by the volume of displaced rocks, which can be up to several million cubic meters.
Collapse - this is the separation and fall of large masses of rocks from steep and steep mountain slopes onto river valleys and sea coasts due to the loss of adhesion of the detached mass to the parent base. Landslides can injure people, destroy transport routes, block equipment, create natural dams with the subsequent formation of lakes, and cause the overflow of huge amounts of water from reservoirs.
Landfalls happen:
· large - weight 10 million m3 And more;
· medium - weight from several hundred to 10 million m3;
· small - several tens of cubic meters.
The formation of landslides is facilitated by the geological structure of the area, the presence of cracks on the slopes, crushing of rocks, and a large amount of moisture.
The collapse begins not suddenly. First, cracks appear on the mountain slopes. It is important to notice the first signs in time and take rescue measures. In 80% of cases, collapses are associated with human activity. They occur when construction work or mining is carried out incorrectly.
ACTIONS OF PERSONNEL
IN AVALANCHE AREAS
Tutorial
The present tutorial developed on the basis of summarizing the experience of military personnel, as well as theoretical knowledge and practical skills acquired during international mountain training camps for military personnel held in Switzerland within the framework of the Partnership for Peace Program.
The Manual provides recommendations to the unit commander on ensuring the safety of personnel when crossing various mountain slopes in avalanche-prone areas, rules for recognizing the likelihood of avalanches and the behavior of personnel in avalanche-prone areas. The order of work of the commander, the combat order of the unit when organizing and conducting the search and rescue of persons buried under the snow during avalanches are also discussed.
Introduction…………………………………………………………………………………..4
1. Conditions for the occurrence of snow avalanches….………………...………………………..5
2. Features of personnel training. Determination methods
avalanche areas…………………...…………………………………………..8
2.1. Preliminary assessment area of upcoming actions, planning
route……………………….…………………………………..……….9
2.2. Assessment of the probability of snow avalanches along the route and
in the area of operation…………………...……………………………………………..…10
2.3. Assessment of sections of the route along selected slopes …………………..12
3. Search and rescue of people buried under snow.………………….……………13
3.1. Organizing a search at the site of an avalanche………………..…………………13
3.2. Responsibilities of officials when organizing and conducting a search……..…..19
3.3. First medical aid for persons who find themselves buried under
snow………..…………..…………………………….…………………………23
Conclusion……….………………………………………………………………………………..28
INTRODUCTION
The word “avalanche” is of German origin from the Late Latin labina - landslide and means a moving, sliding and overthrowing collapse of a mass of snow on mountain slopes.
Avalanches are possible in mountainous areas where there is stable snow cover. The main reasons causing avalanches are:
Overloading of mountain slopes with snow during snowstorms and heavy snowfalls or due to low adhesion force between new snow and the underlying surface during the first two days after the end of the snowfall (dry avalanches);
The appearance of water lubrication during thaws or rains between the lower surface of the snow and the underlying surface of the slope (wet avalanches);
Formation in the lower parts of the snow layer of a loosening horizon consisting of crystals of deep frost unconnected with each other (avalanche of sublimation diaphthoresis - the process of transition of a substance from solid to gaseous state bypassing the liquid state). The reason for the loosening of the snow cover is higher temperatures in the lower snow horizons, from where water vapor migrates to higher (colder) horizons. This entails the evaporation of snow in the warm horizon and its transformation into a sliding horizon.
The speed of an avalanche is, on average, 20 - 30 meters per second. An avalanche is usually accompanied by a kind of low-pitched whistle (in the case of dry snow falling), a grinding sound (in the case of wet snow falling) or a deafening noise (in the case of an air wave). The frequency of avalanches and their volume depend on the morphology (slope surface structure) of the mountain.
Avalanches moving along hollows, ravines and erosion furrows often fall from steep hollows, but reach small volumes.
Avalanches, sliding over the entire surface of the slope outside the channels from destroyed cirques (natural bowl-shaped depressions at the top of the mountains, formed under the influence of small glaciers or snowfields), fall rarely, but reach enormous volumes.
The remnants of avalanches are usually avalanche snowfields.
Avalanches have enormous destructive power and can lead to great disasters, destruction and loss of life.
To protect against avalanches and reduce their consequences, preventive and engineering measures have been developed and, as a rule, carried out.
Preventive measures include the work of units of the mountain avalanche service and mining technical supervision to predict the time of avalanches and carry out their artificial release using shelling and explosions.
Engineering measures consist of preventing snow sliding by planting forests in avalanche catchment areas, building up slopes and strengthening them with support structures; diversion of avalanches from protected objects using guide dams, avalanche cutters and passage of avalanches over objects using canopies and galleries.
For areas prone to avalanche danger, special maps are drawn up, which highlight areas with significant, medium and weak avalanche danger, as well as potentially dangerous areas.
CONDITIONS FOR SNOW AVALANCHES
Depending on the nature of snow movement along the slopes, three types of avalanches are distinguished:
“wasp” (snow landslides), sliding along the entire surface of the slope outside the channels;
“trough” moving along hollows, logs and erosion furrows;
"jumping" along ledges, that is, freely falling.
The formation and descent of snow avalanches is influenced by the following factors:
1.1. Terrain conditions.
1.2. The amount of surface snow on a mountainside.
1.3. Wind power.
1.4. The structure of the surface layer of snow.
1.5. Air temperature.
Due to the fact that even short-term weather changes significantly increase the likelihood of avalanches, the unit commander must constantly monitor weather changes in the area where his unit is conducting combat operations.
1.1. Terrain conditions
The probability of a snow avalanche directly depends on the steepness of the mountain slope. The critical slope steepness at which a landslide avalanche of dry snow is possible is 30 degrees. The critical steepness of the slope for a wet snow avalanche is 25 degrees.
There is a simple way to determine the steepness of a slope using ski poles:
An avalanche is more likely to occur on a darkened mountainside than on a sunny mountainside.
1.2. Amount of surface snow on a mountainside
The greater the amount of surface layer of snow on a mountain slope, the greater the likelihood of an avalanche. The critical depth of the surface layer of snow is considered to be: 10 – 20 cm in bad weather conditions; 20–30 cm under average weather conditions; 30 – 50 cm in good weather conditions.
1.3 Wind power
In conditions of snowfall in windy weather, a very fragile snow cover is formed on the surface of the mountain slope under the influence of the wind, which sharply increases the likelihood of an avalanche. The presence of such a new snow cover formed by the wind can be easily determined by the characteristic snow peak on the top of the mountain shown in the figure.
1.4. Structure of the surface layer of snow
The surface layer of snow exerts strong pressure on the inner snow layers. The delicate balance between the surface and inner layers of snow on an avalanche-prone mountainside can be easily upset. To initiate an avalanche on such a slope, a small amount of new surface layer of snow or the presence of even a lone skier is sufficient. Unmistakable signs of a high probability of an avalanche on such a slope may be traces of a recent avalanche or unexpected snow falls in areas of deep snow cover.
1.5. Air temperature
Predominance low temperatures air after snowfalls helps to strengthen the structure of the snow cover and, thus, reduces the likelihood of avalanches for a long period of time. Increasing temperatures, on the contrary, weaken the structure of the snow cover and temporarily increase the likelihood of avalanches. At the same time, a short increase in air temperature creates favorable conditions for strengthening the structure of the snow cover, which in most cases reduces the risk of an avalanche.
In the spring, due to rising air temperatures and increasing background radiation from the earth's surface, the danger of snow avalanches increases during the daytime.
On the first sunny day after heavy snowfall, the likelihood of avalanches is greatest. This likelihood increases significantly when the snow becomes heavier and wetter in one day, especially on the first clear day following several days of cloudy weather.
A typical slope on which an avalanche is likely to occur is a steep, shady slope close to the top of a mountain that has a large amount of windblown snow on it.
The most dangerous are the so-called landslide avalanches. In a matter of seconds the result sudden change structure and pressure of the inner and outer layers of snow, a large area of snow cover moves downslope. People caught in such avalanches, as a rule, find themselves completely buried under the snow and have minimal chances of being rescued.
