R. Lienerth, R. Bednarik, The. Nádvorník, On the. Kořízek (2004), www.lezec.cz
Photo: Stock R. Lienertha, R. Bednarik and materials. Kořízka

The basic mechanism of avalanche

The snow cover has a number of factors and runs a number of physical processes. Avalanche is one of the mechanical processes, but until the very last in a series. The snow layer is the subsidence, plazení, or flake, or tongue. Subsidence and crawling, like other processes, mainly due to the gravitational force, but no clear eye. Flake tongue and we are able to visually locate, but usually achieve very small dimensions.

Avalanche is the latest in mechanical processes which we set, but it is also most destructive process. Avalanche from a geomorphological point of view we can distinguish three zones - zone of adhesion, range transport and deposition zone.

As we mentioned, avalanche develops on the basis of physical processes in the snow layer. These processes lead to two particular types of stress. Tension tension - in places, where the snow tends to form a breakaway, and pressure at, where the layers of snow and stabilize each other based on their mass. Two illustrative examples of the profile of the slope and on it lying snow layers are shown in Figure:

The optimum condition is higher compressive stress to tensile. At the moment, when the pressure exceeds the tensile stress, released the avalanche. It should be noted, that this factor may be a skier, or entering before the rise climber. However,, that it was not so easy, plays a role in this process, many other factors. Coherence snow, layer, which is stored, load slope, temperature, etc.

Most significant factors influencing the development of avalanches
- slope
- surface snow layers
- History snow layers - new snow
- prevailing wind direction (návětrný, závětrný)
- disposition slope (North, South)
- the presence of frost hollow in the lower layers of snow

Slope

The slope is one of the most obvious factors, we know. Generally we can say, avalanches that fall mostly on the slopes 30 - Of 60 stupňů. The steeper slopes of the snow kept odsypává, the usually mild enough not to tensile stress. However, it is a case of, when the avalanche fell on a gradient 12 stupňů!!! It was a spate of wet and hedegrees.

The most dangerous avalanche then fall just on the slopes with a slope of around 30 degrees, which tend to have built up enough layers of snow and, if adhesion is a move in great masses of snow.

Hazardous materials snow layer

Definitely the most dangerous underlying snow layers are grassy slopes. The grass under the snow lay so to form an optimal sliding surface. The security we have neither the slopes covered with dwarf mountain pines. Where, although on a smaller scale, but still the same phenomenon occurs. According to previous monitoring sparsely located small trees, or rocks in the hillside slope stabilize, but instead of an avalanche can be developed,. Markedly prominent group of rocks can be considered as elements of slope stability, as well as sufficiently dense forest. However, careful, provided they manage to pass through the forest with no problems jumping, Avalanche can do it too!!!

History of snow layers

The history of the song snow layers think snow layers of snow periods. Any change in temperature, snow, wind forms a layer of snow, leaving characters, We are able to make these facts from the properties of individual layers read.

The snow on the ground after the impact due to physical processes changes, One of the possible forms of snow metamorphism

If you think of new snow, we should always sharpen your focus. If it snows the wind, consider it a critical mass 30 – 50 centimeters, according to the substrate. The wind can lead to the creation of a situation from lavinézní 10 cm of fresh snow.

Base distinguish snow depending on the moisture in the dry (teplota < -2/3 C) and wet (tepltemperature; -2/3 C). Curthermore, we distinguish hardness, particle size, cohesion and adhesion.

Determining the hardness of the layers of snow on the ground:
- fist (1)
- four fingers (2)
- finger (3)
- pencil (tip sticks) (4)
- knife (5)
- led

According to the structure of snow crystals, then we distinguish the following basic types of snow.
New Snow ( + ) Snow, Falling or freshly fallen. The crystals are dependent on climatic conditions. New snow is characterized by large discontinuities. It is the basis of dust avalanches.
Fuzzy snow ( / ) The new snow will initially operate wind. He begins to interfere with the initial crystal structure (so-called. destructive phase). But still maintained a greater extent, the original hexagonal system. In contrast, the breaking of the original crystals causing impaction of larger crystals, thereby increasing the overall consistency. Fuzzy snow slab avalanches is based.
Okrouhlozrný snow ( • ) It is a dry snow, arising from the influence of temperature plstnatého. At first glance, the apparent destruction of the crystal lattice, and snow flakes no longer look like a regular bullet, matt white mat. Represents a transition stage between collapsing and výstavbovou metamorphosis. Thanks to its binding ability to participate in the creation of boards.
Hranatozrný snow ( ‚ ) Due to temperature changes are warped the original crystal and enters its new form. If there is a large temperature changes, increases the diffusion of water vapor and the supersaturation. This is an essential prerequisite for the new change výstavbovou. This generates ice grains of angular shape. This kind of snow is quite problematic, it is very moving. It is strange and dangerous type of avalanche snow especially in combination with cavity frost.
Dutinová jinovatka ( ^ ) It is formed exclusively within the snow layer. Form in the protracted indoor frost (at -10 ° C). The crystals have the shape calycifloral, which is produced by evaporation of water vapor from the angular crystal. Thus penetrated the airspace is no longer sufficient to face the onslaught of the top layer and there is distortion. (see below)
Firn ( ° ) It is a rounded ice grains, of any further change in crystal. This occurs mainly due to prolonged exposure to elevated temperatures. The occurrence of firn occurs mainly in spring.
Led, ice sheet ( It ) It firnová or ice floe on the different strengths. It can occur at any depth firn. Its depth and thickness depends on the weather. This is dangerous ground for new snow. The ice layer is very dangerous to thaw!

Direction of prevailing winds
Prevailing wind direction draws our attention to the leeward slopes, where the significant layers of snow deposited by wind transferred.

Disposition slope
The disposition slope, or orientation. An important factor to influence the snow layer is the sun. Generally, the southern slopes quickly stabilize, the influence of solar radiation heating is more pronounced interconnection layers of snow. Generally, During winter the southern slopes are safer, on the contrary, the northern spring, because, on the southern slopes occur podtékání snow layer water is significantly higher risk of avalanches base.

The presence of frost hollow.
Frost hollow is created in the snow layer, usually of very low temperatures, sometimes as a border gives -10 C or less. To its formation occurs in the lower parts of the snow layer, mainly due to temperature difference between surface and bottom vrsty, where there is always a bit warmer. Furthermore, the tightness of a snow layer of vapor generated by sublimation of snow crystals.

Frost Hollow behaves like sand, If you or your ball-bearing overburden. It is about snow crystals with zero cohesion.

As we consider the critical amount corresponding 5% the amount of snow layers deposited over a layer of frost hollow height of snow layers deposited over a layer of frost hollow.

Top demonstrate a model example.
The snow layers are excavated at a depth 1,5 m encountered a layer of frost on the cavity power 0,05 m. In this case, the risk of avalanches release on this layer is relatively small, because the slope is stabilized her weight.
The snow layers are excavated at a depth 0,8 m encountered a layer of frost on the cavity power 0,10 m. In this case we should be most cautious and possible hikes in places in excess slope 30 degrees abandon.

Division of avalanches