Snow avalanches are a natural process of unloading mountain slopes associated with snow accumulation and redistribution of snow. All year round, the transfer of geological material from the upper layers of the relief to the lower with the subsequent unloading of mountain slopes.
What are snow avalanches in the mountains, the reasons for their occurrence and descent, classification of snow avalanches, the impact of weather on avalanche danger in the mountains.
Any movement of snow mass down the slope at a speed of more than one meter per minute is called an avalanche. For avalanche specialists, the lower threshold for the size of an avalanche is 10 cubic meters. Under certain conditions, this volume is enough to cause injury or death. The bulk of accidents in mountaineering, mountaineering, and freeriding occur in small avalanches ranging in volume from 100 to 500 cubic meters. m.
According to statistics, most of the avalanche incidents involving humans occur through his own fault. That is why it is important to have minimal knowledge and skills to assess the avalanche situation. Balance your goals with the risks that inevitably exist in the mountains.
According to statistics, 25-30% of those bursting in an avalanche die from injuries during movement along with snow mass. Of those who survived but were completely buried in the snow, fewer than half survive..
Reasons for avalanches.
Snow cover is heterogeneous. It accumulates on the slope in layers. Each of the layers is formed in a particular period and usually due to snowfall or wind carrying, which redistributes the previously fallen snow. Or because of snowfall with the wind together (blizzard).
Also, new layers appear due to the formation of surface hoarfrost (an analogue of dew, but in a frozen state). Under the influence of the sun (insolation crusts). Or due to a metamorphism of processes within the snowy layer itself.
Gravity acts on all the scrap lying on the slope, which tries to move the snow down. As soon as in some particular place the gravity exceeds the resistance, destruction occurs, which under certain conditions begins to spread in all directions.
Primary failure usually occurs not in the thickness of any main layer, but at the boundary of the layers (“weak layer”). Moreover, the destruction of the “weak layer” is the destruction of the shift. Only after the destruction along a weak layer does the “cutting” of the “board” from the snow slope take place. The characteristic sound of “wumps” or “wow” and sometimes perceptible drawdown is the destruction of a weak layer.
There are a fairly large number of snow avalanche classifications. However, the basic ones are:
Moreover, the classification of avalanches by their type is the most common in the world. She divides all avalanches by type of separation into:
Snow avalanches from the line (“boards”).
Snow avalanches from the point.
What are snow avalanches from the line (“boards”).
The most dangerous type of avalanches. Since immediately a large amount of snow enters the movement over a large area. If you find yourself in such an avalanche, then get out of it is extremely difficult. In addition, the blocks into which the “board” is crushed are traumatic. It is the “boards” that account for the main percentage of accidents in avalanches.
Snow avalanches from the point.
Start with a little snow and move at a relatively low speed first. This often allows you to leave the danger zone. Particularly dangerous in the spring and summer, as they consist of wet or wet snow.
Dry and wet snow avalanches.
Both types of avalanches are divided into dry and wet. Dry winters predominate in winter, but the closer to spring, the higher the percentage of wet avalanches. Other things being equal, wet snow avalanches are always more dangerous than dry ones. In the event of a complete burial, the chances of survival in any wet avalanche are practically zero.
Avalanche classification by size.
The effect of weather on avalanche danger and snow avalanches.
Snow layers experience constant processes of structural change (metamorphism), and the main driving force behind these changes is meteorological factors. Some of them lead to increased stability of the snow cover and a decrease in avalanche hazard. Others entail a decrease in snow cover stability and increase the likelihood of snow avalanches.
The effect of snowfall on avalanches.
An increase in the thickness of the snow cover leads to the addition of a load on the snow mass and an increase in stresses in it. It is believed that if more than 30 cm of fresh snow falls per day, the likelihood of natural avalanches.
It is important not only the amount of snow that has fallen, but also the rate of snowfall. The higher it is, the greater the likelihood of a bad avalanche. The critical threshold is considered to be snowfall at a speed of 1.5-2 cm / hour or more.
Thus, it is necessary to take into account both the total amount of snow that has fallen over the last day and the rate of snowfall. Especially at peak times. 20 cm of snow falling in a few hours can be more dangerous than 1 meter of snow, but evenly falling within 3 days.
The effect of wind on avalanches.
The wind is able to move significant masses of snow. And the higher his speed, the greater his transporting ability. Even after a light snowfall, if accompanied by a strong wind, avalanche danger may occur on the leeward slopes, since a critical mass of snow can accumulate there. Constant monitoring of the wind transfer and its signs (cornices, zastrugi, “snow flags”, cushions of shadow zones, granular frost on vertical objects, etc.) is necessary..
The threshold value of wind at which strong wind transport begins on the usual alpine terrain of 5-7 m / s. But in the foothills and midlands, it starts at a speed of 3-4 m / s.
The effect of temperature on avalanches.
The effect of temperature on the snow cover and, accordingly, on the avalanche situation is very diverse. For convenience, there are 5 main temperature scenarios that are regularly repeated during the winter.
Strong rapid warming. Going through 0 degrees dramatically increases avalanche hazard.
Slow, moderate warming, without going over 0 degrees, gives stabilization.
Gradual warming during the day and cooling at night are signs of stabilization..
Cold weather causes the process of conservation of those conditions that were, or their gradual deterioration.
A long frosty period leads to the formation of weak layers.
The effect of solar radiation on avalanches.
In general, if we talk about the winter period, the sun has a stabilizing effect by reducing the average daily temperature gradient in the snow mass and starting the process of metamorphism by the type of rounding of snow crystals. The deep frost that has formed earlier and a number of other weak layers, while maintaining sunny weather with near-zero air temperatures, can change over time and sometimes even disappear.
But, as soon as the temperature of the surface layer of snow passes through zero due to solar heating, the situation will change dramatically. That is why in the spring, when the influence of the sun is especially great, the avalanche can change in tens of minutes..
The effect of rain on avalanches.
Rain in the highlands is one of the obvious markers of avalanche danger. Like freshly fallen snow, rain increases the load on the snowy slope, which in itself can lead to avalanches, but, in addition, there is also a decrease in adhesion between the snow grains due to water seeping into the snow. There is a likelihood of an avalanche descending over the entire thickness of the snow cover to the ground.
Based on the book School of mountaineering, a training manual.