Avalanches occur because the weather supplies the avalanche material snow through precipitation. Weather and avalanches are therefore inextricably linked. The weather patterns in the Alps are complicated, which is why we will limit ourselves here to the weather and avalanche basics. Even with relatively little weather knowledge and experience, the avalanche danger can be assessed on the basis of indications. As the weather leaves typical traces in the snowpack, attentive freeriders can also check whether the avalanche report is correct or possibly wrong. Snow and rain, the wind as an avalanche architect as well as temperature and solar radiation are components of the weather and are just as responsible for the avalanche danger as the terrain and people.
Every snowfall increases the avalanche risk!
The extent to which the avalanche risk increases depends on the type of snowfall, the temperature, the wind force and the old snow base. The decisive factor is the yield [intensity] of the snowfall, i.e. how much snow has fallen in what time. The rule is: the faster, the less favorable. Above a certain amount of fresh snow, it becomes dangerous for freeriders. If a so-called critical amount of fresh snow falls [according to W. Munter], there is at least a considerable avalanche risk [level 3]. If the critical amount of fresh snow is reached or exceeded, a great deal of caution and defensive behavior is required.
If it snows under favorable conditions, it becomes dangerous from a fresh snow accumulation of 30-50 cm.
Favorable conditions are:
Weak wind - no or little drifting snow
Rain that turns into snow
Temperature - especially at the beginning of the snowfall +/-0° C
Regular and frequently used slopes, e.g. freeride slopes.e.g. freeride slopes: skiing solidifies the snow cover and stabilizes the slope
When snowfall occurs under unfavourable weather conditions, the critical amount of fresh snow can be reached at 10-20 cm.
Particularly unfavorable conditions are:
stormy wind
very cold temperatures, below - 8° C
surface frost, hard snow, ice or very old snow as a base for the fresh snow
almost untracked slopes - backcountry!
From a fresh snow accumulation of 20-30 cm, a dangerous avalanche situation must be expected under average conditions. In medium weather conditions, there are both good and bad signs. In all cases, the danger will persist for at least the next few days until the fresh snow has bonded sufficiently with the base.
Avalanche danger often decreases quickly after heavy snowfall, as the snow cover settles quickly under its own weight. Heavy snowfalls form thick and stable snowpacks. Even without snowfall, the avalanche risk can increase and become critical, e.g. due to fair weather or foehn storms, as this example of a ski tour in strong winds in the Bedretto Valley showsThe first fine day after several days of bad weather is particularly dangerous and accident-prone. Therefore: If you go freeriding on such days, you must set your danger sensors to alarm and, if in doubt, be prepared to give up a slope. Note: Freeride and off-piste slopes are often much more stable than the snow cover of the touring terrain in the backcountry due to constant use (old tracks act as barbs for the fresh snow). However, there is of course no guarantee of this, so if in doubt, you should assume an unfavorable situation and behave defensively!
Wind and avalanche danger
Wind is the master builder of snow slabs and therefore the trapper. Anyone who has ever had to battle a full-blown storm knows that wind can be extremely strong in the mountains. It is therefore not surprising that wind is capable of transporting huge amounts of snow. The stronger the wind, the more snow is transported - the larger the accumulations of drifting snow can be expected. If the wind speed doubles, the snow drifts increase dramatically.
The windward side of a mountain is called the windward side, the leeward side the leeward side. The wind always transports the snow from windward to leeward. Particularly large and dangerous accumulations of drifting snow are usually found on the leeward side of a mountain.
In the Alps, the wind usually blows from westerly directions: West/northwest/southwest. This is why the largest drift snow fields are often found on east-facing slopes. If the weather comes from the west, the high altitude wind blows from the west. Unfortunately, however, the winds on the ground - which are responsible for the snow drifts - often blow from completely different directions, as turbulence and deflections occur in the mountains. Therefore, after a snowfall from the west, we must never rely on the fact that dangerous drift snow traps are only hanging on the eastern slopes. After heavy snowfall under the influence of wind, there are dangerous drift snow traps on all slopes. Fortunately, it is possible to determine the wind direction on an individual slope based on the wind signs on the snow surface!
Vines
...hang above the dangerous lee slopes. The cornice always indicates the side to which the drift snow has been transported. The dangerous drift snow wedge is located under the cornice. This is often prone to failure: huge masses of snow are located at the steepest point of the slope. The snow cover becomes thinner towards the bottom of the slope, where the snow cover is only weakly supported. An avalanche can be triggered remotely from the [pressurized] foot of the slope - a particularly disastrous avalanche trigger - as escape is often not possible. Even if the cornices are often very stable - they are always a sign of danger!
Waves and dunes
on the snow surface are the result of heavy snow drifting.
Strong winds have created large amounts of drifting snow. These waves can be up to 1 m thick. The wind blows at right angles to the waves. The flat side of the waves and dunes always points in the direction of the wind and the steeply sloping side is in the lee. This is no longer powder, as the snow is already bound by the wind. Waves and dunes are an alarm signal: Attention, avalanche danger!
Wind dunes/castrugis
are depressions in the snow surface carved out by the wind. In contrast to the waves, the steep sides of the wind gullies point towards the wind: "You have braved the wind? They are often very hard and therefore unpleasant to ride. If you come across a windswept slope while freeriding, ask yourself: Where is all the snow now that the wind gangs have been milled out of?
Wind scours
Blow-out cavities, known as wind scours, often form around rocks and other obstacles.
"Comet tail":
Drift snow is deposited in the lee of obstacles. This can create long, small snow banks. They look similar in shape to the tail of a comet and are therefore called "comet tails". The tail running out in the slipstream indicates the wind direction.
Low/snow-free ridges and crests
Slope areas that are heavily exposed to the wind can be snow-free even in high winter. Here, the wind sweeps over them with particular force and speed and carries the snow into gullies, hollows and other depressions. It can be wrongly assumed that there is hardly any snow anyway, so there is no danger of avalanches. It is extremely dangerous to avoid the gullies and hollows filled with drift snow now: the brittle drift snow can be under tension and often easily triggered as a snow slab.
Drift snow gullies and hollows are particularly dangerous, as even relatively little snow is enough to bury a freerider deep under certain circumstances. In steep gullies and other extreme terrain, even small loose snow avalanches can become extremely nasty if they wash you down the gully. In the event of an avalanche, you have little chance of getting out of the gully.
Foehn and fair weather storms
You often hear the expression that the weather is foehn. Foehn refers to a [relatively] warm, strong wind that blows over the Alps and transports or blows away large amounts of snow. When the foehn blows on the northern side of the Alps, the wind blows from the south; when the foehn blows on the southern side of the Alps, it blows from the north. Under clear or almost cloudless skies, very mild temperatures are often reached on the northern side of the Alps. Now the south föhn transports large amounts of drift snow, especially to north-facing slopes, so that after a föhn storm the dangerous drift snow traps are located on the northern slopes. However, the wind direction can also vary greatly during foehn storms. In any case, you must independently assess where the drift snow accumulations are located. The combination of warmth and strong winds makes the foehn an effective snow eater and the avalanche danger often increases dramatically.
Snow plumes on ridges and summits
indicate a fair weather storm [or foehn storm]. Do not confuse snow plumes with clouds! Like the Föhn, these storms can become very strong and transport large amounts of snow to leeward slopes.
Solar radiation and temperature
Everyone knows that the sun and temperature are connected. So it is only logical that temperature and solar radiation often influence the avalanche risk together. As already mentioned, the solar radiation of a slope is strongly dependent on its exposure and steepness. Even in the height of winter, it is often possible to relax comfortably on a sunny slope, as the low-lying sun provides the areas inclined towards it with particularly intense heat radiation.
The solidification and transformation of the snow cover are strongly dependent on the temperature. Therefore, south-facing slopes can stabilize faster than shaded slopes [northwest/north/northeast/east slopes] due to degrading transformation and melting transformation. After snowfall, warming causes a [short-term] increase in the risk of avalanches. Self-triggering avalanches must be expected, especially on sunny slopes after heavy snowfall.
In the shady slopes, strong temperature differences between the ground [near 0°C] and the snow surface [very cold - deep minus degrees] often lead to strong build-up transformation. The foundation of the snowpack then becomes dangerously unstable. Thick layers of surface frost often form on north-facing slopes in high winter. No sunlight reaches the snow cover and the frost cannot thaw. If a layer of surface frost is snowed in, it forms an extremely dangerous avalanche slide layer. Steep shaded slopes are by far the most dangerous slopes. Powder is particularly tempting here, but death and misfortune are also at home on the steep north-west/north/north-east and east-facing slopes.
A rule of thumb
Long-lasting cold preserves dangers - over a long period of time! The snow cover solidifies extremely slowly, as the decomposition process is very slow. In addition, when the snow depth is low, there is a risk that the build-up transformation will make the snowpack even more unstable.
Slow and moderate temperature increases - when the snowpack is dry - reduce the risk of avalanches as the snowpack settles and relaxes. Rapid and strong warming due to thaw, foehn and rain intensifies the danger in the short term.
After snowfall in cold temperatures, a rise in temperature causes an increase in the avalanche danger.
Cooling and refreezing solidifies a moist snowpack and reduces the danger. This happens overnight when the sky is clear, so that very favorable conditions prevail in the morning - especially in spring - when the snow cover is frozen.
However, the night must have been so cold that the harsh cover [the frozen layer] is stable enough that you don't break through. As the warmth causes the snow to soften again during the day, the risk of avalanches often increases quickly and significantly.
