SnowFlurry 1 2024/25 | Spring skiing

What to consider

It can generally be assumed that the snowpack stability relaxes a few days after a precipitation event, as a degrading metamorphosis of the snow crystals (i.e. rounding of the crystals) and thus a compaction of the snowpack as well as a connection between the individual snow layers occurs. However, rising temperatures and possible weak layers (e.g. angular layers or sleet) become relevant for avalanche activity.  In the following, four factors are discussed which should be taken into account when skiing in spring.

1.    Daytime warming:

Daytime warming is one of the key factors to watch out for when the weather is consistently fine and temperatures are warm. Over the course of a day, the snow cover is exposed to a changing temperature gradient. This is largely determined from below, i.e. from the ground, by the latent heat emitted by the earth. Thus, a constant temperature of 0° Celsius prevails on the ground, or on the underside of the snow cover. The surface of the snow cover, in turn, is significantly influenced by solar radiation. On one hand, the exposition, i.e. the orientation to the sun, and the slope inclination are decisive. In the northern hemisphere, the sun rises in the east, takes its course in the south and sets in the west. This means that southeast, south and southwest-facing slopes in particular are warmed more by the sun than north-facing slopes, for example. The slope inclination plays another important role in this context, as there is an optimum angle of inclination for the maximum heat absorption of a surface. This is 90°. This means that energy transfer is greatest when solar radiation hits a surface at a right angle. However, as the position of the sun in mid-latitudes changes seasonally, the angle of the slope on which the sun's rays strike at a 90° angle also varies. This February, the optimum angle in Innsbruck is around 30-35°, in March 35-45° and in April up to 55°. In other words: the later in winter, the faster the snow cover on steep slopes warms up and the faster the snow cover is soaked. In addition, steep slopes are often interspersed with rocks or embedded in rock faces. The darker surfaces have a lower albedo, i.e. a higher absorption property compared to snow. As a result, they heat up more and give off additional heat to the surrounding snow fields. More precisely, this means that when planning a tour, it is not only important to consider the orientation and inclination of the slope to be climbed, but also the surrounding and overlying slopes and rock faces. But what specific events can result from an increase in temperature during the course of the day?

On one hand, wet loose snow avalanches and on the other, sliding snow avalanches can occur. Both types of avalanches are caused by the addition of heat to the snowpack, with rising temperatures triggering thawing processes on the snow surface. Over time, this leads to an increasing soaking of the snowpack.

With the stable high-pressure situation that has prevailed in recent weeks, positive temperatures have also been recorded at night at high altitudes. The lack of a drop below freezing point (i.e. the zero degree limit) means that the snow cover does not freeze through completely at night and therefore favours increasing moisture penetration. To prevent the risk of loose or sliding snow avalanches, good time management, keeping an eye on the prevailing temperatures and terrain, and recognising danger signs are therefore essential.

2.   Weak layers

Within a season, weak layers can form in the snowpack in various ways (see also: here). These are often snowed in and the snow cover stabilises over time, which means that their existence is often forgotten.

Intensive soaking of the layers close to the surface can destabilise the previously stabilised connection between a weak layer and the layers above, which can ultimately lead to slab avalanches. As there are no obvious danger signs in the terrain regarding this risk, it is advisable to monitor the avalanche situation throughout the winter, for example with the help of the avalanche bulletin.

3.     Danger of falling

However, in the event of a persistent high-pressure weather situation (which can be accompanied by both warm and cold temperatures), the avalanche situation is not always the main risk to be considered. Often there is even a stable to very stable avalanche situation within such a period. This opens the door to steep and exposed descents that are not possible with a higher danger level. However, one existing risk that can be underestimated is that of falling. A stable high pressure area results in strong heat radiation from the snow surface at night, as there is no diffuse reflection of heat energy through clouds). If negative temperatures prevail at night, the snow cover freezes through overnight. The daily melting and nightly refreezing result in the formation of ice flakes or ice sheets. The static friction of skins and ski boots is  severely reduced on these, which can result in slipping. In addition, a fall  is very difficult to stop. In this respect, it is advisable to take snow or crampons and an ice axe with you, because "it is better to invest 10 minutes more in fitting such equipment than to lose your life in 10 minutes" (Antoine Patet).

4.     Danger of falling rocks

In spring, the risk of falling rocks also increases significantly, a danger that is well known to the climbing scene. The process responsible for this is frost blasting. Water penetrates into crevices and cracks, freezes at negative temperatures (expands in the process) and thus blasts rock material off the rock. Warming favours the thawing process and changes the stability of the rock layers, as the blasted rocks can no longer be held and fall down. When choosing a route directly under steep walls, always listen out for cracking/surring noises and mute witnesses, i.e. boulders and rocks lying in the terrain.

5.    Additional influences  - Inversion weather situation and Sahara dust

Additional influences that can favour a soaking of the snow cover are, for example, the formation of an inversion weather situation or Saharan dust. In an inversion weather situation, which can often form during a stable high-pressure weather situation, there is a reversal of the temperature gradient with increasing altitude. The temperature normally decreases in the troposphere with increasing altitude, but in an inversion the air temperature increases with altitude. The layers of air near the ground are colder than those above, which prevents the convection (rising) of air masses and creates a blockage. Inversion weather conditions have no effect per se on the avalanche situation, but they can cloud the perception of prevailing temperatures, as contrary to "normality" it is warmer in the mountains than in the valley. Saharan dust, on the other hand, has a similar effect on heat input as the previously described effect of stones on the snowpack. The dust, which is transported northwards by the African Easterly Jet in certain weather conditions and thus reaches the Alpine region, is deposited on the snow surface. As a result, the albedo, i.e. the reflectivity  of the snow cover, changes and more heat  is absorbed by the snow cover. Both phenomena have been observed in the Western Alps in recent weeks. As already mentioned, they do not have a direct effect on avalanche problems, but they act as additional factors.