Avalanche problems and danger patterns have one thing in common: They point out typical, repeating and most often obvious dangerous situations.
The difference is at which level we look at it. Avalanche problems give a first, rough overview of sources of danger (e.g. new snow) whereas avalanche danger patterns give deepening background knowledge about the cause of the problem (e.g. problem due to extra load on a weak layer). Danger patterns therefore describe possible scenarios resp. processes that lead to avalanche problems.
The objective is clear: With the help of avalanche problems and danger patterns, dangerous situations can be identified faster, the behaviour can be adapted and avalanche accidents prevented.
danger pattern (dp) 1 - deep persistent weak layer
Major snowfall during the autumn generally remains on the ground only at high altitudes (> 2000m) and in high alpine regions (> 3000m) in shady, very steep terrain. If, following such snowfall, a stable high pressure front without precipitation persists, this snow (through processes of nocturnal outgoing radiation) can become faceted, thereby forming a ground-level weak layer which can endure for long periods of time.
During early winter, other altitudes and expositions are also affected. The rule of thumb is: ground-level weak layers from early winter can frequently be triggered by skiers once winter gets underway. The likelihood of triggering tends to diminish over the course of the winter as snow depths increase. Caution is urged both after very intense snowfall, which can cause a fracture in the weak layer; and wherever the snow is shallow. In springtime, the likelihood of avalanches triggering from the weak ground-level layer is again heightened as that layer becomes thoroughly wet. In such a case, avalanches can easily reach large to very large size.
The weak layer forms due to outgoing longwave radiation during night, without the need for a prolonged cold period, which is the main difference to dp.5.
danger pattern (dp) 2 - gliding avalanche
Gliding avalanches are usually unleashed down towards the valley across steep, smooth slopes. Before they are released, glide cracks form, i.e. easily visible fissures in the snowpack, often several meters deep. Quite opposed to an age-old belief which is still difficult to dispel, such glide cracks are now known to be not favourable signs, but on the contrary, thoroughly unfavourable harbingers of gliding avalanches. A glide crack points to the possibility of a gliding avalanche, though gives no indication about whether a snow mass will actually be triggered and, if so, when. Gliding avalanches are among the most difficult types of avalanche to predict, in terms of their time of triggering, because they can be released literally at any time of day or night even in generally stable snow conditions, on the coldest day of winter or the warmest. Furthermore, gliding avalanches are not unleashed by additional loading. There does exist one sole criterium for a series of gliding avalanches to trigger: the increasing wetness of the snowpack. The wetter the snowpack, the more frequent gliding avalanches are. Early, heavy snowfall exerts a favourable, restraining impact; warm ground temperatures unleash the processes leading to gliding snow.
danger pattern (dp) 3 - rain
Rain is considered the classic danger signal in snow analysis and avalanche science, since on the one hand, it funnels additional weight into the snowpack, and on the other, rapidly diminishes the snowpack’s firmness. Avalanches are then bound to be triggered. Rainfall can occur at any time of the winter. The biggest advantage is: this is the simplest danger pattern of all to recognize.
danger pattern (dp) 4 - cold following warm / warm following cold
Since ages past in avalanche lore, the opinion was handed down that a big temperature change during heavy snowfall (regardless whether cold following warm or vice versa) had a beneficial influence on the avalanche situation. In fact, this is the case only under certain, very specific conditions. Far more often, such a temperature change has a negative effect, since it enhances the faceting of snow crystals inside the snowpack, as a rule leading to the formation a thin, weak layer which is highly prone to triggering. Such layers are often found in south facing terrain. They are highly treacherous, not least because immediately following the snowfall they don’t yet exist, but form only over the course of subsequent days.
danger pattern (dp) 5 - snowfall after a long period of cold
A classic scenario in avalanche situations: following a long period of low temperatures, it begins to snow. In addition, a strong wind is blowing, which transports the freshly fallen snow. Within the shortest imaginable time, an extremely treacherous situation for skiers and boarders arises. Yet this is equally true when, following a long period of low temperatures, ‘merely’ a strong wind blows, without any snowfall. The underlying problem: on wind protected slopes, fresh snowdrift accumulates which usually is deposited atop an old snowpack consisting of depth hoar. The old snowpack and new snowdrift are very poorly bonded to each other. The snowpack then just waits to be triggered by a new disturbance. In case of heavy snowfall or snow transport, avalanches trigger naturally and they can frequently grow to large size. It is even more dangerous if, in addition, the temperature swiftly rises.
danger pattern (dp) 6 - cold, loose snow and wind
“Wind is the architect of avalanches”: this classic adage of Wilhelm Paulcke from the 1930s still has unaltered validity today. Wind influences both falling snow and already deposited snow, it is one of the major formative factors in potential avalanches. If the snow is loosely packed and dry, wind always leads to its transport, thus increasing the danger of avalanches. The colder the transported snow is, the more sensitively it reacts to additional loading, since it becomes even more brittle. Fresh new snow forms the weak layer that is superimposed by snowdrift, which is characteristic for this danger pattern. That means, it either snowed just previous, in low temperatures without wind, and subsequently the wind begins to blow; or it begins to snow without wind, and the wind increases in velocity during the snowfall. This pattern is easy to recognize and of short persistence. One exception is the case when faceted snow is transported by the wind leading to hard, brittle and persistent slabs.
danger pattern (dp) 7 - snow-poor zones in snow-rich surrounding
The snow layering is generally far better in snow-rich than in snow-poor regions caused by the increased metamorphism within the snow pack. Weak layers are not so deep down in snow-poor regions and thus, it is far more likely that avalanches get triggered by backcountry skiers and snowboarders in those zones. You can observe this phenomenon also during a winter with an average snow-cover, if there are snow-poor zones in a snow-rich surrounding. Problems will mostly occur on convex slopes or in the vicinity of ridges.
danger pattern (dp) 8 - surface hoar blanketed with snow
Surface hoar, aesthetically, numbers among the most beautiful types of snow. It is not a potential danger in itself. However, when it is covered over by new, bonded layers of snow it becomes a peril and is thus considered, with reason, to be one of the most critical weaknesses in snow analysis and avalanche science. Surface hoar often forms during cold, long-enduring periods of good weather. One special form of surface hoar creates the so-called Nigg-Effect, named after a Swiss mountain guide: relatively warm, moist air brushes by a crest or ridge. If the snowpack surface is much cooler on the back flank of the mountain (usually a shady slope) due to shade or outgoing radiation, the vapor in the warm air is then deposited at the ridge (and only there), which then forms surface hoar. This effect occurs frequently in late winter and springtime. The Nigg-Effect is considered a classic trap for experts.
danger pattern (dp) 9 - graupel blanketed with snow
In avalanche instruction courses, weak layers inside the snowpack are often compared with ball bearings. Yet this image is suitable only for graupel: a ball-shaped form of precipitation which is deposited in thunderstorm-like showers particularly in springtime. It is easy to grasp that snowdrift which collects on top of it is usually inadequately bonded with it, and the avalanche danger thus escalates. Graupel is often spread over small areas and is very difficult to spot, even by experts, without looking inside the snowpack. It is a thoroughly treacherous situation, which fortunately causes problems only for short periods of time.
danger pattern (dp) 10 - springtime scenario
A particular challenge for backcountry skiers and boarders (and for avalanche analysts as well) arises in springtime. Rarely do situations considered “safe” and those considered “unsafe” occur so close together in time. And never is the spectrum of danger levels in a daily cycle as divergent as in spring. On the one hand, the avalanche danger is easy to assess in conditions of stable firn snow; on the other, there are never as many large avalanches registered in the course of a winter as during critical springtime situations. Apart from the snow layering, a complex interaction of air temperature, humidity, solar radiation and wind exerts an enormous impact. Most critical of all is the scenario beneath overcast skies when air temperatures are high, radiation intense, the air very humid and there is no wind. The snowpack in such conditions becomes wet very rapidly, avalanche danger increases in equal measure. Following cool and clear nights, on the other hand, the early morning hours usually provide safe conditions. That is because in dry air, the transition to vapor of the snow on the surface is immediate, which thereby cools the snowpack down. Wind is favourable in these conditions, since it reinforces this ‘sublimation cooling’ and thus prevents the snowpack surface from softening too quickly. For skiers and snowboarders, clocktime-discipline and flexibility in planning backcountry tours take on preeminent importance.