Apart from slope steepness and aspect, terrain type also plays a particularly crucial role in avalanche formation. Fresh drifting snow often collects in gullies, hollows and cornices, and can lead to dangerous avalanches. Apart from these avalanche risk factors, crevices and escarpments present increased risk of falling.
It is vital that you develop your intuition for recognizing and interpreting these danger signs, so that you are then able to select the best possible route.
Here is a selection of the possible terrain danger areas you could encounter.
Rocky terrain presents not only risk of falling, but also increased avalanche danger. Weak layers are particularly likely to form around the boulders.
Fresh snowdrifts often end up in gullies and chutes after long windy periods. These terrain features promote the deposition of drifting snow on the leeward sides of slopes. This is especially treacherous when the snow is deposited by wind blowing across the slope (“cross loading”). Chutes on the windward side can also exhibit compressed snow deposits.
Cornices are formed by drifting snow and can especially be found near sharp terrain breaks such as exposed ridges. When snow is blown from the WINDWARD to the LEEWARD side, it forms an overhanging mass.
An additional load (e.g. an alpinist) can cause the cornice to break off.
Crevasses are formed when glaciers flow over surface irregularities such as mounds and steep slopes. There is a grave danger for alpinists of falling into a crevasse. In winter, a crevasse can be covered by a small amount of snow (falling or drifting snow), forming a type of “snow bridge”. A large amount of snow is better, but it is still not a guarantee that the crevasse can be traversed safely.
The wind is the architect of avalanches. Snow banners, cornices, and wind-eroded snow surfaces are signs of high wind activity and snow drifting.
Even if the snow is still soft and easily navigable here, the wind has already done its job and picked up drifting snow.
Drifting snow is snow that is blown. The wind usually erodes fresh snow when it is falling and deposits it on the lee side, thereby causing snow drifting. However, old snow can also be blown by the wind, mechanically eroded and deposited as a snowdrift. The higher the wind speed, the greater the amount of snow that is blown. Drifting snow is always bonded. If it falls onto a weak layer, it creates just the right conditions for a slab avalanche.
Sastrugi – the wind has eroded the snow and deposited it behind the ridge in the form of a snowdrift.
Small slab already covered by windblown snow on the lee side behind a ridge. Slope steepness = 28°! Caution is required today!
Cracks are obvious signs of danger and are usually accompanied by a settling noise (“WOOMPH”). The weak layer collapses under a load (e.g. an alpinist), and the trapped air escapes, making this noise.
A recent avalanche is a clear indication of an unfavorable and unstable snowpack. The more recent the avalanche, the greater the danger of additional slab avalanches on surrounding slopes with the same or similar aspect.