Safety: reading the snow

During winter and spring snow is transformed by sun and wind. The snowy covering alters. To progress under the safest conditions and anticipate, it’s best to be able to read the snow. Mountain guides give you the right information.

The wind and build-ups

During snowfalls and the days that follow, the wind moves the snow. It will consistently accumulate on the side described as “downwind” (in the opposite direction of the prevailing wind) of hills and peaks, but also on slopes if it is on a rock or unstable ground.

Look around you carefully: little clues allow you to work out the direction of the prevailing wind. This therefore lets you work out on what slopes the build-ups will be on in the form of snowdrifts and, in advance, possible wind slabs.

Snow sintering is one of these clues. Small bulges are formed on the surface of the snowy blanket. They appear rough sided or in stairs. This side is the wind’s direction. Photo: François Rapin / Cemagref

Another clue: The little trees on the subalpine level. They fight the natural conditions to survive The conifers adapt particularly well and develop a shelter described as “feathered”, by allowing their branches that face the direction of the wind to die. The side on which the tree will grow its branches horizontally and very often on ground level, indicates the “downwind” side.

Each peak or marked relief must imply that a snowdrift has formed. It will be more or less high and overhang more or less. It is extremely dangerous to go round it. While traversing the peaks, you always go for the “windward” side, where there isn’t much snow. In any event, you should stay well back, several meters away from the edge of the snowdrift. They are nearly always very overhanging.

Air pockets or thawed areas

The level described as “mountainous” (on average up to 1800m in altitude) is characterised by the presence of bushes (willows, alders, etc.) that bend under the weight of the snow from the first falls and often stay trapped throughout the season.

Fall after fall, they can be completely covered. They therefore form bumps under the snow. A few twigs often signal their presence. You must therefore go well around them at the risk of getting violently stuck in an air pocket that their branches have created.

Even under the snow, rocks capture the sun’s rays and reflect them. This causes the snow to melt almost to ground level, creating a thawed area around the rock. It can surprise you and cause you to fall. It is therefore advisable to avoid rocks. The majority are given away by the presence of a “bump” or characteristic snow that has thawed and frozen again. 

Snow bridges

On rocky reliefs (screes, moraines, etc.), the snow generally doesn’t cover all the rocks. While progressing in these environments, the trekker must always have in mind that an air pocket, sometimes deep, can be covered by a more or less solid “snow bridge”. If the weather has remained very cold for several days, you can assume that the bridge will hold.

In any case, it’s better to avoid suspected snow bridges. If there’s no alternative, after checking with your pole, cross it slowly one by one, walking carefully!   

The Mountain Guides are trained in snow conditions/avalanches and know how to detect the most common risks. During each of their treks, they will give you their analyses and their thoughts to justify their decision to change the route or goal. In any event you will learn to be cautious.

For more information on snow and avalanches - www.anena.org

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