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SnowFlurry 18 2016/17 | Snow in a foehn storm & Sahara dust

A hairdryer is not a hairdryer

by Lukas Ruetz 03/02/2017
For the coming weekend, there are warnings of a Föhn storm on the main Alpine ridge and the areas north of it - including Saharan dust. Almost every skier is familiar with foehn or foehn-like effects on the mountain. Especially those who like to stay in areas on the main Alpine ridge. But what does the downdraft do to the snow cover?

Foehn basics

Discussed in the WeatherBlog 4 2016/17

Foehn effects (primarily dust clouds on the windward side and clearing on the leeward side) occur in all wind directions as soon as there is a mountain range in the way. West föhn, for example, can often be observed in the valleys east of the Arlberg.

Another example: The south föhn only breaks through into the valley in the west-east running Sellraintal, Northern Stubai Alps - home of the Schneestöberer - at around 160km/h wind (not to be confused with gusts) on the Patscherkofel, Innsbruck's local mountain known for its föhn storms. Otherwise it is completely calm in the villages, while there are already gale-force gusts in the valley floor of the Wipptal (running from Brenner to Innsbruck) and in Innsbruck. At summit level in the northern Sellrain mountains (summit heights of 2800m) there is a weak wind on the Patscherkofel (about 20km further east) during storms, while in the southern Sellrain mountains with summit heights of up to 3300m there are strong winds, but no storms yet. In Sellrain you can also observe foehn-like effects with westerly winds: West of the Kühtai Pass (2000m) at the end of the valley it is snowing lightly and overcast, east of this in the valley itself the sky is clear.

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What does the foehn do to the snow?

This is relatively easy to explain: Like any other form of wind, the foehn forms drift snow packs, wind gangs = sastrugi, wind dunes, scours, comet tails, cornices, drifts, snow drifts, snow sweeps, snow drifts and snow plumes. If you can't tell the difference between wind gnats and wind dunes or drifting snow and snow plumes, you should take a look at the EAWS glossary. In addition, like all types of wind, it can cool a moist snow surface by repeatedly bringing in fresh, non-water vapor-saturated air (evaporative cooling) - but usually causes the snowpack to become more soaked due to the increased temperatures.

In principle, the foehn has very different effects on the snowpack depending on the area, regardless of the direction.

Interesting about snow packs that form in connection with south föhn is their susceptibility to disturbance. Due to the usually higher air temperature or high zero degree line, fresh snow packs form at all altitudes - especially in the classic foehn snow lines. However, these are often only disruptible, i.e. brittle, from a certain altitude: On 3.3.2017, the Tyrolean avalanche situation report read: "The main danger comes from fresh and recently formed drift snow packs. On the shaded side, these can be disturbed above about 2200m, on sunny slopes above about 2500m. With increasing altitude, the drift snow accumulations become larger, thicker and more susceptible to disturbance." Although the snow was also transported below these altitude limits, it is much more difficult or even impossible to disturb there, mainly due to the high temperatures.

And the Saharan dust?

We'll keep this night-and-fog poaching short and simple in the WeatherBlog territory: Saharan dust reaches the Alps when low-pressure areas that extend as far as North Africa generate a southerly high-altitude current and dust is stirred up on the ground, which then reaches the high-altitude current. The Alps are located at the front of the trough (= on the eastern side of the low pressure area). The wind or the air masses are transported towards the Alps by the direction of rotation of the low (counter-clockwise in the northern hemisphere). This means that the wind blows from a southerly direction, i.e. foehn north of the main ridge. In most cases, the low pressure development takes place too far north and many low pressure areas slowly lose strength on their way to the southeast, so Saharan dust transport does not occur every time a low pressure area lies southwest of the Alps.

Whether and how the avalanche risk changes due to Saharan dust can be found here at the SLF and here at the Avalanche Warning Service Tyrol.

No matter what layer of Sahara dust is in the snowpack, it will always be on the snow surface at some point when it melts - as the snowpack melts from top to bottom.

Note: Wind (with fresh snow) is the easiest avalanche-forming factor to recognize - but is also responsible for the majority of avalanche accidents. If you regularly visit the same areas, you should get to know the effects of the different wind conditions for each area.

This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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