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

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.

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.