In the Oz Report last week Hansjoerg Truttman gave a highly accurate warning of tumblings and crashes that actually went on to happen less than 48hrs later. But even despite that, some pilots maybe still don't get it: Gerolf Heinrichs (whom designed my glider and I'd certainly give him respect for his judgement on such matters) is reported by Davis Straub
"I asked Gerolf about foehn conditions and he says that it requires an upper level layer of stable air and that you can usually see the lenticulars. There has been no indication of upper level stable air except perhaps a bit on day two"
Well, that's a bit beside the point. Yes, maybe a classical foehn condition requires a venturi to be established (with the mountains providing one side) but in fact the problem in Austria this week has been quite simply "wind" (Foehn is in fact a complex meteorological phenomenon, of which wind is just one component). Lenticulars are a poor indicator indeed of this wind; maybe only one in ten foehn conditions here produce them (you need the right amount of moisture in the air, to begin with).
Indeed, the foehn all this week here (I'm about 400km West of the Worlds location at Greifenburg and south of the Alps) has been gentle and given excellent flying conditions, south of the Alps. This doesn't mean that you don't get a venturi effect in the Alps, and the tropopause still acts as a layer below which the venturi can get established.
Just think about it: the tropopause is usually at around 8 to 10km. The Alps are 3 to 4km high. Without having a stable layer below the tropopause, you create a venturi that is a reduction of 1/3 to 1/2 of the normal space that the air passes through. Add to this a pressure difference across the alps, and you have real potential for high winds, gusts, wind shear. Mix in a decent lapse rate, like reported the whole of this week... tumble, tumble, tumble. Not much fun, I'd say.
11:03:59 AM 
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