Foehn effect

visentin · Joined: 13 Jan 2008 Posts: 322

http://www.summitpost.org/article/466432/foehn-effect.html

scooter12ga · Joined: 29 Jan 2008 Posts: 127

Great article! I always wondered what to call that.

cb294 · Joined: 12 Jun 2008 Posts: 123

Good description of the meteorology, just one minor piece of pointless trivia: The word Föhn in German originally referred (and still does) to the warm southerly wind coming over the Alps. AEG used Fön as brand name for their electrical hairdriers. Subsequently that brand name has replaced the generic word and is now used for all electrical hairdriers.

CB

visentin · Joined: 13 Jan 2008 Posts: 322

MoapaPk · Joined: 13 May 2005 Posts: 4403

"While rising along the mountain slopes, in contact with the ground, it gets colder. "

Actually, it would get colder just from adiabatic expansion. Because of the added latent heat of condensation, the cooling effect (degree per m of elevation change) is less for the rising wet air, than the warming effect for descending dry air.

"spannish" = Spanish?

visentin · Joined: 13 Jan 2008 Posts: 322

den dzjow · Joined: 07 Nov 2004 Posts: 4

There is no other meteorological phenomenon spreading so much confusion and mistakes than the foehn. It all started in the 19th century when an Austrian meteorologist Julius von Hann came up with the "thermodynamical foehn theory" which is summarised in the figure in the article. Unfortunatly his unrealistic theory has been widely accepted throughout history. The result is that similar figures and explanations are still found in almost any meteorological handbook nowadays (and now also here on summitpost).

Unfortunatly this theory is wrong. Or let say it's role in the whole foehn principle is almost negligible. Foehn in the Alps always starts when there isn't precipitation falling on the loefside at all! The airmass doesn't rise entirely from ground level on the loefside to the mountain summits. In reality most of the airmass in the lowest levels is blocked ahead of the mountains while air with a warmer potential temperature from aloft (from around summit level) sinks dry adiabatically on the leeward side in a supercritical flow in terms of fluid dynamics. That's in breef how foehn works in reality.

People who are further interested in the meteorological background should try to read for example the works of Petra Seibert. http://www.meteohistory.org/2004polling_preprints/docs/abstracts/seibert_abstract.pdf
Here you can read about the confusion resulting from Hann's theory.