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Fohn Wind Effect
brentonrule
Topic Author
brentonrule created the topic: Fohn Wind Effect
Hi Bob, Richard et al.
Well its me again. Now starting to get into the Met Issue 5 June 2010.
Page 3.7 talks about the Fohn Effect and I am puzzled. Is this the only time heated air goes 'down' - This diagram shows warm dry air descending?
Is this because it starts off cold, and it is 'heating - relatively speaking' on the way down; and because the cold air at the top drops down the hill?
The first thing to consider is why this air rose in the first place. It never 'wanted' to rise, it was forced to rise because it encountered rising terrain. Also a Fohn wind will only form in a stable atmosphere [usually
in the vicinity of a hight or a ridge].
When we consider stability, temperature is relative. When we say the air coming down the lee side of the range is warm - we mean it is warmer than it was when it began to rise on the windy side. The environment air above the mountain would still be warmer than the air that is forced to rise, so there would be no tendency for thermal activity.
There are two reasons for the higher temperature on the lee side. The clould that formed in the air that was lifted released latent heat into the air. [Latent heat is always released when ever cloud forms]. Also the descending air is suffering an increase in pressure, so it also heats adiabatically.
