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HeliTorque :: View topic - That fekking (I mean Fohn) wind effect
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HeliTorque Forum Index » Ground School

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That fekking (I mean Fohn) wind effect
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tombeeston
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PostPosted: Tue Apr 03, 2012 7:55 pm    Post subject: That fekking (I mean Fohn) wind effect Reply with quote

Hi all

I wonder if anyone can answer the following question please?

At sea level on the windward side of a mountain, the air has a temp of +19C and dewpoint +5C, approx what temperature would you expect the air to be on the lee side at 2000ft?

a) 10
b) 11.5
c) 13
d) 16

I think this is a question about the fohn wind effect. i understand that the lee side air is warmer and drier, and the cloudbase is higher, (and i think i understand why!) but I am really struggling to quantify it.

(I've had the same problem as another post in here about the dry adibiatic lapse rate being modified by -0.5C, but nothing mentioned in the red pooley's book. I don't even know if it's relevant to this question!)

Answer is C by the way, all help greatly appreciated

Thanks

Tom
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tombeeston
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PostPosted: Tue Apr 03, 2012 8:46 pm    Post subject: Reply with quote

another in the same vein:

at sea level on the windward side of a mountain, air has a temp of +19C and dewpoint + 13C, due to rainfall, dewpoint becomes +11.5C on the lee side of the mountain, approx what is the cloud base on the windward side?

a) 2000ft
b) 1800ft
c) 2700ft
d) 3100ft

answer is a, but again i don't really know why!

thanks
Tom
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Ascend_Charlie
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PostPosted: Wed Apr 04, 2012 1:36 am    Post subject: Reply with quote

To both of those, the answer is:
e. Who really gives a stuff about it because it is useless rubbish that clogs up your brain and has between Nil and Zero use in real life.
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ALFA8C
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PostPosted: Wed Apr 04, 2012 9:12 am    Post subject: Reply with quote

Hi Tom,

Hope you are well.

Please tell me these are NOT questions from a PPL(H) examination paper!

If they are, I don't stand a cat in hells chance of passing it!!!!!!

J.
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tombeeston
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PostPosted: Wed Apr 04, 2012 4:36 pm    Post subject: Reply with quote

Ascend_Charlie, never a truer word spoken. <ripple of applause>

A8C, they are example ppl met questions. Apart from not knowing how to answer them, I'm fine! I hope you're well too?
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paddywak
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PostPosted: Wed Apr 04, 2012 10:55 pm    Post subject: Reply with quote

They normally give you a height for the mountain and the other dew point in these type of questions.

The Saturated Adiabatic Lapse rate (SALR)= 1.98C per 1000ft and
The Dry Adiabatic Lapse rate (DALR) = 3 C 1000ft.

This means if the air is unsaturated or dry (same thing) the parcel of air will cool down at the rate of 3 degrees C until it cools down so much it becomes saturated. This point is called the DEW POINT.
This saturated parcel of air will then cool down at a LOWER rate than before, at 1.98C per 1000ft or sometimes written as 1.5C per 1000ft depending on what you read. This is the SALR
Now your parcel of air will form cloud as it cools down on the windward side of the mountain and at a point near to the top of the mountain precipitation occurs and so the parcel of air dries out again and descends down the lee side warming adiabatically at the DALR of 3 C per 100ft.

In your example the question is missing the dew point for the air on the leeward side. If you had this figure you could work out the point where the SALR changes back to the DALR and then arrive at a temperature.

Just out of interest the cloud base can be calculated as temperature-dew point x400 = cloud base in ft
EG: OAT 9 C - DEWPOINT 7 C X 400 = cloud base of 800ft

You can see that the closer the temp is to the dew point the lower the cloud base will be.

I hope this makes some sense, good luck with your exams!
Leigh.
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LoachBoy
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PostPosted: Fri Apr 06, 2012 8:41 am    Post subject: Reply with quote

Which is why you can never see the Exe Valley when temp & dew point get within 1 or 2 degrees of each other!!!
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