Welcome Guest
HeliTorque
  
User Control Panel

Security Code: : Security Code
Type Security Code Here: :
 
Register Here
Lost Password?

Online Stats:
Visitors: 23
Members: 0
Total: 23

Membership:
New Today: 0
New Yesterday: 0
Registering: 0
Members: 6662
Latest: chrisw

Most Ever Online
Visitors: 447
Members: 10
Total: 457


HeliTorque :: View topic - More Fohn effect
Forum FAQ
Forum FAQ
Search
Search
Memberlist
Memberlist
Usergroups
Usergroups
Profile
Profile
Contact Manager
Contact Manager
Log in
Log in
Log in to check your private messages
Log in to check your private messages
HeliTorque Forum Index » Student Pilots & Hour Builders

Post new topic   Reply to topic All times are GMT
More Fohn effect
View previous topic :: View next topic  
Author Message
paddywak
H Addict
H Addict


Offline
Joined: Mar 28, 2009
Posts: 537
Location: UK


uk.gif

PostPosted: Fri May 11, 2012 8:34 pm    Post subject: More Fohn effect Reply with quote

I came across a question which has been irritating me which concerns the fohn effect.

the temperature at the surface is 16c the dew point is 4c, the condensation level on the lee side of a mountain 10000ft high is 8000ft. What will the surface temp be on the lee side.

Simple enough you may think!

I did temp -dewpoint x 400 = cloud base in ft

Cloud base = 1200ft.

surface temp at 16c - 3c/ 1000ft DALR until at 1000ft then reducing at the SALR of 1.8/1000ft = -4.4c at 8000ft.

Warming up at the DALR down from 8000ft to the surface gives 19.6c at the surface.

Does any one else get the same?

The answer marked correct was 18.5c

Puzzled Rolling Eyes
Back to top
View user's profile Send private message
tombeeston
Starting to 'Torque
Starting to 'Torque


Offline
Joined: Jan 18, 2010
Posts: 41
Location: Duns, Berwickshire



PostPosted: Fri May 11, 2012 10:48 pm    Post subject: Reply with quote

No idea!!

Is it another one of those DALR is actually 2.5 deg C things? (ref other posts on helitorque)

Personally I think the actual quantification of this seems to be largely ridiculous - I expect there are a few theories, 1 exam paper quotes from one, 1 from another etc, nothing ever being as simple as it seems.

We need a meteorologist!!!
Back to top
View user's profile Send private message
flip2
High Flying 'Torquer
High Flying 'Torquer


Offline
Joined: Sep 05, 2009
Posts: 225



PostPosted: Sat May 12, 2012 10:15 am    Post subject: Reply with quote

Interesting.

I do these calculations either:

1. Long hand

DALR = 3 / 1,000ft
SALR = 1.8 / 1,000ft

Sea Level Temperature: 16C
1,000 ft 13C (DALR)
2,000 ft 10C (DALR)
3,000 ft 7C (DALR)
4,000 ft 4C (DALR... now temperature is the same as dew point)
5,000 ft 2.2C (SALR)
6,000 ft 0.4C (SALR)
7,000 ft -1.4C (SALR)
8,000 ft -3.2C (SALR)
9,000 ft -5C (SALR)
10,000 ft -6.8C (SALR)

And now we go back down the otherside:

10,000 ft -6.8C
9,000 ft -5C (SALR)
8,000 ft -3.2C (SALR... now at the cloud base given in the question)
7,000 ft -0.2C (DALR)
6,000 ft 2.8C (DALR)
5,000 ft 5.8C (DALR)
4,000 ft 8.8C (DALR)
3,000 ft 11.8C (DALR)
2,000 ft 14.8C (DALR)
1,000 ft 17.8C (DALR)
Sea Level 20.8C (DALR)

So the answer I get is 21C

-- or --

2. Short hand

If you know the cloud base on either side, you can use the formula:
Temperature Rise = Cloud Base on the Lee Side (in '000s) - Cloud base on Windward (in '000s) * 1.2
(Side track: the 1.2 comes from DALR - SALR)

We have already calculated the cloud base on the windward side as 4,000ft.

The question gave the cloud base on the lee side as 8,000ft.

So, Temp Rise = (8-4) * 1.2 = 4.8C

The original temperature was 16C, so the answer = 16 + 4.8 = 20.8C

Which is the same answer as above... but not apparently the "correct" answer.

So my next thought was they are using the old SALR of 1.5 / 1,000ft.
Using my formula above:
4 * 1.5 = 6C
16 + 6 = 22C

Again, not the "correct" answer.

But using Tom's 2.5 DALR (and 1.8 SALR):
4 * 0.7 = 2.8C
16 + 2.8 = 18.8C

That is getting close to the "correct" answer of 18.5C.

Errors do happen in the database... is it possible that this is one of them?
Back to top
View user's profile Send private message
paddywak
H Addict
H Addict


Offline
Joined: Mar 28, 2009
Posts: 537
Location: UK


uk.gif

PostPosted: Mon May 14, 2012 5:00 pm    Post subject: Reply with quote

Yes Flip, I think youre right..there does seem to be quite a few errors in the Bristol bank as well as hordes of fixed wing questions. I dont think they can bothered to separate the questions which may be relevant to us rotary students.

There is a market there for a descent set of heli questions to practice your knowledge on.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    HeliTorque Forum Index » Student Pilots & Hour Builders All times are GMT

 
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Sponsors


Billund Air Center

Visit HeliTorque!