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HeliTorque :: View topic - Meteorology, dewpoint temperature, clouds
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HeliTorque Forum Index » Ground School

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Meteorology, dewpoint temperature, clouds
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Dark007
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PostPosted: Thu Aug 20, 2009 8:43 pm    Post subject: Meteorology, dewpoint temperature, clouds Reply with quote

I'm a little annoyed with my PPL Confuser. Mainly because it's doing exactly what it says on the tin.

Here's the question:
Given a surface temperature of +21 deg C and a dewpoint temperature of +7 deg C, at approximately what height will the base of cumulus cloud be found.
A - 4000 ft
B - 7000 ft
C - 8500 ft
D - 5500 ft

Here's my workings and answer:
21 - 7 = 14 / an Dry Adiabatic Lapse Rate of 3 deg C per 1000 ft
14/3 = 4.6 * 1000 ft which means the nearest height is "A" - 4000 ft

Think about that and I'll post the actual answer a bit further down....


























Apparently it's "D" 5500 ft.
According to the book's explanation:
"Because the surface dewpoint temperature is given, you are required to account for its affect (sic) when calculating the cloud base.
The dewpoint decreases by 0.5 deg C /1000 ft, so the DALR of 3 deg C/1000ft is modified to 2.5 deg C/1000ft.

Cloud base = (surf temp - dp temp)/DALR
= (21 - 7)/2.5
= 5600 ft
The nearest answer in this case is 5500ft."

Now this, to me, appears to be bollocks. As far as I know the dewpoint temperature of the air doesn't change, it is what it is. I can't find anything that cooberates this. DALR is 3 deg C, and SALR is 1.5 deg C, ELR is 2 deg C. So where did the 5 deg C dewpoint lapse rate come from, if there is such a thing.

Any help gratefully received.

I'm going to find a beer...
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niho
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PostPosted: Thu Aug 20, 2009 9:41 pm    Post subject: Reply with quote

Hi Dark,

I'm doing my met at the moment too and find this stuff hard! I agree with your workings, and can't see anything about dewpoint temperature changing in the red pooley's book, however a search of the omniscient wikipedia does suggest that it changes: http://en.wikipedia.org/wiki/Lapse_rate#Significance_in_meteorology see the second paragraph of the "Significance in Meteorology" section:
Quote:
The dew point also drops (as a result of decreasing air pressure) but much more slowly, typically about −2 C per 1,000 m


1000 m = 3281 feet
2 deg per 3281 feet
2/3281*1000 = 0.6 deg per 1000 feet - not the 0.5 as the confuser suggests but close enough...
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Dark007
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PostPosted: Fri Aug 21, 2009 9:54 am    Post subject: Reply with quote

Thanks Niho. It's perplexing as to why that bit of information isn't in the Red Pooley's book, particularly as exam questions rely on it. I'm seeing my instructor for a lesson later, so I'll ask him about it.
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RicardoGarces
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PostPosted: Thu Oct 01, 2009 11:22 am    Post subject: Reply with quote

Hi guys,

I know a lot of time has passed already after this post, but I was checking it out, and I don't think you realized everything properly.

Your first calculations were right. Your result was 4600, not 4000.

So, if you have a cloud base starting after 4600, and there are no cumulus cloud bases above 6500 ft, three answers were wrong, leaving you with the 5500 answer to be correct. Get my drift?
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Dark007
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PostPosted: Thu Oct 01, 2009 12:03 pm    Post subject: Reply with quote

Yes it was wrong, but not for the reason you say. Of the four possible answers the nearest answer to 4600 is 4000.

However, in the posts further down and in the confuser 4000 is wrong because I failed to take into account the dewpoint temperature change per 1000 feet.
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PostPosted: Fri Oct 02, 2009 11:09 am    Post subject: Reply with quote

Yes you are right. Maybe my logic could be dangerous if they had given for instance, closer heights to the correct number. But, you have to agree that, in this particular case, I would get lucky Smile

For everyone out there looking into this post, don't forget:

Calculation for base of clouds

T-Td x 400ft

as simply as that.

Thank you Dark, your post made me look into this and learn it.
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