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Moderator: AndyR

#1548168
Ugh :?

thought I understood this, turns out I don't.

So, from Pooley's Air Pilot's Manual on Meteoroogy:

"On a given day the sea level pressure is 1013.2 hPa. The temperature at 3,000ft AMSL is given as minus 4C. The temperature difference compared with the ISA is ... "

I'd thought from what I'd read that for every 1,000ft increase in altitude the temperature would be expected to decrease by approximately 2C. Which would mean that at 3,000ft we would expect the temperature to be 9C, the fact that it's actually -4C would therefore mean that the differential is ISA (9 at that altitude) - 13 (to get to the -4C).

The book answer though is that the differential is ISA - 10. Which I just don't get at all. If they mean 15 - 10, then that doesn't work. If they mean 9 - 10 then that doesn't work either.

There must be a glaring error in my understanding of the question, some issue with my maths, or a problem with the book answer. I can't work out which it is though :(

If someone could explain the correct answer to me I'd appreciate it. Thanks :)
By johnm
FLYER Club Member  FLYER Club Member
#1548172
It's a while since I did this but as I recall......

The dry adiabatic lapse rate is a constant 3 deg C per 1000ft the moist varies with temperature.

So in your example ISA would be 15 -9 i.e. 6 so your answer would be ISA - 10.

The two degrees per thousand feet represents a typical moist adiabatic lapse rate in our climate AIUI.
#1548175
johnm wrote:The dry adiabatic lapse rate is a constant 3 deg C per 1000ft the moist varies with temperature.

So in your example ISA would be 15 -9 i.e. 6 so your answer would be ISA - 10.

The two degrees per thousand feet represents a typical moist adiabatic lapse rate in our climate AIUI.


That certainly makes the maths work, but the book has made absolutely no mention of 'dry' versus 'moist' adiabatic lapse rates so far. It may mention it later, of course - at the moment though (if that's why the answer is what it is) then they're asking questions based on material that hasn't been covered :(
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By GrahamB
FLYER Club Member  FLYER Club Member
#1548186
Adiabatic lapse rates have nothing to with this question, as they pertain only to when air rises and falls.

Sea level pressure is 1013.25 = ISA standard, so no need for any offset there.
ISA standard SL temperature is 15 deg C, and the standard lapse rate is 1.98 deg C per 1000', hence ISA temp at 3000'amsl = 15-(3 *1.98) deg C = 9 deg C for all intents and purposes.

The temperature given at 3000' is -4 deg C, which is ISA -13 deg C.

The book is wrong.


(edited for typo)
Last edited by GrahamB on Sun Jul 23, 2017 8:08 am, edited 1 time in total.
A le Ron, MercianMarcus liked this
#1548189
GrahamB wrote:Adiabatic lapse rates have nothing to with this question, as they pertain only to when air rises and falls.


Which would explain why I've not read about them so far.

GrahamB wrote:Sea level pressure is 1013.25 = ISA standard, so need for any offset there.
ISA standard SL temperature is 15 deg C, and the standard lapse rate is 1.98 deg C per 1000', hence ISA temp at 3000'amsl = 15-(3 *1.98) deg C = 9 deg C for all intents and purposes.

The temperature given at 3000' is -4 deg C, which is ISA -13 deg C.


Which is what I thought originally.

GrahamB wrote:The book is wrong.


Which is both good, because I got the right answer, and bloomin' annoying :-/ At least I know I'm not going mad.
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By webdevduck
FLYER Club Member  FLYER Club Member
#1557270
This has been baffling me for the past couple of days too. Thanks for the clarification. :thumleft: If only they published an errata.

I have to say I'm unimpressed with the APM books so far. Somewhere I have the AFE Air Law book (which I've managed to mislay) and I found it clearer and better illustrated.