I have problem understanding what kind of answer is expected in meteorology question as follows:
How many feet must you climb through to reach FL 85? Given: FL 85 Departure field elevation 1500 ft QNH 1013 hPa Temperature ISA -10deg C 1 hPa = 27ft
Is it asking about an indicated altitude change or about a true elevation difference?
If this was about indicated, altimeter set to 1013 hPa at 1500ft aerodrome at ISA-10deg C would show as 1560ft (am I applying compensation of 1% per 2.5deg C correctly?). Therefore to change its indication to 8500ft, aircraft must make a climb of indicated 6940ft? That makes answer makes sense for me, since it would represent something that is visible on AI and VSI.
On the other hand, if the question is about true elevation difference, FL85 would be indicated around 8160ft elevation at ISA -10deg C (again, is this correction in the right direction)? To climb from elevation 1500ft, it would require 6660ft elevation change. That answer feels pretty useless, since no instrument would show that (apart from a GPS or some radar).
The answer accompanying the question suggest to calculate difference of 7000ft and compensate 1% per 2.5deg C that to get 6720ft. That does not feel to correct to me at all, since it completely ignores initial elevation/altitude.
If I am using incorrect terminology anywhere, I would be happy to correct that.