[GrahamB]

3 x 2 deg C /1000 = a lapse of 6 deg C

15 - 6 = 9

9 deg C.

[johnm]

Need the context to determine the lapse rate.

The ICAO works on a standard lapse rate of 1.98 deg C per 1000 ft up to altitude nose bleed, but the Dry Adiabatic Lapse Rate (DALR) is 3 deg C per 1000 ft

Moist Adiabatic Lapse Rate is variable to significant degree with temperature and is all about how clouds form...….

However if we're talking standard ICAO assumptions then Graham's calculation is pretty much right.

[pizzytinzel]

Yep, per the above really.

2C per 1,000 ft is the assumed LR for ISA - International Standard Atmosphere, which is mainly used to calculate density altitude for landing/TO calcs.

[webdevduck]

If my memory isn't failing me, the APM Met book has an error in it. It confused me last year and is not helped by the fact they don't seem to have an online errata. I'm going to guess that is where you're going with this. Have I guessed correctly and if so do I get a prize?

[Irv Lee]

General musings:
Students don't seem to like Met. Just generally, try not to confuse the ISA (the Met one, not the financial one) with the real world. It describes a planet that doesn't exist. If I told you there is a place on Earth where sea level temp is 15 deg C 24/7/365, would you believe me? - there isn't such a place, never mind all the other bits like approx. 2 deg lapse rate. Many students don't get on with Met because they don't grasp early on that the ISA is a committee-like invention, that is EXTREMELY useful(and necessary) in aviation for lots of stuff (e.g. making sure during planning that you should not end up running into the airfield boundary fence on landing (can't guarantee it, it depends where you touch down!), not running into the same fence on take off, not crashing into a hill because you can't climb once airborne, etc). What it is useless at is telling you actual real values like real lapse rate in any particular bit of atmosphere on any particular day.