PPRuNe Forums - DenAlt... 1°C = 120 feet and also = 500 feet.

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- - DenAlt... 1°C = 120 feet and also = 500 feet. (https://www.pprune.org/questions/642050-denalt-1-c-120-feet-also-500-feet.html)

DenAlt... 1°C = 120 feet and also = 500 feet.

I am trying to understand WHY...

A: if you go up 500 feet the adiabatic lapse rate will reduce the temperature by 1°C, which is because the air is less dense as you go up... less dense equals less heat over the same volume, so it's cooler.

BUT,

B: if the ISA says standard is 15°C at MSL and your non-standard temp is 16°C, you compensate by 120 feet (higher) because warmer air is less dense. Which is basically the same reason as in 'A'.

So why does one degree equal four times as much altitude difference in A as it does in B? They're both measuring vertical shift to temperature ratio, aren't they? What is it that I don't get?

Could it be that A is a theoretical rule of thumb, application of an approximation whereas B is the application of an empirical fact?
Just guessing, without a great deal of thought, about things long past.

No.

A is lapse rate due to adiabatic pressure change.
B is the equivalent density altitude change due 1 degree C at the same sea level pressure.