If you fly from Marseilles to Palma at FL100 you will follow a surface of constant atmospheric air pressure. The true altitude of this surface will depend on the density of the air below. Flying from Marseilles, where the QNH is lower, to Palma, where the QNH is higher, you would normally expect your true altitude to increase. Since true altitude (height of FL100 above MSL) remains constant, it must be colder in Palma.
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Last edited by hvogt; 9th November 2011 at 17:47.
Reason: Typo