Originally Posted by
tcasdescend
I understand that altitude is dependent on temperature.
But isnt temperature localised?
I mean wouldnt the temperature change due to the weather (e.g. sunny or cloudy or depending on the season?)
Wont the temperature change when you start and stop an engine for instance?
In that case how will a pilot know how to calibrate his or her altimeter?
Aircraft in lower areas and in terminal areas use the local QNH (local pressure setting) to indicate their altitude, this ensures all aircraft in the area are working off of the same reference. When higher up, aircraft all switch to a standard datum called (a permanent QNH of 1013 hPa), and use this pressure setting as the reference.
The actual true altitude with QNH 1013 at flight level 250 when it's -20C at that flight level compared to when it's -40C are of course not the same, but these differences are ignored because they don't usually have any significance, unless you're flying around the Himalayas.
The only time a pilot would really adjust their temperature is at a lower altitude or when around high terrain and the temperature is lower than 0C, it's done using a procedure called temperature compensation. If you are going off an RNP approach chart for example it will have a lower temperature limit, if the OAT goes below this temperature you must use the temperature compensation procedure to still be able to use the chart.