Answer
This is what it is all about.
Altimeters are pressure sensors, calibrated with a feet scale according to ISA scale.
As mentioned in another reply, if it is colder than ISA, the isobars will be closer, resulting in your being lower than what your altimeter reads. Someone heard the saying From high to low, lookout below ? This goes for both low pressure (if flying on 1013) as well as for temperature, both for QNH/1013 (QNE).
If the station is at the same level as yourself, there will be no correction, but as the station is below you, your altitude is only correct at the station level. As mentioned above, your height will deviate with 4% for each 10 degree ISA deviation. The ISA temp is around -5 at FL100 (-4.8 if you want to be anal), and since they say it is -20, you are 15 below ISA. These 15 degrees have to be corrected for the air between the aircraft and the station, in this case 7500 feet. So the calculation is
10000 - 0.06 x 7500 = 9550'
It is confusing to many - was to me for a long time as well - but if you read my explanation above and supply it with sketches of various scenarios, you should be able to see the logic.
If flying FL, remember to correct for QNH also, as well as up to 2000 feet for mountaineous areas, the latter, if we are discussing miminum safe altitudes.
Regarding the QFE/QNH questions raised above ... first of all, at 2500 feet, it is highly unlikely a QNH, as a QFE around 2500 feet should be in the order of 27½ inches, +/- local pressure. But, whether or not it is QNH or QFE does not matter in this question, as the QNH is irrelevant in this case. If the question stated you were in FL100/flying on QNE (which is 1013/29.92), then you would have to correct for it. High to low, look out below.
Last edited by TheDrop; 23rd Sep 2007 at 07:30.