bunyip wrote:
<font face="Verdana, Arial, Helvetica" size="2">If the airplane was flying over an area of extreme cold on the ground, but the air above was, due to an inversion, at ISA, then the altimeter would be indicating a correct value with no errors due to low temperature. If it had the field QNH setting applied it would be above the indicated altitude.</font>
What matters is the "average" temperature of the column of air -- and I'm being a bit vague about what I mean by average. For every slice of the column, the temperature of the slice determines whether the pressure drop is higher or lower than ISA.
Say there was a 1000 ft of ISA-30 degC air and ISA above it. The pressure drop in that 1000 ft would be about 3 hPa (100 ft) more than ISA would predict. In the 1000 to 2000 ft band, the pressure drop in that 1000 ft would be exactly as ISA predicts (and the next 1000 ft, and the next...)
So for any aircraft flying above the 1000 ft of cold air, the altimeter would overread by 100 ft, whether it was at 1000 ft or 10,000 ft.
Hope that helps.