For Bookworm; the column of air over the airfield affects the accuracy of the indicated altitude if it has a standard lapse rate, surely? If the lapse rate was lower than normal then at altitude the oat might be closer to ISA and so the altimeter would not have such a large error as it would at the field elevation.
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.
Am I right? It is easy to confuse me when I really don't know what I am talking about.