QNH by definition is “barometric pressure measured at sea level, corrected for the elevation of an airport”. Add to that the height of a tower or other spot where instrument is located. The idea is to read correct elevation of the airport at the certain point.
Today’s barometers are very accurate. I used VAISALA, which has a temperature dial where ATC enters the present value of outside temperature and barometer automatically makes correction – if any! Having used this barometer for years, I noticed very few correction of not more than 1mb at the time. Not that we used this feature very often – to be honest!
What is important with QNH? First, to be reasonably accurate, and second, that ALL users in the same airspace use the same QNH value. Small inaccuracies related to Altimeter and barometer are irrelevant to a point. For my 33 years of controlling I haven’t heard any pilot correcting QNH for temperature – which doesn’t mean it didn’t happen - except us on the ground. To support this, very often aircraft transponders show 2-3 hundred feet variation to a level maintained and very often ATC reacts only if 300 feet or more is showed. Where is this coming from?
As stated above, my experience says that temperature correction is for max 1 mb, in another word 28 feet. 28 feet goes almost 11 times in 300 feet! Temperature discrepancy, reading error, aircraft altimeter and ground barometer permissible errors combined would never produce error bigger than 2mb and that is very pessimistic value- it would probably remain within 1 mb. 2mb equals to 56 feet! Would this endanger aircraft knowing how much safety is included in procedures and MSA? ILS CAT 1 is different story, there’s no room for 56 feet!
Question for pilots: are you guys going to use barometric altimeter in full IMC, say WX at minima, flying ILS CAT 1 to the minima of 200 feet? I don’t think so, too dangerous! Not only because of possible combined error but barometric altimeter is also very slow – or better said not fast enough to accurately indicate 200 feet. Beside that you can read altimeter only in 100 feet increments. Electronic altimeter, whatever name they are called today, wouldn’t care for outside temperature at all, you will know vertical distance from the runway in a foot – and that is what you are going to use!