Originally Posted by
ZG862
@DB: I *think* I'm with you. Let's see: The elevation of EGBJ's ARP found in the AIP is its measured, geographic distance above actual, wet & salty, mean sea level. The sea level on which QNH is quoted isn't real in that its pressure of 1013.25hPa only occurs on a spherical earth at 15 degrees C in which UK is still in EU. Therefore, calculating QFE from QNH must consider where EGBJ would actually be if the world was a perfect,
Nearly there. You have either a variable atmosphere and a fixed elevation of EGBJ which is real world. Or you have a standard atmosphere and a variable elevation which is the aviation style good enough approximation. As the variance is dependent on atmospheric condition it can't be in the AIP but you would need to ask your meteorologist for the difference QNH vs QFF of the day (hour).
The thing to take away is that QNH is just as field dependent as QFE is. Colder places will have smaller QNH than wormer places in the same air mass. Can be seen in the alps regularly.