Originally Posted by
2 sheds
No it isn't - it depends on prevailing temperature and pressure range.
(my bold)
Actually, QNH does NOT depend on the prevailing temperature. It is the airfield level pressure reduced to MSL based on ISA conditions. So in the calculation from airfield-level pressure (which is measured) to QNH, an ISA temperature and temperature profile is assumed. Why? Because your altimeter is also not temperature aware/compensated. So these two errors cancel each other out, and with the correct QNH set, your wheels touch the runway when the altimeter reads airfield/threshold elevation.
There is another Q-code, QFF, where the actual temperature - and even the actual humidity - is taken into account when converting the observed airfield level pressure to the MSL level pressure. So QFF would be the pressure at the bottom of the hole, when you were to dig a hole at the airfield down to MSL. But QFF is fortunately not used in aviation, since the altimeters would need to be temperature compensated: You would not only need to enter the QFF on the subscale, but also the airfield temperature and humidity to ensure your altimeter reads threshold elevation when you touch down. (And note that even QFF makes a few assumptions that may not be valid. One of which is that the temperature lapse rate from airfield elevation to MSL is according to ISA.)
https://en.wikipedia.org/wiki/QFF
This means that the difference between QNH and QFE only depends on the airfield elevation. In that sense, it is a fixed value per airfield (although the way the rounding works, there may occasionally be a +/- 1 hPa difference, I guess).