It is an example shown at page 4 CPL Meteorology:
[attachment=2448]image.jpg[/attachment]
It is given that the aircraft is 8500 AMSL and the QNH is 1018. Since 8500 is already AT MEAN SEA LEVEL, isn’t it already its Pressure Height, instead of 8350?
[attachment=2449]QNH Question.png[/attachment]
Bob has shown the distinction clearly with the sketch. That is, you need to consider the pressure datum level from which the altimeter reading is taken.
However, keep in mind that the 30 ft/hPa is an approximation used for convenience and is fit for purpose at reasonably low altitudes, say, below 8000-10000 feet. Suggest a look at post #6 in this thread to see how the real numbers vary in the ISA model -
https://www.bobtait.com.au/forum/performance/6777-altimetry-rates#13277
Engineering specialist in aircraft performance and weight control.
