Some random thoughts:
- Q codes were apparently started for Morse radiotelephony… and for some reason somehow stuck!
- It would seem that there is no logic to the letters used as far as any mere mortal can determine. So not an abbreviation, not an acronym, not anything logical.
- Between general communications, aviation and maritime uses, there are more than 200 Q codes (!).
- I used to fly in South Africa years ago. At that time there were a paltry few Q codes in regular use: QNH (as discussed), QFE (to show 0ft on the ground at X airfield), QSY (change frequency), QTR (what is the time), QBI (Instrument Flight Rules in effect at…). That was about it.
- Altimeter settings: as already mentioned just about the whole world uses the metric system. And in that regard: when I started flying in the 70’s, millibar was the norm in South Africa as the country was fully metricated. But some years later it was determined by the “General Conference on Weights and Measure” (which is globally recognised as the “governing body” for weights and measures) that the hectoPascal was the true SI metric unit of measure for pressure. And was thus formally accepted in aviation. Other than in North America…
And yes: in reality a hPa is exactly the same as a mbar.