QNH is the barometric setting which will make your Alt read the mean height above sea level in an area. So inbound you will check the "Area QNH" for the airport you are heading to and set the altimeter to that ...neatly avoiding smacking into a runway whilst your altimeter still reads 500 AGL because the pressure has changed from when you last set the instrument. The altimeter is, of course, a pressure instrument and so the reading will vary as barometric pressure varies. In most of the world QNH is stated in hPa but in the US they use inches of mercury, repective alt settings being 1013 and 29.92.
Another way to set it on the ground is to dial the pressure until the alt reads the known airfield height.
Thanks for the insight Darryl,
wouldn't it make more sense if on the ground I set the altimeter to exactly zero? What if I am 10,000ft AGL and the airport is, lets say 1822ft ASL? Do I simply add or subtract the height of the airport? How does this work?
There must be some literature about such things somewhere available for download. Does anyone know a link by any chance?
Thanks again for your help,