One possible correction here: although an altimeter does only respond to pressure (and measures pressure very well) pressure does not correspond to altitude, except at ISA lapse rates.
If you fly from a hotter region to a colder region your altitude will decrease, although air pressure (and altimeter) may very well not change.
For example, take a procedure commencement altitude of 3000 feet indicated, QNH 1013hPa or 29.92inHg (how convenient yes?). .If the temperature is 30°C (North Africa) true altitude will be around 3210 feet. If the temperature is -20°C (Arctic), true altitude will be 2690-ish feet.
My apologies if I have misunderstood you Max Motor. I'm sure you know all the above, but artflyer may not. In any case, anyone can check my working with a CRP1 or Jeppeson slide rule.
To get back to the original question - has anyone answered it conclusively yet? I couldn't actually tell! But if the air is
hotter than ISA for a particular pressure altitude, you will have to wind up the subscale over QNH to get True Altitude (or aerodrome elevation if you are on the ground at your mountain strip.) This would make mountain QNH higher than "real" sea level pressure at the bottom of the cliff.
In other words, as per my example, your altimeter says 3000 feet at your local North African 30°+ mountain airfield, but the airfield is actually at 3210 feet AMSL. So wind up the QNH subscale to get the altimeter to read 3210 feet...
cheers,. .O8 <img src="smile.gif" border="0">
[ 15 February 2002: Message edited by: Oktas8 ]</p>