Originally Posted by
n5296s
Supplementary question, why is it 2000' in mountainous terrain (at least in the US)?
Does PITHBBLOT mean anything to you? In case you haven't seen it before it's a handy mnemonic for remembering the errors of the barometric altimeter (it'll certainly familiar to pilots taught within the British mil). The one that we're interested in here is the 'O', orographic error. Windspeed tends to increase over higher terrain (a not dissimilar example is increased speed over an aerofoil section), and as we know as speed of the air increases the pressure decreases. This has the effect of creating a lower atmospheric pressure over mountainous terrain. This in turn would result in an aircraft effectively flying at reduced terrain separation over those mountainous areas if flown purely by reference to a barometric altimeter ("high to low, down you go"). Conventional wisdom suggests that this error starts to become significant when flying over terrain of 3000ft or more, hence many organisations describe increased safety altitudes in those areas.