Originally Posted by
tcasdescend
Why only qnh at lower altitude?
If it is possible why not qne standard throughout the world?
Because at low altitude it is more important to avoid flying into terrain/mountains, but at higher altitude the primary concern is to avoid hitting other aircraft. We could use QNH at every altitude, but then we would have to adjust the altimeter all the time since the pressure is always changing. Therefore when climbing through the "transition altitude" we set the altimeter to standard pressure I.e. 1013hPa, and maintain that setting until the descent. It's just more practical.
Regarding altitude and temp. Yes, density and thus the readout on the aircraft altimeter is affected by temperature. But thing is: how could you(or the altimeter) know to what extent the temperature is affecting the air between the aircraft and the ground. Is the aircraft just flying in a especially cold little pocket of air or is actually the entire column of air between the aircraft and ground cold? This is very difficult to know precisely so instead all aircraft altimeter are calibrated to measure the air according to the "International Standard Atmosphere". It assumes the temperature at sea-level is 15C and decreases by 2C every 1000ft of altitude.
In other words: the altimeter does actually not take into account the temperature and most of the time pilots do not either take it into account. Thus the altitude readout is never exactly accurate, but close enough anyway. As mentioned in another comment: unless it is really cold we don't compensate for the temperature while flying.