for measuring atmospheric pressure of a station ,why in addition to "QFE" PRESSURE "QNH" & "QFF" is necessary?

Question

in measuring of atmospheric pressure of a specific station , it seems "QFE" parameter as actual pressure is enough. Why we need "QNH" as reduced to standard condition or "QFF"as reduced to sea altitude is necessary? it seems they are not useful or practicable information. what is meant if e.g. "QNH" is other than 1013 hpa(milibar)?

Answer 1

All the three give the atmospheric pressure.Among these three values, the QFF value is reported in synoptic messages which are taken in the various observatories(observation is taken usually every three hours )taken all over the worlld at the same time for comparison of pressure values and to draw isobars(line of equal pressure values) over weather charts.QFF is the corrected barometric pressure reduced to m.s.l. according to meteorological conventions.This is used for general forecast of weather.
But QFE and QNH values are taken in the Meteorological offices located in each aerodrome and are used for aviation purpose only.These are usually taken every half an hour and supplied to aircrafts.
QFE gives the pressure value at the aerodrome level(Runway level) which is used by the pilots to set their altimeter in the aircraft.Once this value is set in the altimeter while the aircraft is flying, it will give the height of the aircraft above the aerodrome level(not above the mean sea level) Once the aircraft has landed, it will show zero height.Therefore, the QFE setting is sometimes called zero setting.
QNH is the value of the pressure reduced to mean sea level using the QFE value.If this value is used for setting the altimeter, it will give the height of the aircrat from the sea level during the flight and the height of the aerodrome(runway level) after landing.Both QFE and QNH are supplied to the pilot in every weather message and both are used by them to set the altimeter.QNH value is used by them to know the height of the aerodrome above the sea level.
A hypothetical atmosphere where some standard values of temperature,pressure and lapse rate,gravity etc are assumed is called ICAN atmosphere.Here the standard value of pressure is 1013.25 hpa and QNH is calculted based on this value.

Answer 2

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