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QFF is a Q code. It is the MSL pressure derived from local meteorological station conditions in accordance with meteorological practice. This is the altimeter setting that is intended to produce correct altitude indication (i.e., no error) on an altimeter at the actual sea level elevation, while QNH is intended to have no error at the station elevation (or, especially when applied within a region with a relatively small range of surface elevations, at the altitudes close to the surface elevation within the region).
Meteorological practice of calculating QFF differs between meteorological organizations around the world. Some examples:
- QFF is derived from the barometric pressure at the station location by calculating the weight of an imaginary air column, extending from the location to sea level, assuming the temperature and relative humidity at the location are the long term monthly mean, the temperature lapse rate is according to ISA and the relative humidity lapse rate is zero.
- Another method[which?]:
- The derivation assumes that an isothermal layer at the station temperature extends to the sea level. This is the barometric pressure at the surface reduced to MSL using the observed temperature at the surface (which assumes an isothermal layer from MSL to that surface). QFF accounts for the effect that temperature has on the pressure lapse rate and therefore the resultant calculated pressure.
- Kloth, Ralf D. (1994-12-31). "List of Q-codes". Retrieved 2014-05-16.
- "Communications Instructions Operating Signals" (PDF). Combined Communications-Electronics Board. April 2006. Retrieved 2014-05-16.
- "QFF versus QNH". atpforum.eu. Retrieved 2014-05-16.
- "International Standard Atmosphere (ISA) and Pressure Settings". Retrieved 2014-05-16.