QFF is an
aeronautical code Q code. It is the
MSL pressure derived from local
meteorological station
A weather station is a facility, either on land or sea, with instruments and equipment for measuring atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include tempera ...
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
The Q-code is a standardised collection of three-letter codes that each start with the letter "Q". It is an operating signal initially developed for commercial radiotelegraph communication and later adopted by other radio services, especially ...
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:
* The
Australian Bureau of Meteorology method:
:: 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
The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...
is according to
ISA
Isa or ISA may refer to:
Places
* Isa, Amur Oblast, Russia
* Isa, Kagoshima, Japan
* Isa, Nigeria
* Isa District, Kagoshima, former district in Japan
* Isa Town, middle class town located in Bahrain
* Mount Isa, Queensland, Australia
* Mount Is ...
and the relative humidity lapse rate is zero.
:: QFF is the location value plotted on surface
synoptic chart and is closer to reality than
QNH
The Q-code is a standardised collection of three-letter codes that each start with the letter "Q". It is an operating signal initially developed for commercial radiotelegraph communication and later adopted by other radio services, especially ...
, though it is only indirectly used in aviation.
* Another method:
:: 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.
::The range of QFF so far recorded, low pressure to high pressure, is from 856 to 1083 hPa.
See also
*
Atmospheric pressure
*
QFE
*
QNH
The Q-code is a standardised collection of three-letter codes that each start with the letter "Q". It is an operating signal initially developed for commercial radiotelegraph communication and later adopted by other radio services, especially ...
References
{{reflist
Aviation meteorology
Encodings