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QNH and QFE
cessna888
Topic Author
cessna888 created the topic: QNH and QFE
Hey all, just have the following question which I'm struggling with finding why it's the answer
The correction applied to station level pressure to obtain the QNH assumes:
(a) the density of a column of air extending down to mean sea level is independent of temperature at the station (correct answer).
Why and how is this the case? Can't seem to find info on why QFE is independent of temperature.
See Manual of Meteorology [Aviation Meteorology] pages 3 and 4.
To calculate QNH take QFE and add the extra pressure attributed to an imaginary column of air extending from the elevation of the station to mean sea level. This column of air is assumed to have a pressure lapse rate equal to ISA i.e. 30 feet per hPa.
For example, if a station had an elevation of 1500 feet and the QFE at the station was 958 hP, the extra pressure attributable to a 1500 foot column of air would be 1500 ÷ 30 = 50 hPa. So the QNH would be 958 + 50 = 1008 hPa. In this method, no consideration is given to any deviation from the standard atmosphere. Therefore it is not absolutely accurate, but it is certainly good enough for the purpose of aviation.
There is such a thing as QFF. In this case, an adjustment is made and the column if air extending from the station to mean sea level is assumed to have a density equal to the seasonal average for the location. I believe this method is used for assigning sea level pressure values for isobars on a synoptic chart.
