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I am wondering what is the difference between QFF and QNH?

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  • $\begingroup$ See also this answer (ignore the math equation in the question; the text of the answer is far easier to follow than the text of the question). $\endgroup$
    – Ralph J
    Dec 3, 2023 at 13:25

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Both QNH and QFF are pressure measurements at a meteorological station (e.g. airport), with the pressure height-corrected back to sea level.

The difference is in the assumptions that are used for the correction to sea level reading. QNH assumes standard atmosphere (for temperature, lapse rate and humidity) and QFF assumes actual atmospheric measurements for the correction.

Since a barometric altimeter uses standard atmosphere assumptions, with a pressure correction, the result is that when you put QNH is the Kollsman windows, the altimeter will read measurement station elevation at the measurement station (e.g. the airport). In a non-standard atmosphere, the altimeter will now deviate from 0ft at sea level.

If you put QFF in the Kollsman window, the altimeter will read 0ft at sea level. However now the altimeter will deviate from the airport elevation at the airport.

Since we typically want the altimeter to be correct at the airport, QNH is the normal setting.

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