##### Pressure calculations (QNH-QFE-QFF)

**QFE**: Standard reduction calculation based on a rearrangement of the hypsometric equation;

`P`is the measured pressure,

_{s}`H`is the height of the barometer above station reference level,

_{s}`g`= 9.81 ms

^{-1}

$$QFE = P_s \left (1 + \frac{gH_s}{RT} \right)$$

**QNH**: ICAO describes how to calculate QNH in documents 7488 and 9837. The calculation is in two steps; first the elevation in ISA that has the QFE pressure, then QNH:

$$H_{ISA} = 44330.77 - 11880.32 * QFE^{0.190263}$$ $$QNH = 1013.25 * \left (1 - 0.0065 * \frac{H_{ISA} - H}{288.15} \right )^{5.25588}$$

**QFF**: Calculated using the Swedish Meteorological and Hydrological Institute method:

$$QFF = QFE * e^{\frac{H * 0.034163 (1 - 0.0026373 cos\phi)}{T_1}}$$

`Φ`is the station latitude,

`H`the elevation.

`T`is a way of taking account of winter inversions: If

_{1}`t`is the air temperature:

$$t<-7^\circ C \Rightarrow T_1 = 0.5 * t + 275$$ $$-7^\circ C < t < + 2^\circ C \Rightarrow T_1 = 0.535 * t + 275.6$$ $$t > + 2^\circ \Rightarrow T_1 = 1.07 * t + 274.5$$