57
12.4.7. Annex – formula absolute air humidity
The absolute air humidity (AH) or real air humidity is calculated as follows:
AH = SAH·RH
AH = Absolute air humidity
RH = relative air humidity in %
SAH=10^9·SVP/(CV·RVAP·(TT+273,15))
SAH = maximum absolute air humidity in g steam pro 1 m³ air (g/m³)
SVP(TT) = saturation vapour pressure in mbar over water at the air temperature TT in °C
CV = factor of compressibility for steam; CV can be calculated approximately as follows:
Temperature in °C
CV
-
50,9 ≤ TT < 0,0
1-2 · 10^(-5) · (0,1· (TT+60)-1)^2
0,0 ≤ TT ≤ 100,9
1-10^(-4) · ((0,1·TT+1)^2+4)
RVAP = Universal gas constant for steam
RAVP = 4,6151·10^2 Nm/(kg °C)
TT = air temperature in °C
SVP(TT) = c1·e^(c2·TT/(c3+TT))
The constants c1, c2, c3 see table under “dew point calculation“.
12.4.8. Annex – formula air pressure altitude correction
The altitude correction is calculated as follows:
QFE=B
n
·2,7182183^(H
F
·0,03416356/(T+e·C
h
+(H
F
·0,00325)))
thereby
B
n
= measured air pressure
QFE = corrected air pressure from the measuring point to the reference plane
H
F
= altitude difference to the measuring point in m
T = 273,15 K + t (with t=air temperature in °C)
C
h
= Constant 0.12 K/hPa
e = steam pressure at station altitude (hPa)
e = SVP(TT) ·RH
with
RH = relative air humidity in %
SVP(TT) = saturation vapour pressure in mbar over water at the air temperature TT in °C, SVP(TT)
formula see dew point
In case measured temperature and rel. humidity are not available, they can be calculated with the
ICAO-standard atmosphere. (This should also be configurable)
According to the ICAO-standard atmosphere, the rel. air humidity will be considered 0 % (dry air) and
the temperature will be set to 15 °C (288.15 K). Up to 11,000 m (above sea level), the standard tem-
perature decreases by –0.0065 K/m.
12.4.9. Annex – formula air pressure over sea level QNH
The QNH-calculation is done with the global mean value at the normal sea level (NHN) of the ICAO-
standard atmosphere (rel. air humidity = 0 % (e=0), temperature = 15 °C).
Thus, following formula applies for QNH:
QNH=B
n
·2,7182183^(H
B
·0,03416356/(288,15+e·C
h
+(H
B
·0,00325)))
