2.3 Saturation ratio
The water vapor saturation pressure depends solely on temperature in the
approximation of the Clausius-Clapeyron (List, 1984), which is
applicable to planetary atmospheres. For the computation of the water
saturation in the Martian atmosphere, we use a semi-empirical form of
the Clausius-Clapeyron law, the Goff-Gratch equation (Goff and Gratch,
1946), that yields the vapor pressure over ice :
log P sat = 2.07023 – 0.00320991 T –
2484.896 T -1 + 3.56654 log T ,
where P sat is the saturation vapor pressure in
mbar and T is the temperature in kelvin.
The saturation ratio S is defined asS=PH2O/Psat . The uncertainties ofS are evaluated as the total differential of S , using the
uncertainties of the retrieved temperature, pressure and the watervmr .