Note on calculations

The calculations are made easier if you have a table of saturation pressures for water vapour and perform linear interpolation to get in between values. The partial vapour pressure is given by PsatxRH, where RH is the relative humidity and Psat is the saturation vapour pressure for water vapour. If you heat a parcel of air, the surrounding air remains at more or less the same pressure (unless there are other factors). On heating the parcel the mole ratio of all components in the parcel remains the same, the total pressure P remains the same, so the pressure of all components remains the same (including the partial pressure of the water vapour). Another way to look at is to say that the partial pressure of any component is its mole ratio multiplied by the total pressure.