In order to identify air masses Rossby has recently constructed an adiabatic diagram bearing his name. As the variables of the diagram he prefered two important meteorological elements, potential temperature and mixing ratio, as conservative factors for an air mass. However the potential temperature and mixing ratio change by the radiation, mixing and the condensation of water vapour.
The author investigated how they change by mixing and concluded that when the condensation does not occur they become nearly equal to their mean value but are dependent of the pressure at the final state when the condensation occurs. By using the Rossby diagram, however, their values can easily be estimated even when the pressure of the two air masses is different from each other. As the equivalent potential temperature is independent of the pressure at the final state its deviation from the mean value of the two air masses before mixing is tabulated (table 1). Also the author showed that the temperature of the mixture is easily estimated on the Rossby diagram when the water vapour condenses.