The author tried to derive theoretically a hygrometer equation, and explain in a new way that the wet-bulb temperature T' defined by the above equation should change pseudo-adiabatially in the case of the adiabatic vertical motion of damp air while the air temperature T follows a dry-adiabatic change.
Then by applying the general idea of entropy, as was done by H. Arakawa, the equivalent potential temperature defined by C. G. Rossby was derived approximately as a function of wet-bulb potential temperature. But in applying those formulæ given by the above authors to air mass analysis, it must be remembered that some errors may enter into the calculation in an order of the difference between dew point and wet-bulb temperature.