Abstract: By use of the so-called generalized temperature of the saturated moist air, a characteristic quantity representing the enthalpy of the air and a set of dynamic equations are derived. On the basis of them, the concept of the moistbalanced wind (or moist geostrophic wind) and that of pseudoequivalent temperature wind (or moist thermal wind) are suggested, Finally, some fundamental behaviours of low-level jet streams are studied.