Transient effects of nucleation in steady and unsteady expansions of a vapor in metastable state are investigated in the relaxation time approximation (RTA). Three regions of nucleation with corresponding nucleation times are distinguished for general applications and the unsteady nucleation rate equation is explicitly displayed in each region using an asymptotic method. Applications to shock tube and nozzle experiments for the expansion of moist air show that transient nucleation effects on nucleation rates become more significant near saturation (in the validity of RTA) than near maximum nucleation rates. The position of the nucleation wave front, which corresponds to states of maximum nucleation, on particle paths and the flow field therein seem to be unaltered by transient nucleation effects except for expansions on those particle paths with cooling rates higher than 3 degrees C/mu s near the center of rarefaction waves in shock tubes.