The classical nucleation theory of homogeneous freezing along with a thermodynamic model of aqueous HNO3 is used to calculate the critical temperatures and compositions at which ice crystals will nucleate from an aqueous HNO3 droplet. Assuming that stratospheric aerosols are composed of aqueous HNO3 near the ice frost point, we show that ice particle formation in the stratosphere can only occur if the air mass is cooled to about 2 to 3 K below the equilibrium condensation point of ice. A simple function is given for predicting the nucleation temperature of ice particle formation in the stratosphere from the ambient water vapor concentration. Microphysical calculations show that only fast cooling rates (>500 K day-1) encountered in cold lee waves are rapid enough for nucleating the majority of the background aerosols into ice particles. The likelihood that ice crystals formed in lee waves may result in the subsequent crystallization of HNO3 in the droplets is discussed. The barrier to ice formation by heterogeneous gas phase nucleation is compared to that of homogeneous freezing nucleation.¿ 1997 American Geophysical Union