This work presents a new physically based parameterization of cirrus cloud formation for use in large-scale models which is robust, computationally efficient, and links chemical effects (e.g., water activity and water vapor deposition effects) with ice formation via homogenous freezing. The parameterization formulation is based on ascending parcel theory and provides expressions for the ice crystal size distribution and the crystal number concentration, explicitly considering the effects of aerosol size and number, updraft velocity, and deposition coefficient. The parameterization is evaluated against a detailed numerical cirrus cloud parcel model (developed during this study), the equations of which are solved using a novel Lagrangian particle-tracking scheme. Over a broad range of cirrus-forming conditions, the parameterization reproduces the results of the parcel model within a factor of 2 and with an average relative error of -15%. If numerical model simulations are used to constrain the parameterization, error further decreases to 1 ± 28%. Copyright 2008 by the American Geophysical Union.