Ice may form by both homogeneous and heterogeneous freezing nucleation processes in clouds at temperatures below −35°C. Most investigations have focused on the former process. This paper presents results from adiabatic parcel model calculations that include the effects of both freezing processes in unactivated solution droplets. Uncertainties in predicting the homogeneous freezing rates are discussed and used to select solution drop composition and freezing characteristics that bracket those expected in the upper troposphere. The heterogeneous freezing rates of insoluble atmospheric aerosols are parameterized based on published freezing rates of carbonaceous particles. Process model simulations show that the potential variability in ice formation in cirrus clouds is much greater if heterogeneous freezing nucleation is considered in addition to homogeneous freezing. The impact of insoluble aerosols on ice formation is inferred to increase with insoluble particle size and with the fraction of soluble aerosols containing insoluble components. The maximum impact of heterogeneous nucleation is indicated for vertical motions less than 0.2 m s−1 and for insoluble components being associated with at least 10% of all soluble aerosols. The wide range of ice crystal concentrations observed in cirrus is most consistent with the occurrence of both heterogeneous and homogeneous ice formation processes. These conclusions are partially supported by existing observations of aerosols and cloud microphysical characteristics in upper tropospheric clouds but require new measurements for confirmation.