A physically based parameterization of cirrus cloud formation by heterogeneous freezing is developed along with a novel method to compute associated nucleation rates. The analysis is restricted to immersion freezing, possibly the dominant pathway for heterogeneous cirrus formation under cold (<235 K) conditions. The size of ice nuclei (IN) immersed in a liquid particle does not significantly affect the heterogeneous freezing threshold (the saturation ratio over ice where ice formation is initiated) of the mixed particle. If perfect IN were present at cirrus altitudes, almost all of them would freeze near ice saturation, even in slow updrafts. If only one type of less potent IN with freezing thresholds >1.3–1.4 triggers cirrus formation, cloud properties are not very susceptible to changes of IN properties, as in the case of homogeneous freezing. In contrast, much stronger indirect aerosol effects on cirrus clouds are possible if at least two types of IN with distinct freezing thresholds compete during the freezing process, most likely leading to a suppression of ice crystal concentrations.