We use a Lagrangian microphysical aerosol-cloud model to simulate cirrus clouds along trajectories at northern hemisphere midlatitudes. The model is constrained by recent in situ observations in terms of aerosol size distributions, freezing relative humidities, cooling rates, and cirrus particle sedimentation rates. Key features include competition between insoluble and volatile aerosol particles and temperature perturbations induced by high-frequency gravity waves. Recent analyses of field measurements have revealed the crucial roles both factors play in cirrus formation. We show that most cirrus form in synoptic cold pools, but with microphysical properties determined by mesoscale variability in vertical velocities. Heterogeneous ice nuclei (IN) present in concentrations probably typical for northern midlatitude background conditions (<0.01–0.03 cm−3) significantly modify cirrus properties but do not control cirrus formation. The key effect of IN on cirrus clouds is a reduction of the number of ice crystals. This indirect aerosol effect results in reduced cloud albedo due to increased effective radii and decreased ice water contents, as well as in nonlinear changes of cirrus occurrence, optical extinction, and fraction of clouds that are subvisible. The nonlinear dependence of the three latter quantities appears when IN concentrations rise above a threshold concentration of some 0.01 cm−3, the exact value depending on the cloud formation temperature, cooling rate, and IN freezing relative humidity. In such conditions, IN become the controlling factor in cirrus formation, diminishing the role of homogeneous freezing. Ice nuclei with freezing thresholds near ice saturation are capable of introducing strong changes of cloud properties, even at low concentrations. Optically thin and subvisible cirrus are particularly susceptible to IN. The presence of a small number of IN (0.001 cm−3) can significantly increase their occurrence frequencies. If such clouds predominantly form on IN, they might be affected by anthropogenic activities. Changes in upper tropospheric cooling rates and ice-forming aerosols in a future climate may induce changes in cirrus occurrence that are comparable in magnitude to observed decadal trends in global cirrus cover.