The formation and development of polar stratospheric clouds are examined using a comprehensive microphysical model applied along air parcel trajectories. During Antarctic winter specific parcels are sampled over a time span of several days using extinction data from the POAM II satellite instrument. The observed evolution of polar stratospheric cloud spectral opacity is compared to predictions from model simulations. Hence, aerosol growth and decay in response to changing environmental conditions can be observed and interpreted. In some instances good agreement between observations and predictions has been obtained over periods of many days. The analysis provides evidence for the role of temperature history in cloud formation and properties, in agreement with previous studies. On occasion, the apparent behavior of a PSC along a trajectory is inconsistent with current theoretical concepts. ¿ 1999 American Geophysical Union