Balloonborne backscatter measurements of polar stratospheric clouds (PSC) from four Arctic winters have been used together with isentropic airparcel trajectories to assess recent theories of PSC formation. Assuming unperturbed HNO3 concentrations, observations performed in the month of January, reveal that PSC formation initiates 3--4 below the nitric acid trihydrate (NAT) condensation temperature (TNAT). According to a suggested scenario for PSC formation [Tabazadeh et al., 1995> liquid supercooled ternary solution particles may freeze upon an increase in temperature after previously being close to the ice frost point. Liquid particles, identified by the temperature histories according to this scenario, show a relatively compact relationship between measured temperature and backscatter ratio, indicating a substantial growth at 3--4 K below the NAT-temperature as expected by ternary solution particles. Newly formed solid particles are of small size, in contrast to aged solid particles which appear to retain large sizes up to the NAT-temperatures during evaporation and grow by condensation at temperatures slightly below TNAT, indicating the composition to be NAT. ¿ American Geophysical Union 1996