The Improved Limb Atmospheric Spectrometer (ILAS) captured many polar stratospheric cloud (PSC) events in the Northern Hemisphere during the winter and early spring of 1997. Simultaneous measurements of nitric acid and aerosols by ILAS made it possible to infer PSC composition. The aerosol extinction coefficient and nitric acid data were compared with the theoretically predicted values for supercooled ternary solution (STS), nitric acid dihydrate (NAD), and nitric acid trihydrate (NAT) at thermodynamic equilibrium to classify PSC types. The observations showed that in 1997, both nitric-acid-containing solid and liquid PSCs formed over the Arctic during winter and early spring, until mid-March. The STS PSCs were observed early in the PSC season, in mid-January. Most of the PSCs observed late in the PSC season had features of nitric-acid-containing hydrates. An intensive analysis of the temperature histories suggested that most of the STS events observed in January had experienced the thermal conditions necessary for the formation of liquid PSCs. The nitric-acid-containing hydrates observed in March seemed not to have been influenced by any mountain-induced lee waves. The process of nitric-acid-containing hydrate formation based on synoptic scale temperature change is discussed.