Observations of Polar Stratospheric Clouds (PSCs) were carried out with an airborne lidar on the stratospheric M55 Geophysica aircraft during a flight from Rovaniemi, Finland, on 9 January, 1997. The clouds were observed at the zenith, downwind from the Norwegian Alps: three PSCs, of somewhat different characteristics, were detected at heights between 23 and 28 km. In two of the clouds, different types of particles seem to coexist: echoes attributable to types I and II PSCs are found in different portions of the clouds. The formation of the PSCs is related to an orographic lee-wave, whose development was forecast by a mesoscale dynamical model used to plan the flight path. The largest observed PSC displays a complex structure, that appears to be influenced by waves of different wavelengths. In particular, lidar and in situ data suggest the presence of a wave having a relatively short length (about 18 km) that overlaps on the main lee-wave. The short wavelength oscillation is thought to play a major role in the cloud development, determining the rapid formation and evaporation of particles and therefore the non-stationary character of the PSC. (C) 2003 Elsevier Science Ltd. All rights reserved.