The two most recent balloon flights devoted to studying the stratospheric aerosol vertical structure by comparison with winds, temperature and ozone vertical structures took place on 7 May and on 5 June 1980. At 15.23 h GMT on 18 May Mount St Helens volcano (46 °N, 122 °W) erupted with a tremendous explosion, projecting ash into the stratosphere. An explosion of this size occurs only about once a decade1. This sudden introduction of material into the atmosphere offers the opportunity to study air motions both horizontally and vertically. The last such large-scale opportunity was offered by the Mount Fuego eruption which took place in 1974. In this latter case, the enhancement of stratospheric aerosols was observed by means of ground-based lidars and of balloon-borne particle counters. The time development of the aerosol event in 1974 and 1975 has been described elsewhere2-4. Mount St Helens material can now also be traced by various satellite-borne instruments. This new stratospheric aerosol event appears to be spectacular. As shown by the photographs discussed here, it leads in its early stage of development to an increase by a factor of three of the Earth limb reflectivity at 15-km altitude after a period of several years of low stratospheric aerosol content.