IN the stratosphere the attenuation in the radiance of a light beam is predominantly due to scattering by air molecules and aerosols, and absorption by ozone. For the height range 10–30 km, aerosols can account for at least 60% of the attenuation at 700 nm (ref. 1) and are responsible for most of the long and short term variations2. During 1969–76 the stratospheric optical thickness due to aerosol scattering was measured at Adelaide 35°S, using a lidar3. The observations reported here show that the aerosol optical thickness varied by more than a factor of three during that time.