During the cruises ANT XVII/4 (Punta Arenas Bremerhaven, 2000) and ANT XX1/1 (Bremerhaven - Cape Town, 2003) continuous measurements of clouds and aerosol have been performed by a Mobile Aerosol Raman Lidar (MARL). This system measures backscatter of light at 355 nm and 532 nm and is able to detect even extremely thin layers of particles in the atmosphere. Cloud base and top height, depolarisation optical depth and color index can be deduced from this data. During the Polarstern cruises very thin layers of ice particles have been observed frequently in the tropical tropopause layer (TTL). Cases of persistent cloud layers with an optical depth below 10^-3 are reported. Based on the data of radiosondes which were launched daily aboard the vessel, the conditions at which these type of clouds form are analysed. It is shown, that these clouds form in a layer below the cold point, but above the lapse rate tropopause. The relation between cloud occurrence and temperature and wind in the TTL as well as state of the QBO are discussed.