Our two lidar systems MARL (Mobile Aerosol Raman Lidar) and ComCAL (Compact Cloud and Aerosol Lidar) have been operated during measurement campaignsaboard the research vessel Polarstern (2003 and 2005) and at Paramaribo/Suriname (5.8°N, 55°W) in 2004/2005. The lidar systems detect cirrus in the upper troposphere with a very high sensitivity by the depolarization measurement. Cirrus properties like altitude, optical depth, particle phase are derived from the lidar signals. Using scattering theory and an estimate of the particle size, the number concentration of ice particles can be calculated. In almost 90% of the measurements performed at Paramaribo thin or subvisible cirrostratus were detected in the tropical transition layer (TTL). Occasionally, extremely thin clouds with optical depths below 10-4 were observed at the cold point tropopause (CPT). The condensed mass concentration was 0.5 ŠÌg/m3. If we assume a particle size of 5 ŠÌm (effective radius) this corresponds to a number density of only a few particle per liter. The extremely thin clouds which were observed at - or even slightly above - the cold point tropopause seem to dwell in subsaturated air. Our findings indicate that these thin layers of particles, are not composed of pure water ice.