Cirrus clouds were observed with a Mobile Aerosol Raman Lidar (MARL) during field campaigns in the tropics in 2004/2005 (Paramaribo/Suriname, 6°N, 55°W) and aboard our research vessel Polarstern. Our lidar system is capable of detecting thin cirrus including extremely thin clouds with an optical depth below 10-3. The system measures cloud altitudes with high vertical and temporal resolution and determines the depolarisation and optical depth. It can measure during day - and night-time. In daytime cloud types including contrails are classified by means of a video camera. During the campaigns, water vapour was measured by means of balloon borne probes (Vaisala RS80/RS90, "Snow white" frost point hygrometer) and by the LIDAR using the Raman technique.