With our mobile Aerosol Raman Lidar (MARL) we have performed field campaigns in the mid latitudes in 2003 (Lindenberg/Germany, 53°N, 15°E) and in the tropics in 2004/05(Paramaribo/Suriname, 6°N, 55°W). The lidar system detects aerosols and clouds in the UTLS region. It is capable of detecting thin cirrus including extremely thin clouds with optical depth below 10-3. The system measures cloud altitudes with high vertical and temporal resolution and determines the depolarisation and optical depth. It operates day and night. During daytime cloud types including contrails are classified with the help of a video camera. During the campaigns, water vapour was measured by means of balloon borne sondes (Vaisala RS80/RS90, 'Snow white' frost point hygrometer) and by the LIDAR using the Raman technique. We investigated the occurrence of ice supersaturated regions and their relation to the occurrence of clouds. The accuracy of the water vapour measurements needs to be critically evaluated for this purpose. The cloudiness in the upper troposphere was found to be very high in the tropics where in about 90% of all measured profiles cirrus was present. In the mid latitudes cirrus were found in 55% of the measurements
