Cirrus clouds were observed with a mobile Aerosol Raman Lidar (MARL) during field campaigns in the mid latitudes in2003(Lindenberg/Germany, 53°N, 15°E) and in the tropics in 2004/05 (Paramaribo/Suriname, 6°, 55°W). 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 depolarisationand 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 bymeans of balloon borne probes (Vaisala RS80/RS90, Snow white frost point hygrometer) and by the LIDAR using the Raman technique. In the mid latitudes cirrus were found in 55% of the measurements. In the tropics the cloudiness in the upper troposphere was found to be very high, since in about 90% of all measured profiles cirrus was present. Transport processes in the tropical tropopause layer were investigated with a newly developed trajectory model which is coupled with a radiative transfer model. Cloud occurrence and dehydration of the air in the model agrees well with the observations.