Aerosols and cirrus clouds were observed with a mobile Aerosol Raman Lidar (MARL) during 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). Our lidar system, 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. 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 formation and dehydration in the model compares well with the observations.