From April to September 2003 extensive investigations of upper tropospheric water vapour and clouds have been conducted at the Meteorological Observatory in Lindenberg (MOL) / Germany (14.5°E, 52.5°N). Cloud occurrence as well as their optical parameters like optical depth, extinction and depolarisation have been measured with a Mobile Aerosol Raman Lidar (MARL) during day and night-time. With sky cameras visual characterisation of the clouds was possible at day time and allowed the identification of contrails. In cloud free regions, the aerosol backscatter profile can be determined from the lidar data.At the observatory, four radiosondes (Vaisala RS80A) were launched a day and were calibrated once a week using the MOL-Research sonde. This method allows the precise measurement of the water vapour content even in the cold upper troposphere and thus the determination of the relative humidity above ice (RH_I). Additionally, water vapor concentration profiles were measured by the lidar based on the Raman technique.Based on this data, the conditions at which cirrus clouds and/or persistent contrail were observed are analysed and the difference of the optical properties between natural cirrus and contrails are investigated.