Lidar measurements along with radiosonde data and video observations allow a detailed investigation of thin or subvisual cirrus clouds and contrails. The optical depth (OD), base and top altitude, the color index and the depolarization of clouds are determined from lidar backscatter profiles at 532 nm and 355 nm. Temperature and humidity is provided by radiosondes. Cirrus and contrails are distinguished with the help of sky images that are taken along with the lidar measurements. Measurements with this type of instrumentations have been performed in Lindenberg/German (52.2°N, 14.1°E) from May to October 2003.During the exceptionally warm and dry summer of that year a high occurrence of thin and subvisual cirrus clouds have been detected. During about 50% of all "clear sky conditions" (e.g. no clouds with optical depth >1 are present) thin clouds prevail in the upper troposphere, half of which are subvisual. Episodes with high occurrence of persisting contrails are investigated in detail. During most / all of theses episodes thin cirrus prevail near the tropopause.. Most often contrails occur within the persisting cirrus. Isolated contrails seem to be the exception. According to the available radiosonde humidity measurements the atmosphere is saturated but not strongly supersaturated with respect to ice in this region. Analysis of synoptic weather charts show, that the patterns favorable for persisting contrail formation are connected to stable high pressure systems or to warm conveyor belts.