Thin ice clouds were frequently observed in the tropoical tropopause layer (TTL) around 17 km altitude during our recent ground based (Paramaribo, Suriname, 6°N, 55°W) and ship-borne (RV Polarstern) campaigns. These clouds play an important role in the freeze-drying process that determines the low water vapor content of the stratosphere. In the tropics, the cloudiness in the upper troposphere was found to be very high, since cirrus was present in 88% of all our measured profiles. Also, we frequently observed extremely thin cloud layers (ETTCi) with optical depths below 10-4 at the cold point tropopause.Transport processes in the TTL were investigated with a newly developed trajectory model which is coupled with a radiative transfer model. This analysis suggests that clouds are always present in the TTL where the air is saturated with respect to ice due to adiabitc cooling. Also, we find evidence, that thin cirrus efficiently dehydrate ascending air. The trajectory analysis as well as in situ water vapor measurements indicate that the ETTCi exist in subsaturated air. It is likely, that these clouds consist of nitric acid trihydrate (NAT) rather than pure ice.