Cirrus cloud measurements have been performed during the INCA field campaigns in Punta Arenas/Chile (53.12°S, 70.88°W) and in Prestwick /Scotland (55.51°N, 4.60°W) in each hemispheres fall in the year 2000. Additional measurements are currently performed at the Meteorological Observatory Lindenberg (MOL). From lidar backscatter profiles at 532 nm and 355 nm the optical depth (OD) of the clouds is retrieved as well as base and top altitude of the clouds and the phase of the particles.One difference observed between the northern and southern hemisphere is the occurrence of very faint layers of particles in an altitude range of 5 to 8 km which were seen only in the North. However, for the cirrus itself no difference has been detected as far as the frequency of occurence (dt) of thin or subvisible clouds is concerned. In both campaigns about 35 % of all cirrus were subvisible (OD<0.03) and about the same fraction of thin cirrus was detected (0.03<OD<0.3).Differences in the results from the southern and the northern hemisphere are found in the wavelength dependence of the backscatter coefficient and the depolarization behaviour. These results suggest, that there are clouds consisting of rather large particles in the South (Punta Arenas), which have not been detected in the North (Prestwick). A detailed analysis of these data requires a non-spherical scattering theory which is difficult to conduct and currently under investigation.In summary we can state that our data suggest that the higher concentration of aerosol (including anthropogenic aerosol) in the northern hemisphere does not have an impact on the abundance of cirrus, including those in the subvisible range under the prevailing meteorological conditions of the campaigns. However, aerosols seem to have an important influence on the microphysical properties of high tropospheric clouds.