A phenomenon known also beyond atmospheric sciences is the springtimeAntarctic ozone hole and the fact that ozone is catalytically destroyedby products of man-made CFCs. Yet, ozone depletion is tightly connectedwith natural processes in the polar stratosphere. Below a certainthreshold temperature, polar stratospheric clouds (PSCs) can form which provide the surface for heterogeneous reactions that lead to theactivation of initially bound chemical reservoir species. As theactivation is a key process for consequent succeeding ozone depletion, polarstratospheric clouds are a main factor for ozone chemistry in the polar stratosphere.PSCs occur at very low temperatures found in the stratosphere duringpolar night. Different cloud types, consisting of ice crystals, nitricacid trihydrate (NAT) particles, or H2O/HNO3/H2SO4 droplets form underdifferent temperature conditions and cause different chemical effects.In the Antarctic stratosphere, wintertime temperatures are much lowerthan in the Arctic due to different atmospheric wave patterns. Thedifference in temperature is reflected in frequency and persistence ofPSC types.By ground-based lidar it is possible to determine altitude and extent ofPSCs, and the retrieved backscatter ratio and depolarisation allow todistinguish the observed cloud type. Our study is based on lidarobservations from Ny-?lesund in the Arctic [79°N, 12°E] and McMurdo inthe Antarctic [78°S, 167°E].As expected, we find large differences in the occurrence frequency ofthe various PSC types. Yet, the statistical analysis reveals somesurprises. It is found that the most common PSC type at the Antarcticstation is made from solid NAT particles, while only a small fraction ofthe observed cloud layers consist of ice particles. On the other hand,the majority of PSCs at the Arctic station is found to consist of liquidparticles.As the PSC type has an influence on the heterogeneous activation rate,these results have to be carefully validated and taken into account byozone chemistry models.