LIDAR depolarization is commonly used for discriminating liquid and ice particles. Since depolarization depends in a complicated manner on particle shape and size, in-cloud variability of depolarization has been used as an indicator of the microphysical homogeneity of cirrus. The comparison between midlatitude (Florence, Italy, 43. 60¿N) and polar (Dumont d'Urville, Antarctica, 66. 68¿S) cirrus showed a lower mean depolarization and a higher in-cloud uniformity of cloud depolarization for polar clouds in the (-80, -50¿C) temperature range. A wider in-cloud variability of depolarization was observed in polar clouds at higher temperatures (-50, -30¿C), reflecting the presence of supercooled liquid layers. The large in-cloud variability of depolarization in Florence cirrus could be explained with a microphysics that is dynamically and chemically perturbed as compared with the polar site. Aged jet contrails are, in fact, present in many Florence cirrus records.