By means of a scanning backscatter and depolarization lidar, contrails have been investigated for ages between a few seconds and 60 min. At environmental temperatures ≤-60 ¿C the depolarization measured in the contrails rises from about 0.1 to 0.5 with time, whereas at higher temperatures around -50 ¿C already the youngest contrails exhibit depolarization values of about 0.5. This indicates a strong dependence of the aerosol formation and growth on the environmental temperature which might influence their behavior in the atmosphere. ¿ American Geophysical Union 1996