During the April 1996 SUCCESS field campaign, our ground‐based polarization lidar studied a variety of contrails ranging from a new NASA DC‐8 contrail to persisting contrails from commercial jet aircraft that had spread into an almost invisible cirrostratus sheet. What distinguishes these contrails from natural cirrus is their tendency to remain thin (∼50–500 m), and to generate strong laser backscattering and depolarization, often along with solar coronas, even an hour or more after formation. Lidar depolarization ratios in persisting contrails ranged from ∼0.3–0.7. Contrail‐cirrus clouds composed of high numbers of small (20–30 µm) particles would satisfy these findings, which has implications for comprehending their radiative and climatic effects.