1] In situ measurements of light scattering and microphysical characteristics of young and 20-min-aged persistent contrails and of frontal cirrus clouds were carried out with the airborne polar nephelometer and microphysical Particle Measuring Systems instruments. Optical and microphysical properties of contrails at different stages of evolution and of cirrus clouds sampled in ice-supersaturated air masses at ambient temperatures near À60°C are examined. The results show that quasi-spherical ice particles with diameters smaller than 5 mm control the optical properties of the plume shortly after formation. In slightly aged contrails the optical properties are governed by larger nonspherical ice crystals. Very similar optical properties are observed in frontal cirrus but with a different ice particle size distribution. The contrail optical and microphysical ice particle data obtained in this study are representative of the visible persistent contrail created by a midsized airliner after the wake vortices have decayed.