Contrail cirrus were observed by airborne lidar and multispectral infrared remote sensing. Results of microphysical analysis were obtained for two cases of persistent contrail cirrus which form in the same area but of differing maturity. The cross section area of the older contrail was 18 times greater than the newer case. In both cases the inferred number of particles per distance along the flight track dimension of contrails was nearly similar, close to 26¿1011/m. The average liquid water concentration of the two cases was similar, but the amount of water per distance along the flight line dimension was much greater for the older contrail, 34 g/cm and 680 g/cm. In both cases the amount of liquid water in the contrail was much greater than the amount generated by the aircraft which produced the contrail. These results are consistent with a model of contrail evolution where the effect of the jet exhaust is to generate a dense concentration of small ice particles. Mixing then maintains the total number of particles, and particles grow to the extent of the available vapor in the ambient air. ¿ 1998 American Geophysical Union