Laser radar observations of cirrus clouds were carried out during the spring of 1987 and the winter of 1987/88 at Nagoya, Japan. In most cases the integrated backscattering coefficient of the clouds ranged from 0.001(/strad) to 0.1(/strad). The maximum value of the backscattering coefficient of the clouds smoothly decreased with temperature below -40°C and -20°C in the spring and winter respectively, and the value of the winter coefficient was smaller than that of the spring in the temperature range below -40°C. The temperature dependence of cloud depth was different in the two seasons, and the minimum depth was observed near -40°C. The lowest cloud temperature was about -75°C in the spring, whereas it was limited to near -50°C in the winter. Double-layered cirrus clouds were frequently seen during the observation period. The upper layers were located near the tropopause, and the height differences of the two layers were about 2-3km. Most of them were observed in the spring, and they sometimes continued for longer than a few hours.
The relation between cirrus clouds and jet streams was studied. The results showed that the lower limitation of the winter cloud temperature was due to the facts that a jet stream was usually located over Japan and only the clouds below the jet stream could be seen in the winter; the clouds were frequently far below the tropopause with high wind speed area between; and the tropopause on several occasions was located below the jet stream. The winter cirrus clouds were observed around two types of jet stream. One was in the anticyclonically curved area of a meandering jet stream, and the other was in the vicinity of a strong and straight jet stream. On the basis of these results, we discuss the optical properties and generation mechanisms of the cirrus clouds in the vicinity of a jet stream.