Contrails of small and very large optical depth

This work is divided into three distinct parts. We first develop a means of displaying contrails on a plan position map from calibrated photographs and their height determined from a lidar. The contrails found near the tropopause are very weak with optical depth (τ<0.2). The photographs also indicate the turbulent nature of the layer in which the fallstreaks are evaporating and chilling the air. A single very strong lidar echo layer also appears in the 6 to 8 km layer. Observations from GOES show that this echo is due to a single contrail that fortuitously passed over the lidar during a major portion of its life. The observed optical depth (τ>2) exceeds any in the literature. However, it has several of the physical features reported previously. Significantly, the trail is found along the boundary of the region in which the relative humidity with respect to ice just exceeds 100%.