give unprecedented detail on vertical cloud structure. Cloud radar in space can provide good vertical resolution of reasonably thick clouds, including the tops and bottoms of layered clouds (Stephens et al. 2002). Lidar in space can provide very sensitive measurements of thin layers of clouds or aerosols (Winker 1997). These data have provided an unprecedented view of cloud structure, particularly in showing how clouds are layered vertically, which was not possible with visible, infrared, or microwave passive instruments.
The CloudSat radar and the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar provide a new dimension in observing the atmosphere. Rather than providing horizontal distributions of cloud and aerosol features typical of more conventional satellite sensors, these new nadir-pointing active sensors measure