The primary objective of the 2B-TAU algorithm is to produce estimates of the optical depth and its uncertainty for each radar profile determined to contain cloud. The algorithm retrieves the optical depth and effective radius using radar reflectivity measured by the CloudSat CPR, upwelling top of the atmosphere radiance measured by MODIS in several channels, and other ancillary data (eg. geolocation, time, and surface albedo). The retrieved values are spectral and represent the optical depth and effective 4adius in the short wave. Using CPR information about the distribution of cloud in the atmospheric column, estimates are then produced of the profile of cloud optical depth versus height in the column.