This paper compares two methods of cloud flagging that were developed at the Deutscher Wetterdients (DWD) and at the Laboratory of Atmospheric Physics, University of Thessaloniki (LAP). The two methods are applied to the same data set to uncover their similarities and differences. The LAP method aimed at flagging the quality of global UV irradiance spectral measurements with respect to the purity of their spectral characteristics. while the DWD flags describe the sky conditions and their effects on the radiation field in an absolute sense as well as their short-time variability. In this respect, the two methods appear to have distinct differences, and also similarities, in describing when the sun's disk is occluded or not. (C) 2001 Elsevier Science B.V. All rights reserved.