A ground-based photometer was used to monitor the temporal variation of the zenith-scattered skylight radiance (at &lgr;=752 nm) with a temporal resolution of 2 Hz. In detail, eight data sets for different optically thick (&tgr;c≥1/(1-g) with &tgr;c being the cloud's optical thickness and g the asymmetry factor for Mie scattering) cloud covers were analyzed with standard spectra methods. A general feature of all cloudy sky radiance power spectra is the presence of a remarkable scale break occurring at spatial scales that approximately correspond to the vertical cloud extension. This finding is consistent with theoretical estimates by Marshak et al. [1995> and Davis et al. [1997b> based on radiative smoothing by photon diffusion in vertically extended, optically thick and homogeneous clouds. ¿ 1999 American Geophysical Union