A method for inferring cloud optical depth is introduced and assessed using simulated surface radiometric measurements produced by a Monte Carlo algorithm acting on fields of broken, single-layer, boundary layer clouds derived from Landsat imagery. The method utilizes a 1D radiative transfer model and time series of zenith radiances and irradiances measured at two wavelengths, 1 and 2 , from a single site with surface albedos Ͻ . Assuming that clouds transport radiation in accordance with 1D theory and have spectrally invariant ␣ ␣ 1 2