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tokens: Table 1 Parameters of the model atmosphere. The spatial and temporal variability in the cloud cover is derived from observations by MODIS, the Moderate Resolution Imaging Spectroradiometer (for a detailed description of the MODIS data sets, including retrieval algorithms, see King et al. 2004 ; Parkinson Greenstone 2000 ). We describe a cloud by the pressure of its top, p c , its optical thickness, b c , and the cloud particle size distribution and composition. Each cloudy model pixel has a cloud fraction. This is zero for a cloud-free pixel. Even with as many as 100 pixels along the light equator of the model planet, the pixels are much larger than typical horizontal variations in clouds, and they are much larger than MODIS ground pixels. For a mixture of cloud-free and partly and fully cloudy MODIS ground pixels within a model pixel, we computed the locally reflected Stokes vector for a cloud-free and a cloudy model pixel, and took their weighted sum to compute the reflected Stokes vector for the pixel as a whole. In this way, we accounted for the cloud fraction as reported for all cloudy MODIS ground pixels. The mean cloud fraction of our model planet is about 66%. The MODIS database provides a cloud top pressure for all of its pixels, and we computed p c of a cloudy model pixel by taking the average of the cloud top pressures of the MODIS pixels within our model pixel, weighted by their cloud fractions. To limit the number of different atmosphere-surface models, we divided p c for the model pixels into three categories: 500 mbar for 0 ≤ p c 600 mbar, 700 mbar for 600 ≤ p c 800 mbar, and 850 mbar for p c ≥ 800 mbar (see Table 1 ). We computed b c for a cloudymodel pixel by taking the mean of the cloud optical thickness values of the MODIS ground pixels within the model pixel. In particular, we take the mean of the MODIS logarithmic daily mean data set (see Table 2 ). This data set provides the cloud optical thickness for a given MODIS pixel i as log b c i , where b c i is the cloud optical thickness. When we computed the mean b c , we weighted log b c i of each MODIS pixel with its cloudiness fraction. This logarithmic mean as a good approximation for the cloud optical thickness has been proposed by Hubanks et al. (2015) and Oreopoulos et al. (2007) . The wavelength at which a cloud optical thickness is provided in the MODIS data set depends on the surface: b c pertains to 645 nm above a dry surface, to 858 nm above an ocean, and to 1240 nm above snow or ice. We obtain b c at another wavelength by multiplying the value from the data set with the ratio of the cloud particle extinction cross-section at that other wavelength and at the given wavelength. The cloud optical thickness depends somewhat on the wavelength: for a cloud with b c = 10, composed of particles with r eff = 12.5 μ m and v eff = 0.1, the cloud optical thickness, for example, ranges from 9.775 to 10.002, from 350 to 865 nm. The extinction cross-sections of the cloud