Clouds increase the complexity of the radiative transfer process, particularly for aerosol retrievals in clear regions in the vicinity of clouds. This study focuses on identifying mechanisms responsible for the enhancement of reflectance in clear regions in the vicinity of cumulus clouds, and quantifying the relative importance of each mechanism. Using cloud optical properties and surface albedo derived from ASTER and MODIS, we perform extensive Monte Carlo simulations of radiative transfer in two cumulus scenes in a biomass burning region in Brazil. Our primary results show that the interaction between clouds and molecular Rayleigh scattering above clouds is the dominant mechanism for the enhancement of visible reflectance in clear regions of boundary layer cumulus over dark surfaces. The Rayleigh scattering contributes ~80% and ~50% to the total enhancement for the wavelengths 0.47µm and 0.66µm, respectively. Out of the total contribution of molecular scattering, ~90% arises from the atmosphere above clouds. There is little contribution to the enhancement from the atmosphere below clouds. The dominant mechanism responsible to the cloud-induced enhancement of clear sky reflectance in the vicinity of clouds provides theoretical basis and simplifications for future aerosol remote sensing from satellite.