Aerosols serve as cloud-drop condensation nuclei (CCN) and thus significantly affect cloud properties and the initiation of precipitation. Large concentrations of manmade aerosols have been reported to both decrease and increase rainfall owing to their radiative and CCN activities. Here we briefly review the subject and propose a conceptual model which explains this apparent dichotomy. At one extreme, pristine tropical clouds with low CCN concentrations rain out too quickly to mature into long-lived clouds. Heavily polluted clouds, on the other hand, evaporate much of their water before precipitation can occur, if they can form at all given the reduced surface heating resulting from the aerosol haze layer. The optimum precipitation enhancement thus occurs at moderate aerosol concentrations, while at greater aerosol loading microphysical and radiative effects collude to suppress precipitation. This reconciles the apparent contradictory reports of aerosol impacts on precipitation amount.