Andreae, Flood or drought: How do aerosols affect precipitation

Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud properties and the initiation of precipitation. Large concentrations of human-made aerosols have been reported to both decrease and increase rainfall as a result of their radiative and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rain out too quickly to mature into long-lived clouds. On the other hand, heavily polluted clouds 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. We propose a conceptual model that explains this apparent dichotomy. Cloud physicists commonly classify thecharacteristics of aerosols and cloudsinto “maritime ” and “continental ” regimes, where “continental ” has become synonymous with “aerosol-laden and polluted. ” Indeed, aero-sol concentrations in polluted air masses are typically one to two orders of magnitude greater than in pristine oceanic air (Fig. 1) (1). How-ever, before humankind started to change th