Reshaping theory of cloud formation

As predicted by Köhler some 80 years ago, droplet activation places an upper limit on the supersaturation of water vapor that can be reached in the atmosphere. Given sufficient solute or enough depression of surface tension, or a combination of the two, the supersaturation in a given situation will decrease. At high aerosol and soluble trace gas concentrations and for low cooling rates, strict activation is not necessary for formation of a visible cloud; indeed, a continuum exists from ambient aerosol to wetter and wetter particles to unactivated clouds to activated ones. What is seen as "cloud" can, in reality, be a collection of droplets ranging from fully activated to unactivated. By affecting cloud optical properties, these chemical phenomena may lead to nonnegligible global negative forcing and may be as important regionally as the Twomey effect itself. To assess the importance of the indirect climatic effect of aerosols, one seeks a robust connection between cloud droplet population and a prognostic variable from global aerosol models. How that link might depend upon chemical cloud activation effects, including variations in aerosol chemical composition, solute water solubility, solute surface tension lowering, and condensation of trace gases, remains to be determined. Lack of global data on these activation effects poses additional uncertainty beyond that already recognized by the Intergovernmental Panel on Climate Change , making the largest uncertainty in estimating climate forcing even larger.