Cloud-controlling Factors: Low Clouds

The way in which meteorological and aerosol factors conspire to determine the statistics and climatology of layers of shallow (boundary layer) clouds is reviewed, with an emphasis on factors that may be expected to change in a perturbed climate. The paramount role of theory is identified, both in service of advancing our understanding, but also in modeling and attributing specific causes and effects. In particular, it is argued that limits to current understanding of meteorological controls on cloudiness make it difficult, and in many situations perhaps impossible, to attribute changes in cloudiness to aerosol perturbations. Suggestions for advancing our understanding of low cloudcontrolling processes are offered; these include renewing our focus on theory, model craftsmanship, and increasing the scope and breadth of observational efforts