Marine stratocumulus clouds such as those shown in the figure exhibit an intriguing behavior: In sparsely covered regions like the one indicated by the arrow, clouds seem to occasionally vanish and then reappear several kilometers away. That disappearing act, first noticed last year by Graham Feingold (National Oceanic and Atmospheric Administration) and colleagues, is one of many surprising dynamic behaviors observed in clouds. Scientists have also witnessed abrupt changes in cloud morphology and seemingly unprovoked transitions from drizzles to downpours. Although clouds’ complex dynamics affect long-term climate trends—they play a part in determining Earth’s albedo and rainfall distribution—most climate models gloss over them. More detailed simulations that couple microphysics with fluid dynamics are too computationally intensive to use for long-term predictions. Now, Feingold and coauthor Ilan Koren (Weizmann Institute of Science) have captured some of the complexity of cloud dynamics in a model simple enough for pen-and-paper analysis. Inspired by a century-old population-dynamics model, the researchers liken clouds to prey that are “eaten” by precipitation. High aerosol concentrations forestall the onset of rain or, in predator–prey vernacular, increase the difficulty of the hunt. Building on those basic concepts, the researchers produced three relatively simple equations that predict key emergent phenomena, including time-periodic oscillations that may be associated with the disappearing and reappearing clouds. (I. Koren, G. Feingold, Proc. Natl. Acad. Sci. USA 108, 12227, 2011.)—Ashley G. Smart
