A new theory may solve a longstanding puzzle surrounding the formation of rain from clouds. In the 10 January print issue of PRL, a team suggests that raindrop formation is a kind of nucleation phenomenon–like bubbles forming in soda or sugar crystals in honey. The theory seems to explain one effect of pollution on rain clouds over land and may eventually lead to better weather and climate forecasts.
Traditionally, atmospheric scientists have described rain development in two steps: Droplets with radii smaller than about 20 microns grow through condensation, acquiring water molecules from water vapor in the surrounding cloud. At around 20 microns gravity begins to take over, and the droplets fall slowly and erratically, growing through so-called collection by accreting smaller droplets. But according to this theory, a droplet should take longer than an hour to grow from 10 microns to 100 microns–a full-sized drizzle drop–whereas a typical precipitating cloud lasts only thirty minutes. Atmospheric scientists believe that other factors, such as cloud turbulence, allow droplets to form more quickly, but the details are not fully understood.
Now Robert McGraw and Yangang Liu of Brookhaven National Laboratory in Upton, NY, have proposed an entirely new way to approach the problem: they use so-called statistical methods, which describe the behavior of a large collection of elements, such as molecules in a gas. They treat drizzle drop formation as a kind of nucleation, like the process that forms sugar crystals in honey. Random fluctuations in honey at the molecular level cause sugar molecules to continually form small clumps, but below a certain size, they can’t grow. Once a clump reaches the critical size by random fluctuations, it becomes energetically favorable to add additional molecules and grow.
McGraw and Liu apply this theory at the larger scale of drizzle drops. Droplets constantly grow through condensation and evaporate at random, but once a droplet reaches a critical radius it can begin to grow more quickly through collection. In the team’s model, the rate at which droplets cross the size threshold can be calculated based on the cloud’s turbulence and the concentration and size distribution of the droplets. According to McGraw, enough of the droplets would cross the size threshold quickly to account for drizzle’s fast formation.
McGraw says the work also helps explain one effect that airborne pollutant particles have on the environment. These particles increase the concentration of droplets in clouds, which according to the theory, increases the critical radius they need to attain before they can grow. So polluted clouds are more stable and less likely to produce rain. This prediction agrees with observations: clouds over land–where there is more pollution–have longer lifetimes than do maritime clouds.
Marcia Baker, of the University of Washington in Seattle, says that McGraw and Liu’s theory is an interesting new formulation of an older idea–that cloud turbulence and fluctuations in water vapor content allow drizzle to form more quickly than it otherwise would. But she cautions that researchers still need to test the theory by comparing it with aircraft and remote-sensing data on drizzle formation. To make the theory more useful in practice, the team also hopes to better account for cloud turbulence in future versions. “That might allow weather forecasters and climate modelers to improve their predictions,” McGraw says.