With grants from NASA andthe U.S. Department of Energy, two physicists at Brookhaven National Laboratory have proposed a new theory on raindrop formation. This new theory explains how rain forms in warm rain clouds.

Robert McGraw, achemical physicist, and Yangang Liu, a cloud physicist, researched the relationship between raindrop formation and the phenomenon of nucleation. This new approach used statistical modeling and raised several questions about the traditional theory of raindrop formation.

Rain development has generally been described in two stages. Small droplets grow through condensation, acquiring water molecules from water vapor in the surrounding cloud. Larger droplets grow by collecting smaller droplets as they fall through the cloud; a full-sized raindrop takes over an hour to form.

This theory, however, does not coincide with the 30-minute average lifetime of a precipitating cloud. Researchers think that other variables such as cloud turbulence allow droplets to form more quickly, and McGraw and Liu have studied the details of how this might occur.

McGraw and Liu found that once a droplet reaches a certain size, it can grow more quickly by collecting other raindrops. This model calculates the rate at which droplets cross the energy barrier and begin to grow based on the concentration and size distribution of the droplets.

Another significant aspect of the scientists’ research is the effect of aerosols on raindrop formation. Aerosols increase the concentration of droplets in clouds.

According to the new theory, the aerosol effect increases the critical size droplets need to attain before they begin to grow and increases the kinetic barrier height. Therefore, aerosol-polluted clouds are more stable and less likely to produce rain.

‘We already know that clouds over land have longer average lifetimes than do clouds over oceans. That makes sense because clouds over land contain pollutants like aerosols,’ McGraw said.