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BROOKHAVEN SCIENTISTS DESCRIBE NEW STATISTICAL APPROACH TO PREDICTING RAINDROP FORMATION
Chemical physicist Robert McGraw and cloud physicist Yangang Liu compare raindrop formation to the phenomenon of nucleation, the same process that forms sugar crystals in honey. Their approach, based on statistical modeling, raises questions about the traditional deterministic view of raindrop formation. Rain development is generally 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 would take over an hour to form. This timescale, however, does not resolve with the 30-minute average lifetime of a precipitating cloud. Scientists believe that other factors, like cloud turbulence, allow droplets to form more quickly, but the details of how this would occur have never been fully explored, until now. McGraw and Liu treat raindrop formation as a kind of homogeneous nucleation. In honey, for example, constant random fluctuations at the molecular level cause sugar molecules to clump together. Below a certain size, the clumps don’t continue to grow because they lose more energy due to surface tension than they gain due to volume for each molecule added. At a certain critical size, the crystal begins to grow because it becomes energetically favorable to add molecules. The rates at which these sugar clumps grow can be calculated based on the molecular properties of the honey. Applying a similar theory to raindrops in a cloud, McGraw and Liu explain that once a droplet reaches a certain size, it can grow more quickly by collecting other raindrops. Their model can calculate the rate at which droplets cross the energy barrier (in this case, it’s a kinetic barrier that needs to be overcome) and begin to grow based on the concentration and size distribution of the droplets. While the pair’s work may give meteorologists a new tool for weather forecasting, in their field of atmospheric chemistry it helps explain one effect that aerosols have on the environment. Aerosols increase the concentration of droplets in clouds. According to their theory, this increases the critical size droplets need to attain before they begin to grow and increases the kinetic barrier height. Hence, aerosol-polluted clouds are more stable and less likely to produce rain. Says McGraw, “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.”
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