Bring On the Rain


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A fast-paced rain dance inside clouds brings on rainfall, shows a new Weizmann study. The findings, published in Nature, may provide an effective tool for rain prediction.
The Weizmann team has revealed that turbulent whirlpools within clouds spin heavy droplets outward, much like a sling whirled around to discharge a stone by centrifugal force, a phenomenon the scientists call the “sling effect.”
Rain dance . illustration
Droplets hurled by a turbulent whirlpool are more likely to collide with one another than are droplets floating peacefully about. Colliding droplets form heavy, rain-producing drops. The team – Prof. Gregory Falkovich of the Physics of Complex Systems Department, graduate student Alexander Fouxon and visiting scientist Dr. Michael Stepanov – has developed a formula that makes it possible to calculate the speed with which tiny droplets in clouds cluster into the rain-producing drops. By predicting the collision rate of droplets in a turbulent cloud, the formula makes it possible to forecast when the cloud will produce rain.
Emergence of raindrops from the cloud occurs in two stages. First, tiny moisture droplets condense and gradually grow until they reach a diameter of about 20 micrometers (20 thousandths of a millimeter). At this size, the droplets begin crashing into one another and gathering into larger drops about a millimeter across.
The collisions are caused mainly by turbulent airflow, creating whirlpools and eddies inside the cloud. Since turbulent flows of different magnitudes exist in all clouds, the Weizmann Institute formula, which includes such variables as temperature, humidity and wind speed, may prove useful for improving the precision of numerous meteorological forecasts.
Prof. Gregory Falkovich’s research is supported by the Gabriel Alhadeff Research Fund and the Edward D. and Anna Mitchell Family Foundation.