Focus: How Raindrops Form
Although the TV weatherman can say where raindrops might fall, researchers still have difficulty explaining why they form. A paper in the 26 March PRL sheds light on this cloudy subject by showing that wind turbulence can play a crucial role. Inside clouds tiny vortices created by the wind spin water-sodden dust particles into clusters, where they meld to form raindrops, say the authors. This discovery may eventually help meteorologists predict storms with better accuracy.
Raindrops begin forming when water vapor condenses on micrometer-sized particles of dust floating in the atmosphere. The dust particles grow to millimeter-sized droplets, which are heavy enough to begin falling. As they fall, the droplets accumulate more and more moisture, until they become the large raindrops that we see here on the ground.
Theorists and experimentalists understand this progression, but they cannot agree on how long it takes. “When you estimate the typical time you need to grow from micron- to millimeter-sized droplets, it would take maybe ten or fifteen hours,” says Gregory Falkovich of the Weizmann Institute of Science in Israel. “And empirically people noticed that often rain starts long before this–say in half an hour.”
Theorists have suspected for nearly forty years that wind was a catalyst helping the raindrops form more quickly. The wind, it was believed, increased the relative velocities of the micrometer-sized droplets and caused them to collide and stick together until they became large enough to begin falling. But even after theorists included wind velocities in their models, they could not make their predictions match observation.
Now Falkovich and his colleagues believe they may have found a key factor in the formation of raindrops: wind turbulence. The inside of a cloud is full of turbulence that creates many swirling eddies of air. These tiny vortices, according to Falkovich, act as centrifuges, spinning the micrometer-sized particles out to the edges, where they cluster together. “In these dense clusters,” he says, “there is a much higher probability for them to collide and create bigger droplets.” According to the team’s new calculations, these clusters appear to be about a millimeter in size–just the size needed for raindrops to begin falling.
“This paper is a reflection of a long series of works on the motion of particles in flows,” says Leo Kadanoff of the University of Chicago. Kadanoff says that in the past five years sophisticated analytic models like this one have been developed, but this paper is one of the first attempts to apply them. “It’s a good piece of work,” Kadanoff says. “It points the right way towards the future.”
Falkovich believes that the model may apply to other systems, such as fuel sprays in combustion engines and pollutant concentrations in the atmosphere, but his real hope is for the weather. “Ultimately,” he says, “I would love to give meteorologists a simple formula that says, ‘if wind is of a certain magnitude, then rain will come in forty minutes’–but this is still a dream.”