Tefnut's Environmental and Drought News Article
Tuesday, 21 July 2009 AFP
French scientists armed with ultra-fast video footage say raindrops come in such a bewildering array of sizes for reasons much simpler than previously thought.
Dr Emmanuel Villermaux and Benjamin Bossa of the Aix-Marseille University in southern France, report their findings online in the journal Nature Physics.
Way back in 1904, scientists noted that raindrops came in different sizes.
For generations, schoolchildren were taught that raindrops start as micro-droplets that then gather together in clouds with their neighbours to become bigger droplets.
The common wisdom was that raindrops come in such a remarkable range of sizes through a complex interaction between droplets as they fall.
Droplets which bump into other droplets coalesce and become bigger, while smaller droplets are those that do not bump into as many other droplets, so the idea went.
But Villermaux and Bossa say the explanation for why raindrops can range from fine droplets to chubby plops, is much simpler than this.
Their movies of falling water droplets shows the diversity of raindrops is caused by fragmentation of individual, non-interacting raindrops.
They have found a raindrop starts to fall as a sphere, but then flattens out into a pancake shape.
Eventually, as the pancake widens and thins, the onrush of air causes it to hollow out, like an upturned bag, they say.
Finally, the bag inflates beyond the ability of the water's tension to hold things together and bursts into lots of smaller droplets.
The whole process takes only several thousands of a second.
But when the droplets hit the ground, the most numerous by far are very small, while large drops are comparatively few.
"Rainfall does indeed start through coalescence in the clouds but something quite different happens on the way down, and this explains the diversity of raindrop sizes," says Villermaux.
"Each drop breaks up individually, independently of its neighbours, on its way to earth."
The findings could have practical applications in understanding rainfall patterns and in crop spraying, says Villermaux.
"Take pesticide spraying, for instance. With most sprayers in use today, all it takes is a slight breeze for half of the pesticide to end up in the neighbouring field," he says.
"Understanding how droplets will be distributed can be very important."
Source: ABC Science News
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