The hypothesis was that droplets which bump into other droplets coalesce and become bigger, while smaller droplets are those that did not bump into as many other droplets.īut the new paper, published online in the journal Nature Physics, says the process is in fact much simpler than thought. Way back in 1904, scientists noted that raindrops came in different sizes. "Each drop breaks up individually, independently of its neighbours, on its way to earth." "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," said Emmanuel Villermaux, who co-wrote the study with Benjamin Bossa of the Aix-Marseille University in southern France. The whole process takes only several thousands of a second.īut when the droplets hit the ground, the most numerous by far are very small, while large drops are comparatively few. The bag then inflates beyond the ability of the water's tension to hold things together and bursts into lots of smaller droplets. Initially, a raindrop starts to fall as a sphere, but then flattens out into a pancake shape.Įventually, as the pancake widens and thins, the onrush of air causes it to hollow out, like an upturned bag. Complex interaction between these droplets as they fall explains why raindrops come in such a remarkable range of sizes, goes this idea.īut French scientists armed with ultra-fast video footage say something else happens on the way down - and this is why rainfall can range from fine droplets to chubby plops.
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