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August 03, 2010

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Origin of raindrop size “revealed”

Four stages in the frag­men­ta­tion of a fall­ing, 5-millimeter drop, tak­ing six hun­dredths of a sec­ond in all. (Cred­it: Em­man­u­el Viller­maux)

July 20, 2009
Courtesy Nature Physics
and World Science staff

The sizes of rain­drops result from the break­up of larg­er drop­lets, re­ports a study pub­lished on­line this week in the re­search jour­nal Na­ture Phys­ics. This is a much sim­pler mech­an­ism than was pre­vi­ously thought. 

When rain­drops hit the ground they do so in a wide range of sizes. It was thought that this size dis­tri­bu­tion was the re­sult of a com­plex se­ries of in­ter­ac­tions be­tween the drop­lets as they fall. 

But by an­a­lys­ing high-speed movies of fall­ing wa­ter drop­lets, Em­man­u­el Viller­maux and Ben­ja­min Bossa of Aix-Marseille Un­ivers­ity in France found that this dis­tri­bu­tion is caused by the frag­menta­t­ion of in­di­vid­ual, non-interacting rain­drops. 

The movies show that as an in­i­tially glob­u­lar drop­let falls, it flat­tens out in­to a pan­cake shape. As it gets wid­er and thin­ner, it even­tu­ally cap­tures the air in front of it to form the shape of an up­turned bag. Fi­nal­ly, as the bag in­flates to a cer­tain size, it breaks apart in­to many smaller drop­lets—drop­lets whose size dis­tri­bu­tion mim­ics that of nat­u­ral rain­fall, ac­cord­ing to the sci­en­tists.

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The sizes of raindrops are caused by the break up of larger droplets, reports a study published online this week in the research journal Nature Physics. This is a much simpler mechanism than was previously thought. When raindrops hit the ground they do so in a wide range of sizes. It was thought that this size distribution was the result of a complex series of interactions between the droplets as they fall. But by analysing high-speed movies of falling water droplets, Emmanuel Villermaux and Benjamin Bossa of Aix-Marseille University in France found that this distribution is caused by the fragmentation of individual, non-interacting raindrops. The movies show that as an initially globular droplet falls, it gradually flattens out into a pancake shape. As it gets wider and thinner, it eventually captures the air in front of it to form the shape of an upturned bag. Finally, as the bag inflates to a certain size, it breaks apart into many smaller droplets—droplets whose size distribution mimics that of natural rainfall, according to the scientists.