July 21, 2009

New Research Explains Differences In Raindrop Size

Scientists have long sought to explain why raindrops come in such a variety of different sizes, with prevailing wisdom saying the rain begins as tiny micro-droplets that join with nearby droplets as they fall to form even larger drops.

However, after filming a single falling raindrop about six millimeters in diameter, French scientists now have an explanation for the wide variety of raindrop sizes.

Using ultra-fast video footage, the researchers say the complex interaction between the drops as they fall is indeed partially responsible for the variety of sizes.  However, something else happens on the way down.

The raindrops begin their fall as tiny spheres, but then flatten out into a pancake shape, they say.  The pancake-shaped drops then widen and thin, with an onrush of air causing them to hollow out in the shape of an upturned bag.

The bag-shaped droplet eventually expands beyond the ability of the water's tension to hold itself together, resulting in a bursting of the drop into many smaller droplets. 

The entire process takes just six thousandths of a second.

Each large, heavy drop accelerates and "has to displace the air molecules" as it falls, Dr. Villermaux told BBC News.

"This produces the air resistance or drag."

At a certain speed, the intensity of this drag is greater than the surface tension holding the round drop together, so the drop starts to change shape.

"When it bursts, the fragments match exactly what we find in raindrops," Dr. Villermaux said.

"This is a precise, quantitative explanation for their distribution and size."

The majority of raindrops that hit the Earth are very small, with large drops comprising just a small percentage of the total.

"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 Dr. Villermaux, who along with Benjamin Bossa of the Aix-Marseille University co-authored a report about the study.

"Each drop breaks up individually, independently of its neighbors, on its way to earth," he said.

As far back as 1904 scientists wrote about the wide variety of raindrop sizes. The hypothesis was that droplets collide into other droplets and form the large raindrops, while smaller droplets were merely those that did not bump into as many other droplets.

But Villermaux said there were always "shortcomings" in this theory.

"The drops are not likely to collide that often," he told BBC News.

Since real raindrops are so sparse, a drop would likely "fall on its own and never see its neighbors," he added.

The study's findings could have applications in understanding rainfall patterns and in crop spraying, an AFP news agency report cited Villermaux as saying.

"Understanding how droplets will be distributed can be very important. 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 neighboring field," he said.

The study was published online in the journal Nature Physics.


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