Why raindrops come in many sizes

By Victoria Gill
Science reporter, BBC News

Advertisement

A drop deforms as it falls through the air, and eventually inflates and bursts. Footage courtesy of Emmanuel Villermaux, Aix-Marseille University

We might never consider the size of the raindrops as we hurry for cover, but their variety has puzzled scientists for many years.

Now, by filming one falling raindrop, researchers in France have explained why the drops are an array of so many different sizes.

Reporting in the journal Nature Physics, the team described how the drop deformed and burst as it fell.

Its fragments matched the size and distribution of drops in natural rain.

Scientists previously believed that the drops collided with each other as they descended, and that these interactions produced a variety of drop sizes.

But the lead author of this study, Emmanuel Villermaux from Aix-Marseille University, explained that there were always "shortcomings" in this idea.

"The drops are not likely to collide that often," he told BBC News. Real raindrops are so sparse, he said, that it is likely a drop would "fall on its own and never see its neighbours".

"So we said OK - let's look at what's happening on the scale of a single drop."

With a high-speed camera, Dr Villermaux and his colleagues filmed a single falling drop of water - about six millimetres in diameter.

They recorded how air resistance caused it to deform and eventually break up.

Air resistance causes a raindrop to deform as it falls, and eventually to fragment

The large, round drop fell, gradually flattened out and, as it got wider, eventually "captured" the air in front of it to form the shape of an upturned bag.

This bag finally "inflated" and burst apart into many smaller droplets - all within six hundredths of a second.

This happened because drops were too large and heavy to remain intact.

Each large, heavy drop accelerates as it falls and "has to displace the air molecules" on its way down, explained Dr Villermaux. "This produces the air resistance or drag."

At a certain speed, the number of air molecules - and therefore the intensity of this drag - is greater than the surface tension holding the round drop together, so the drop starts to deform.

"When it bursts, the fragments match exactly what we find in raindrops," said Dr Villermaux. "This is a precise, quantitative explanation for their distribution and size."

Dr Ewan O'Connor, a scientist from the University of Reading, who studies clouds - taking measurements to improve weather modelling and forecasting - described this as a a very nice way of showing exactly what happens.

"But this is unlikely be what happens all of the time in the UK (for example), as we don't get raindrops of this size that often," he told BBC News.

"When raindrops get to a certain size... you will get this break-up. And this is likely to happen often in the tropics."

But, Dr O'Connor added, "this doesn't explain drizzle, where the droplets are much smaller, but there are many more of them."