Interesting

Most people can detect the distinctive fresh, earthy aroma of an approaching rain storm, but now scientists have worked out why.

Researchers using high speed cameras have found that drops of water release clouds of tiny particles when they hit surfaces like soil and leaves.

Their study showed that a raindrop hitting an uneven surface, they trap bubbles of air that shoot upwards and burst from the top of the water droplet like fizz in a champagne glass.

These tiny bubbles carry minute amounts of aromatic particles of oil and dust from the surface that can then be blown for miles by gusts of wind ahead of rain storms.

This, the scientists say, explains why it is possible to smell a rain storm long before it arrives, even when it has been dry for several days.

The effect, known as Petrichor, is often most pronounced during the summer, accompanying the first rain after a long dry smell when more dust and oils have accumulated on plants and on the ground.

The new research, which was conducted by scientists at the Massachusetts Institute of Technology, found that different types of rainfall could alter the smell.

The scientists found that light showers and moderate seemed to trigger more aerosols compared with heavy rain that might accompany thunder storms.

They also found that the type of soil could also influence how many aerosols were released and was particularly pronounced on clay or sandy soil.

Dr Youngsoo Joung, one of the scientists at MIT's department of engineering who conducted the research, said the findings could also help to explain how some soil-based bacteria can spread disease.

He said: 'Until now, people didn’t know that aerosols could be generated from raindrops on soil.

'When moderate or light rain hits sandy or clay soils, you can observe lots of aerosols, because sandy clay has medium wetting properties.

'Heavy rain (which has a high) impact speed, means there’s not enough time to make bubbles inside the droplet.

This finding should be a good reference for future work, illuminating microbes and chemicals existing inside soil and other natural materials, and how they can be delivered in the environment, and possibly to humans.

'To prevent transmission of microorganisms from nature to humans, we need to know the exact mechanism. In this work, we provide one possible way of transmission.'

Scientists in Australia were the first to coin the word 'petrichor' for the smell of approaching rain and characterised it as the release of plant oils along with a compound called geosmin, which is produced by soil dwelling bacteria.

However, the new research is the first to explain the mechanism that causes these compounds to become airborne.

The scientists, whose work is published in the journal Nature Communications, conducted 600 experiments on 28 different types of surfaces - 12 man made surfaces, like aluminium, and 16 soil surfaces.

These included soil collected from alongside the Charles River and sandy soil from Nahant Beach in Nahant, Massachusetts.

The researchers measured the permeability of each soil by adding water to the bottom of a sample within a test tube and measuring how long it took the water to rise upwards.

They then dropped single droplets of water from different heights to simulate varying intensities of rainwater.

They used high speed cameras to capture what happened to the raindrops on impact.

They found that as a raindrop hits a surface, it flattens and tiny bubbles trapped in pores in the surface rise up through the droplet and burst into the air.

With light droplets of rain, more of these bubbles become aerosols, producing a cloud of tiny 'frenzied' droplets within a few microseconds, particularly in porous material like sandy and clay based soil.

When a light breeze was simulated by blowing air across the top of the soil, the scientists found the aerosols were carried away.

They believe that along with the air and water that form the aerosols, tiny amounts of oil and dust from the surface are also trapped inside.

The researchers also found the impact speed of a droplet and the surface 'wetability' were crucial in determining how much aerosols were produced.

Dr Cullen Buie, an assistant professor of mechanical engineering who led the work, said:

'Rain happens every day - it's raining now, somewhere in the world.

'It's a very common phenomenon, and it was intriguing to us that no one had observed this mechanism before.'

He said that while the Australian researchers had first identified the phenomenon of Petrichor in 1964, they had not explained what might cause it to occur.

He added: 'They talked about oils emitted by plants, and certain chemicals from bacteria, that lead to this smell you get after a rain following a long dry spell.

'Interestingly, they don't discuss the mechanism for how that smell gets into the air. One hypothesis we have is that that smell comes from this mechanism we've discovered.'

James Bird, an assistant professor of mechanical engineering at Boston University who also studies the behaviour of fluids when they drop onto surfaces and was not involved in the latest study, said the work could explain how microbes end up high in the atmosphere.

He said: Microbes from soil have been observed high in the atmosphere.

'This paper provides an elegant mechanism by which these microbes can be propelled past the stagnant layer of air around them to a place where the breeze can take them elsewhere.'