Most people can detect the distinctive fresh, earthy aroma of an approaching rainstorm, 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 rainstorm 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 thunderstorms.
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 characterized 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.
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